From 42accc99e6c4ad83876460afbda9d0adab750930 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 16:04:12 +0200
Subject: [PATCH 01/35] let_chains: Remove ast::ExprKind::{IfLet, WhileLet} and
 introduce ::Let.

---
 src/libsyntax/ast.rs | 20 +++++---------------
 1 file changed, 5 insertions(+), 15 deletions(-)

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index b1a62ac81d033..4f2fccd20e5f5 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -1032,10 +1032,9 @@ impl Expr {
             ExprKind::Unary(..) => ExprPrecedence::Unary,
             ExprKind::Lit(_) => ExprPrecedence::Lit,
             ExprKind::Type(..) | ExprKind::Cast(..) => ExprPrecedence::Cast,
+            ExprKind::Let(..) => ExprPrecedence::Let,
             ExprKind::If(..) => ExprPrecedence::If,
-            ExprKind::IfLet(..) => ExprPrecedence::IfLet,
             ExprKind::While(..) => ExprPrecedence::While,
-            ExprKind::WhileLet(..) => ExprPrecedence::WhileLet,
             ExprKind::ForLoop(..) => ExprPrecedence::ForLoop,
             ExprKind::Loop(..) => ExprPrecedence::Loop,
             ExprKind::Match(..) => ExprPrecedence::Match,
@@ -1116,26 +1115,17 @@ pub enum ExprKind {
     Cast(P<Expr>, P<Ty>),
     /// A type ascription (e.g., `42: usize`).
     Type(P<Expr>, P<Ty>),
+    /// A `let pat = expr` pseudo-expression that only occurs in the scrutinee
+    /// of `if` / `while` expressions. (e.g., `if let 0 = x { .. }`).
+    Let(Vec<P<Pat>>, P<Expr>),
     /// An `if` block, with an optional `else` block.
     ///
     /// `if expr { block } else { expr }`
     If(P<Expr>, P<Block>, Option<P<Expr>>),
-    /// An `if let` expression with an optional else block
-    ///
-    /// `if let pat = expr { block } else { expr }`
-    ///
-    /// This is desugared to a `match` expression.
-    IfLet(Vec<P<Pat>>, P<Expr>, P<Block>, Option<P<Expr>>),
-    /// A while loop, with an optional label
+    /// A while loop, with an optional label.
     ///
     /// `'label: while expr { block }`
     While(P<Expr>, P<Block>, Option<Label>),
-    /// A `while let` loop, with an optional label.
-    ///
-    /// `'label: while let pat = expr { block }`
-    ///
-    /// This is desugared to a combination of `loop` and `match` expressions.
-    WhileLet(Vec<P<Pat>>, P<Expr>, P<Block>, Option<Label>),
     /// A `for` loop, with an optional label.
     ///
     /// `'label: for pat in expr { block }`

From f0ab633e4dd56ae2445ec1806d582f50f42117e9 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 16:06:58 +0200
Subject: [PATCH 02/35] let_chains: Remove ast::ExprKind::{IfLet, WhileLet}
 from visitors and introduce ::Let.

---
 src/librustc_save_analysis/dump_visitor.rs | 12 ++----------
 src/libsyntax/mut_visit.rs                 | 16 ++++------------
 src/libsyntax/parse/classify.rs            |  2 --
 src/libsyntax/visit.rs                     | 16 ++++------------
 4 files changed, 10 insertions(+), 36 deletions(-)

diff --git a/src/librustc_save_analysis/dump_visitor.rs b/src/librustc_save_analysis/dump_visitor.rs
index f67241ef23efc..09406f7306dd7 100644
--- a/src/librustc_save_analysis/dump_visitor.rs
+++ b/src/librustc_save_analysis/dump_visitor.rs
@@ -1580,17 +1580,9 @@ impl<'l, 'tcx, 'll, O: DumpOutput + 'll> Visitor<'l> for DumpVisitor<'l, 'tcx, '
                 self.visit_expr(subexpression);
                 visit::walk_block(self, block);
             }
-            ast::ExprKind::WhileLet(ref pats, ref subexpression, ref block, _) => {
+            ast::ExprKind::Let(ref pats, ref scrutinee) => {
                 self.process_var_decl_multi(pats);
-                debug!("for loop, walk sub-expr: {:?}", subexpression.node);
-                self.visit_expr(subexpression);
-                visit::walk_block(self, block);
-            }
-            ast::ExprKind::IfLet(ref pats, ref subexpression, ref block, ref opt_else) => {
-                self.process_var_decl_multi(pats);
-                self.visit_expr(subexpression);
-                visit::walk_block(self, block);
-                opt_else.as_ref().map(|el| self.visit_expr(el));
+                self.visit_expr(scrutinee);
             }
             ast::ExprKind::Repeat(ref element, ref count) => {
                 self.visit_expr(element);
diff --git a/src/libsyntax/mut_visit.rs b/src/libsyntax/mut_visit.rs
index 5a5b633e3151f..35aa2eead0b2b 100644
--- a/src/libsyntax/mut_visit.rs
+++ b/src/libsyntax/mut_visit.rs
@@ -1110,28 +1110,20 @@ pub fn noop_visit_expr<T: MutVisitor>(Expr { node, id, span, attrs }: &mut Expr,
             vis.visit_ty(ty);
         }
         ExprKind::AddrOf(_m, ohs) => vis.visit_expr(ohs),
+        ExprKind::Let(pats, scrutinee) => {
+            visit_vec(pats, |pat| vis.visit_pat(pat));
+            vis.visit_expr(scrutinee);
+        }
         ExprKind::If(cond, tr, fl) => {
             vis.visit_expr(cond);
             vis.visit_block(tr);
             visit_opt(fl, |fl| vis.visit_expr(fl));
         }
-        ExprKind::IfLet(pats, expr, tr, fl) => {
-            visit_vec(pats, |pat| vis.visit_pat(pat));
-            vis.visit_expr(expr);
-            vis.visit_block(tr);
-            visit_opt(fl, |fl| vis.visit_expr(fl));
-        }
         ExprKind::While(cond, body, label) => {
             vis.visit_expr(cond);
             vis.visit_block(body);
             visit_opt(label, |label| vis.visit_label(label));
         }
-        ExprKind::WhileLet(pats, expr, body, label) => {
-            visit_vec(pats, |pat| vis.visit_pat(pat));
-            vis.visit_expr(expr);
-            vis.visit_block(body);
-            visit_opt(label, |label| vis.visit_label(label));
-        }
         ExprKind::ForLoop(pat, iter, body, label) => {
             vis.visit_pat(pat);
             vis.visit_expr(iter);
diff --git a/src/libsyntax/parse/classify.rs b/src/libsyntax/parse/classify.rs
index dfd6f451c28d7..6ebfab3a133ef 100644
--- a/src/libsyntax/parse/classify.rs
+++ b/src/libsyntax/parse/classify.rs
@@ -14,11 +14,9 @@ use crate::ast;
 pub fn expr_requires_semi_to_be_stmt(e: &ast::Expr) -> bool {
     match e.node {
         ast::ExprKind::If(..) |
-        ast::ExprKind::IfLet(..) |
         ast::ExprKind::Match(..) |
         ast::ExprKind::Block(..) |
         ast::ExprKind::While(..) |
-        ast::ExprKind::WhileLet(..) |
         ast::ExprKind::Loop(..) |
         ast::ExprKind::ForLoop(..) |
         ast::ExprKind::TryBlock(..) => false,
diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index 8132024416a30..4d961142ff1c7 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -719,6 +719,10 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
             visitor.visit_expr(subexpression);
             visitor.visit_ty(typ)
         }
+        ExprKind::Let(ref pats, ref scrutinee) => {
+            walk_list!(visitor, visit_pat, pats);
+            visitor.visit_expr(scrutinee);
+        }
         ExprKind::If(ref head_expression, ref if_block, ref optional_else) => {
             visitor.visit_expr(head_expression);
             visitor.visit_block(if_block);
@@ -729,18 +733,6 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
             visitor.visit_expr(subexpression);
             visitor.visit_block(block);
         }
-        ExprKind::IfLet(ref pats, ref subexpression, ref if_block, ref optional_else) => {
-            walk_list!(visitor, visit_pat, pats);
-            visitor.visit_expr(subexpression);
-            visitor.visit_block(if_block);
-            walk_list!(visitor, visit_expr, optional_else);
-        }
-        ExprKind::WhileLet(ref pats, ref subexpression, ref block, ref opt_label) => {
-            walk_list!(visitor, visit_label, opt_label);
-            walk_list!(visitor, visit_pat, pats);
-            visitor.visit_expr(subexpression);
-            visitor.visit_block(block);
-        }
         ExprKind::ForLoop(ref pattern, ref subexpression, ref block, ref opt_label) => {
             walk_list!(visitor, visit_label, opt_label);
             visitor.visit_pat(pattern);

From 3fc964201486f9fdb006bfb7d0caf9b1c3e6cb71 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 16:05:58 +0200
Subject: [PATCH 03/35] let_chains: Handle it in AST pretty printing.

---
 src/libsyntax/print/pprust.rs | 54 +++++++++--------------------------
 1 file changed, 14 insertions(+), 40 deletions(-)

diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index 0aac4f83658b9..c525f2efb49fb 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -1715,6 +1715,16 @@ impl<'a> State<'a> {
         self.ann.post(self, AnnNode::Block(blk))
     }
 
+    pub fn print_let(&mut self, pats: &[P<ast::Pat>], scrutinee: &ast::Expr) -> io::Result<()> {
+        self.s.word("let ")?;
+
+        self.print_pats(pats)?;
+        self.s.space()?;
+
+        self.word_space("=")?;
+        self.print_expr_as_cond(scrutinee)
+    }
+
     fn print_else(&mut self, els: Option<&ast::Expr>) -> io::Result<()> {
         match els {
             Some(_else) => {
@@ -1729,19 +1739,6 @@ impl<'a> State<'a> {
                         self.print_block(then)?;
                         self.print_else(e.as_ref().map(|e| &**e))
                     }
-                    // Another `else if let` block.
-                    ast::ExprKind::IfLet(ref pats, ref expr, ref then, ref e) => {
-                        self.cbox(INDENT_UNIT - 1)?;
-                        self.ibox(0)?;
-                        self.s.word(" else if let ")?;
-                        self.print_pats(pats)?;
-                        self.s.space()?;
-                        self.word_space("=")?;
-                        self.print_expr_as_cond(expr)?;
-                        self.s.space()?;
-                        self.print_block(then)?;
-                        self.print_else(e.as_ref().map(|e| &**e))
-                    }
                     // Final `else` block.
                     ast::ExprKind::Block(ref b, _) => {
                         self.cbox(INDENT_UNIT - 1)?;
@@ -1762,20 +1759,10 @@ impl<'a> State<'a> {
     pub fn print_if(&mut self, test: &ast::Expr, blk: &ast::Block,
                     elseopt: Option<&ast::Expr>) -> io::Result<()> {
         self.head("if")?;
+
         self.print_expr_as_cond(test)?;
         self.s.space()?;
-        self.print_block(blk)?;
-        self.print_else(elseopt)
-    }
 
-    pub fn print_if_let(&mut self, pats: &[P<ast::Pat>], expr: &ast::Expr, blk: &ast::Block,
-                        elseopt: Option<&ast::Expr>) -> io::Result<()> {
-        self.head("if let")?;
-        self.print_pats(pats)?;
-        self.s.space()?;
-        self.word_space("=")?;
-        self.print_expr_as_cond(expr)?;
-        self.s.space()?;
         self.print_block(blk)?;
         self.print_else(elseopt)
     }
@@ -2052,12 +2039,12 @@ impl<'a> State<'a> {
                 self.word_space(":")?;
                 self.print_type(ty)?;
             }
+            ast::ExprKind::Let(ref pats, ref scrutinee) => {
+                self.print_let(pats, scrutinee)?;
+            }
             ast::ExprKind::If(ref test, ref blk, ref elseopt) => {
                 self.print_if(test, blk, elseopt.as_ref().map(|e| &**e))?;
             }
-            ast::ExprKind::IfLet(ref pats, ref expr, ref blk, ref elseopt) => {
-                self.print_if_let(pats, expr, blk, elseopt.as_ref().map(|e| &**e))?;
-            }
             ast::ExprKind::While(ref test, ref blk, opt_label) => {
                 if let Some(label) = opt_label {
                     self.print_ident(label.ident)?;
@@ -2068,19 +2055,6 @@ impl<'a> State<'a> {
                 self.s.space()?;
                 self.print_block_with_attrs(blk, attrs)?;
             }
-            ast::ExprKind::WhileLet(ref pats, ref expr, ref blk, opt_label) => {
-                if let Some(label) = opt_label {
-                    self.print_ident(label.ident)?;
-                    self.word_space(":")?;
-                }
-                self.head("while let")?;
-                self.print_pats(pats)?;
-                self.s.space()?;
-                self.word_space("=")?;
-                self.print_expr_as_cond(expr)?;
-                self.s.space()?;
-                self.print_block_with_attrs(blk, attrs)?;
-            }
             ast::ExprKind::ForLoop(ref pat, ref iter, ref blk, opt_label) => {
                 if let Some(label) = opt_label {
                     self.print_ident(label.ident)?;

From dff1e379fc55429ea9a4794634e02c17cebee603 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 16:03:22 +0200
Subject: [PATCH 04/35] let_chains: Add support for parsing let expressions.

---
 src/libsyntax/parse/parser.rs | 67 ++++++++++-------------------------
 src/libsyntax/parse/token.rs  |  1 +
 src/libsyntax/util/parser.rs  |  7 ++--
 3 files changed, 22 insertions(+), 53 deletions(-)

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index fa697e06d269d..8cf05059e8a64 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -2215,13 +2215,8 @@ impl<'a> Parser<'a> {
                     } else {
                         ex = ExprKind::Yield(None);
                     }
-                } else if self.token.is_keyword(kw::Let) {
-                    // Catch this syntax error here, instead of in `parse_ident`, so
-                    // that we can explicitly mention that let is not to be used as an expression
-                    let mut db = self.fatal("expected expression, found statement (`let`)");
-                    db.span_label(self.token.span, "expected expression");
-                    db.note("variable declaration using `let` is a statement");
-                    return Err(db);
+                } else if self.eat_keyword(kw::Let) {
+                    return self.parse_let_expr(attrs);
                 } else if is_span_rust_2018 && self.eat_keyword(kw::Await) {
                     let (await_hi, e_kind) = self.parse_await_macro_or_alt(lo, self.prev_span)?;
                     hi = await_hi;
@@ -2483,15 +2478,13 @@ impl<'a> Parser<'a> {
                 attrs.extend::<Vec<_>>(expr.attrs.into());
                 expr.attrs = attrs;
                 match expr.node {
-                    ExprKind::If(..) | ExprKind::IfLet(..) => {
-                        if !expr.attrs.is_empty() {
-                            // Just point to the first attribute in there...
-                            let span = expr.attrs[0].span;
-
-                            self.span_err(span,
-                                "attributes are not yet allowed on `if` \
-                                expressions");
-                        }
+                    ExprKind::If(..) if !expr.attrs.is_empty() => {
+                        // Just point to the first attribute in there...
+                        let span = expr.attrs[0].span;
+
+                        self.span_err(span,
+                            "attributes are not yet allowed on `if` \
+                            expressions");
                     }
                     _ => {}
                 }
@@ -3163,9 +3156,6 @@ impl<'a> Parser<'a> {
 
     /// Parses an `if` or `if let` expression (`if` token already eaten).
     fn parse_if_expr(&mut self, attrs: ThinVec<Attribute>) -> PResult<'a, P<Expr>> {
-        if self.check_keyword(kw::Let) {
-            return self.parse_if_let_expr(attrs);
-        }
         let lo = self.prev_span;
         let cond = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
 
@@ -3197,22 +3187,18 @@ impl<'a> Parser<'a> {
         Ok(self.mk_expr(lo.to(hi), ExprKind::If(cond, thn, els), attrs))
     }
 
-    /// Parses an `if let` expression (`if` token already eaten).
-    fn parse_if_let_expr(&mut self, attrs: ThinVec<Attribute>)
-                             -> PResult<'a, P<Expr>> {
+    /// Parses a `let $pats = $expr` pseudo-expression.
+    /// The `let` token has already been eaten.
+    fn parse_let_expr(&mut self, attrs: ThinVec<Attribute>) -> PResult<'a, P<Expr>> {
         let lo = self.prev_span;
-        self.expect_keyword(kw::Let)?;
         let pats = self.parse_pats()?;
         self.expect(&token::Eq)?;
-        let expr = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
-        let thn = self.parse_block()?;
-        let (hi, els) = if self.eat_keyword(kw::Else) {
-            let expr = self.parse_else_expr()?;
-            (expr.span, Some(expr))
-        } else {
-            (thn.span, None)
-        };
-        Ok(self.mk_expr(lo.to(hi), ExprKind::IfLet(pats, expr, thn, els), attrs))
+
+        let expr = self.with_res(
+            Restrictions::NO_STRUCT_LITERAL,
+            |this| this.parse_assoc_expr_with(1 + AssocOp::LAnd.precedence(), None.into())
+        )?;
+        Ok(self.mk_expr(lo.to(expr.span), ExprKind::Let(pats, expr), attrs))
     }
 
     /// Parses `move |args| expr`.
@@ -3299,9 +3285,6 @@ impl<'a> Parser<'a> {
     fn parse_while_expr(&mut self, opt_label: Option<Label>,
                             span_lo: Span,
                             mut attrs: ThinVec<Attribute>) -> PResult<'a, P<Expr>> {
-        if self.token.is_keyword(kw::Let) {
-            return self.parse_while_let_expr(opt_label, span_lo, attrs);
-        }
         let cond = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
         let (iattrs, body) = self.parse_inner_attrs_and_block()?;
         attrs.extend(iattrs);
@@ -3309,20 +3292,6 @@ impl<'a> Parser<'a> {
         return Ok(self.mk_expr(span, ExprKind::While(cond, body, opt_label), attrs));
     }
 
-    /// Parses a `while let` expression (`while` token already eaten).
-    fn parse_while_let_expr(&mut self, opt_label: Option<Label>,
-                                span_lo: Span,
-                                mut attrs: ThinVec<Attribute>) -> PResult<'a, P<Expr>> {
-        self.expect_keyword(kw::Let)?;
-        let pats = self.parse_pats()?;
-        self.expect(&token::Eq)?;
-        let expr = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
-        let (iattrs, body) = self.parse_inner_attrs_and_block()?;
-        attrs.extend(iattrs);
-        let span = span_lo.to(body.span);
-        return Ok(self.mk_expr(span, ExprKind::WhileLet(pats, expr, body, opt_label), attrs));
-    }
-
     // parse `loop {...}`, `loop` token already eaten
     fn parse_loop_expr(&mut self, opt_label: Option<Label>,
                            span_lo: Span,
diff --git a/src/libsyntax/parse/token.rs b/src/libsyntax/parse/token.rs
index cc34883e2e815..98e06ad73c94f 100644
--- a/src/libsyntax/parse/token.rs
+++ b/src/libsyntax/parse/token.rs
@@ -134,6 +134,7 @@ pub(crate) fn ident_can_begin_expr(name: ast::Name, span: Span, is_raw: bool) ->
         kw::Continue,
         kw::False,
         kw::For,
+        kw::Let,
         kw::If,
         kw::Loop,
         kw::Match,
diff --git a/src/libsyntax/util/parser.rs b/src/libsyntax/util/parser.rs
index fcecee8c57fc9..81b5b08593726 100644
--- a/src/libsyntax/util/parser.rs
+++ b/src/libsyntax/util/parser.rs
@@ -260,6 +260,7 @@ pub enum ExprPrecedence {
 
     Box,
     AddrOf,
+    Let,
     Unary,
 
     Call,
@@ -277,9 +278,7 @@ pub enum ExprPrecedence {
     Path,
     Paren,
     If,
-    IfLet,
     While,
-    WhileLet,
     ForLoop,
     Loop,
     Match,
@@ -318,6 +317,8 @@ impl ExprPrecedence {
             // Unary, prefix
             ExprPrecedence::Box |
             ExprPrecedence::AddrOf |
+            // Here `let pats = expr` has `let pats =` as a "unary" prefix of `expr`.
+            ExprPrecedence::Let |
             ExprPrecedence::Unary => PREC_PREFIX,
 
             // Unary, postfix
@@ -338,9 +339,7 @@ impl ExprPrecedence {
             ExprPrecedence::Path |
             ExprPrecedence::Paren |
             ExprPrecedence::If |
-            ExprPrecedence::IfLet |
             ExprPrecedence::While |
-            ExprPrecedence::WhileLet |
             ExprPrecedence::ForLoop |
             ExprPrecedence::Loop |
             ExprPrecedence::Match |

From 70a65e9d1ffa6c3e39bb0eff3c653f6329e2f16b Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 16:05:39 +0200
Subject: [PATCH 05/35] let_chains: Handle in resolve.

---
 src/librustc_resolve/lib.rs | 62 ++++++++++++++-----------------------
 1 file changed, 24 insertions(+), 38 deletions(-)

diff --git a/src/librustc_resolve/lib.rs b/src/librustc_resolve/lib.rs
index 0fbd0666ad1c1..fdef707e347ca 100644
--- a/src/librustc_resolve/lib.rs
+++ b/src/librustc_resolve/lib.rs
@@ -485,8 +485,8 @@ type BindingMap = FxHashMap<Ident, BindingInfo>;
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 enum PatternSource {
     Match,
-    IfLet,
-    WhileLet,
+    // FIXME(54883): Consider fusing with `Let` below once let-statements support or-patterns.
+    LetExpr,
     Let,
     For,
     FnParam,
@@ -496,9 +496,7 @@ impl PatternSource {
     fn descr(self) -> &'static str {
         match self {
             PatternSource::Match => "match binding",
-            PatternSource::IfLet => "if let binding",
-            PatternSource::WhileLet => "while let binding",
-            PatternSource::Let => "let binding",
+            PatternSource::Let | PatternSource::LetExpr => "let binding",
             PatternSource::For => "for binding",
             PatternSource::FnParam => "function parameter",
         }
@@ -3057,13 +3055,7 @@ impl<'a> Resolver<'a> {
     fn resolve_arm(&mut self, arm: &Arm) {
         self.ribs[ValueNS].push(Rib::new(NormalRibKind));
 
-        let mut bindings_list = FxHashMap::default();
-        for pattern in &arm.pats {
-            self.resolve_pattern(&pattern, PatternSource::Match, &mut bindings_list);
-        }
-
-        // This has to happen *after* we determine which pat_idents are variants.
-        self.check_consistent_bindings(&arm.pats);
+        self.resolve_pats(&arm.pats, PatternSource::Match);
 
         if let Some(ast::Guard::If(ref expr)) = arm.guard {
             self.visit_expr(expr)
@@ -3073,6 +3065,16 @@ impl<'a> Resolver<'a> {
         self.ribs[ValueNS].pop();
     }
 
+    /// Arising from `source`, resolve a sequence of patterns (top level or-patterns).
+    fn resolve_pats(&mut self, pats: &[P<Pat>], source: PatternSource) {
+        let mut bindings_list = FxHashMap::default();
+        for pat in pats {
+            self.resolve_pattern(pat, source, &mut bindings_list);
+        }
+        // This has to happen *after* we determine which pat_idents are variants
+        self.check_consistent_bindings(pats);
+    }
+
     fn resolve_block(&mut self, block: &Block) {
         debug!("(resolving block) entering block");
         // Move down in the graph, if there's an anonymous module rooted here.
@@ -3151,8 +3153,7 @@ impl<'a> Resolver<'a> {
                 );
             }
             Some(..) if pat_src == PatternSource::Match ||
-                        pat_src == PatternSource::IfLet ||
-                        pat_src == PatternSource::WhileLet => {
+                        pat_src == PatternSource::LetExpr => {
                 // `Variant1(a) | Variant2(a)`, ok
                 // Reuse definition from the first `a`.
                 res = self.ribs[ValueNS].last_mut().unwrap().bindings[&ident];
@@ -4345,41 +4346,26 @@ impl<'a> Resolver<'a> {
                 visit::walk_expr(self, expr);
             }
 
-            ExprKind::IfLet(ref pats, ref subexpression, ref if_block, ref optional_else) => {
-                self.visit_expr(subexpression);
+            ExprKind::Let(ref pats, ref scrutinee) => {
+                self.visit_expr(scrutinee);
+                self.resolve_pats(pats, PatternSource::LetExpr);
+            }
 
+            ExprKind::If(ref cond, ref then, ref opt_else) => {
                 self.ribs[ValueNS].push(Rib::new(NormalRibKind));
-                let mut bindings_list = FxHashMap::default();
-                for pat in pats {
-                    self.resolve_pattern(pat, PatternSource::IfLet, &mut bindings_list);
-                }
-                // This has to happen *after* we determine which pat_idents are variants
-                self.check_consistent_bindings(pats);
-                self.visit_block(if_block);
+                self.visit_expr(cond);
+                self.visit_block(then);
                 self.ribs[ValueNS].pop();
 
-                optional_else.as_ref().map(|expr| self.visit_expr(expr));
+                opt_else.as_ref().map(|expr| self.visit_expr(expr));
             }
 
             ExprKind::Loop(ref block, label) => self.resolve_labeled_block(label, expr.id, &block),
 
             ExprKind::While(ref subexpression, ref block, label) => {
                 self.with_resolved_label(label, expr.id, |this| {
-                    this.visit_expr(subexpression);
-                    this.visit_block(block);
-                });
-            }
-
-            ExprKind::WhileLet(ref pats, ref subexpression, ref block, label) => {
-                self.with_resolved_label(label, expr.id, |this| {
-                    this.visit_expr(subexpression);
                     this.ribs[ValueNS].push(Rib::new(NormalRibKind));
-                    let mut bindings_list = FxHashMap::default();
-                    for pat in pats {
-                        this.resolve_pattern(pat, PatternSource::WhileLet, &mut bindings_list);
-                    }
-                    // This has to happen *after* we determine which pat_idents are variants.
-                    this.check_consistent_bindings(pats);
+                    this.visit_expr(subexpression);
                     this.visit_block(block);
                     this.ribs[ValueNS].pop();
                 });

From fcffac5eea664834b5b869032761aa0501e6b63b Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 16:03:43 +0200
Subject: [PATCH 06/35] let_chains: Handle in unused parenthesis lint.

---
 src/librustc_lint/unused.rs | 36 +++++++++++++++++++++---------------
 1 file changed, 21 insertions(+), 15 deletions(-)

diff --git a/src/librustc_lint/unused.rs b/src/librustc_lint/unused.rs
index f84ce2f015edf..33b2289593f19 100644
--- a/src/librustc_lint/unused.rs
+++ b/src/librustc_lint/unused.rs
@@ -324,20 +324,28 @@ impl UnusedParens {
                                 value: &ast::Expr,
                                 msg: &str,
                                 followed_by_block: bool) {
-        if let ast::ExprKind::Paren(ref inner) = value.node {
-            let necessary = followed_by_block && match inner.node {
-                ast::ExprKind::Ret(_) | ast::ExprKind::Break(..) => true,
-                _ => parser::contains_exterior_struct_lit(&inner),
-            };
-            if !necessary {
-                let expr_text = if let Ok(snippet) = cx.sess().source_map()
-                    .span_to_snippet(value.span) {
-                        snippet
-                    } else {
-                        pprust::expr_to_string(value)
-                    };
-                Self::remove_outer_parens(cx, value.span, &expr_text, msg);
+        match value.node {
+            ast::ExprKind::Paren(ref inner) => {
+                let necessary = followed_by_block && match inner.node {
+                    ast::ExprKind::Ret(_) | ast::ExprKind::Break(..) => true,
+                    _ => parser::contains_exterior_struct_lit(&inner),
+                };
+                if !necessary {
+                    let expr_text = if let Ok(snippet) = cx.sess().source_map()
+                        .span_to_snippet(value.span) {
+                            snippet
+                        } else {
+                            pprust::expr_to_string(value)
+                        };
+                    Self::remove_outer_parens(cx, value.span, &expr_text, msg);
+                }
+            }
+            ast::ExprKind::Let(_, ref expr) => {
+                // FIXME(#60336): Properly handle `let true = (false && true)`
+                // actually needing the parenthesis.
+                self.check_unused_parens_expr(cx, expr, "`let` scrutinee", followed_by_block);
             }
+            _ => {}
         }
     }
 
@@ -399,8 +407,6 @@ impl EarlyLintPass for UnusedParens {
         let (value, msg, followed_by_block) = match e.node {
             If(ref cond, ..) => (cond, "`if` condition", true),
             While(ref cond, ..) => (cond, "`while` condition", true),
-            IfLet(_, ref cond, ..) => (cond, "`if let` head expression", true),
-            WhileLet(_, ref cond, ..) => (cond, "`while let` head expression", true),
             ForLoop(_, ref cond, ..) => (cond, "`for` head expression", true),
             Match(ref head, _) => (head, "`match` head expression", true),
             Ret(Some(ref value)) => (value, "`return` value", false),

From a7b00f5807dc02f9990c649572d76a91a850110c Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 16:02:51 +0200
Subject: [PATCH 07/35] let_chains: Handle disallowing of let chains in places
 lowering won't support.

---
 src/librustc_passes/ast_validation.rs | 124 ++++++++++++++------------
 src/librustc_passes/lib.rs            |   1 +
 2 files changed, 67 insertions(+), 58 deletions(-)

diff --git a/src/librustc_passes/ast_validation.rs b/src/librustc_passes/ast_validation.rs
index 00b6db0ed9a12..96f7c9853795e 100644
--- a/src/librustc_passes/ast_validation.rs
+++ b/src/librustc_passes/ast_validation.rs
@@ -17,13 +17,11 @@ use syntax::attr;
 use syntax::feature_gate::is_builtin_attr;
 use syntax::source_map::Spanned;
 use syntax::symbol::{kw, sym};
-use syntax::ptr::P;
 use syntax::visit::{self, Visitor};
 use syntax::{span_err, struct_span_err, walk_list};
 use syntax_ext::proc_macro_decls::is_proc_macro_attr;
 use syntax_pos::{Span, MultiSpan};
 use errors::{Applicability, FatalError};
-use log::debug;
 
 #[derive(Copy, Clone, Debug)]
 struct OuterImplTrait {
@@ -76,20 +74,34 @@ struct AstValidator<'a> {
     /// these booleans.
     warning_period_57979_didnt_record_next_impl_trait: bool,
     warning_period_57979_impl_trait_in_proj: bool,
+
+    /// Used to ban `let` expressions in inappropriate places.
+    is_let_allowed: bool,
+}
+
+/// With the `new` value in `store`,
+/// runs and returns the `scoped` computation,
+/// resetting the old value of `store` after,
+/// and returning the result of `scoped`.
+fn with<C, T, S>(
+    this: &mut C,
+    new: S,
+    store: impl Fn(&mut C) -> &mut S,
+    scoped: impl FnOnce(&mut C) -> T
+) -> T {
+    let old = mem::replace(store(this), new);
+    let ret = scoped(this);
+    *store(this) = old;
+    ret
 }
 
 impl<'a> AstValidator<'a> {
     fn with_impl_trait_in_proj_warning<T>(&mut self, v: bool, f: impl FnOnce(&mut Self) -> T) -> T {
-        let old = mem::replace(&mut self.warning_period_57979_impl_trait_in_proj, v);
-        let ret = f(self);
-        self.warning_period_57979_impl_trait_in_proj = old;
-        ret
+        with(self, v, |this| &mut this.warning_period_57979_impl_trait_in_proj, f)
     }
 
     fn with_banned_impl_trait(&mut self, f: impl FnOnce(&mut Self)) {
-        let old = mem::replace(&mut self.is_impl_trait_banned, true);
-        f(self);
-        self.is_impl_trait_banned = old;
+        with(self, true, |this| &mut this.is_impl_trait_banned, f)
     }
 
     fn with_banned_assoc_ty_bound(&mut self, f: impl FnOnce(&mut Self)) {
@@ -99,9 +111,11 @@ impl<'a> AstValidator<'a> {
     }
 
     fn with_impl_trait(&mut self, outer: Option<OuterImplTrait>, f: impl FnOnce(&mut Self)) {
-        let old = mem::replace(&mut self.outer_impl_trait, outer);
-        f(self);
-        self.outer_impl_trait = old;
+        with(self, outer, |this| &mut this.outer_impl_trait, f)
+    }
+
+    fn with_let_allowed(&mut self, v: bool, f: impl FnOnce(&mut Self)) {
+        with(self, v, |this| &mut this.is_let_allowed, f)
     }
 
     fn visit_assoc_ty_constraint_from_generic_args(&mut self, constraint: &'a AssocTyConstraint) {
@@ -319,52 +333,36 @@ impl<'a> AstValidator<'a> {
         }
     }
 
-    /// With eRFC 2497, we need to check whether an expression is ambiguous and warn or error
-    /// depending on the edition, this function handles that.
-    fn while_if_let_ambiguity(&self, expr: &P<Expr>) {
-        if let Some((span, op_kind)) = self.while_if_let_expr_ambiguity(&expr) {
-            let mut err = self.err_handler().struct_span_err(
-                span, &format!("ambiguous use of `{}`", op_kind.to_string())
-            );
-
-            err.note(
-                "this will be a error until the `let_chains` feature is stabilized"
-            );
-            err.note(
-                "see rust-lang/rust#53668 for more information"
-            );
-
-            if let Ok(snippet) = self.session.source_map().span_to_snippet(span) {
-                err.span_suggestion(
-                    span, "consider adding parentheses", format!("({})", snippet),
-                    Applicability::MachineApplicable,
-                );
-            }
-
-            err.emit();
-        }
-    }
-
-    /// With eRFC 2497 adding if-let chains, there is a requirement that the parsing of
-    /// `&&` and `||` in a if-let statement be unambiguous. This function returns a span and
-    /// a `BinOpKind` (either `&&` or `||` depending on what was ambiguous) if it is determined
-    /// that the current expression parsed is ambiguous and will break in future.
-    fn while_if_let_expr_ambiguity(&self, expr: &P<Expr>) -> Option<(Span, BinOpKind)> {
-        debug!("while_if_let_expr_ambiguity: expr.node: {:?}", expr.node);
+    /// Visits the `expr` and adjusts whether `let $pat = $expr` is allowed in decendants.
+    /// Returns whether we walked into `expr` or not.
+    /// If we did, walking should not happen again.
+    fn visit_expr_with_let_maybe_allowed(&mut self, expr: &'a Expr) -> bool {
         match &expr.node {
-            ExprKind::Binary(op, _, _) if op.node == BinOpKind::And || op.node == BinOpKind::Or => {
-                Some((expr.span, op.node))
-            },
-            ExprKind::Range(ref lhs, ref rhs, _) => {
-                let lhs_ambiguous = lhs.as_ref()
-                    .and_then(|lhs| self.while_if_let_expr_ambiguity(lhs));
-                let rhs_ambiguous = rhs.as_ref()
-                    .and_then(|rhs| self.while_if_let_expr_ambiguity(rhs));
-
-                lhs_ambiguous.or(rhs_ambiguous)
+            // Assuming the context permits, `($expr)` does not impose additional constraints.
+            ExprKind::Paren(_) => visit::walk_expr(self, expr),
+            // Assuming the context permits,
+            // l && r` allows decendants in `l` and `r` to be `let` expressions.
+            ExprKind::Binary(op, ..) if op.node == BinOpKind::And => visit::walk_expr(self, expr),
+            // However, we do allow it in the condition of the `if` expression.
+            // We do not allow `let` in `then` and `opt_else` directly.
+            ExprKind::If(ref cond, ref then, ref opt_else) => {
+                self.with_let_allowed(false, |this| {
+                    this.visit_block(then);
+                    walk_list!(this, visit_expr, opt_else);
+                });
+                self.with_let_allowed(true, |this| this.visit_expr(cond));
             }
-            _ => None,
+            // The same logic applies to `While`.
+            ExprKind::While(ref cond, ref then, ref opt_label) => {
+                walk_list!(self, visit_label, opt_label);
+                self.with_let_allowed(false, |this| this.visit_block(then));
+                self.with_let_allowed(true, |this| this.visit_expr(cond));
+            }
+            // Don't walk into `expr` and defer further checks to the caller.
+            _ => return false,
         }
+
+        true
     }
 
     fn check_fn_decl(&self, fn_decl: &FnDecl) {
@@ -494,18 +492,27 @@ fn validate_generics_order<'a>(
 impl<'a> Visitor<'a> for AstValidator<'a> {
     fn visit_expr(&mut self, expr: &'a Expr) {
         match expr.node {
+            ExprKind::Let(_, _) if !self.is_let_allowed => {
+                self.err_handler()
+                    .struct_span_err(expr.span, "`let` expressions are not supported here")
+                    .note("only supported directly in conditions of `if`- and `while`-expressions")
+                    .note("as well as when nested within `&&` and parenthesis in those conditions")
+                    .emit();
+            }
+            _ if self.visit_expr_with_let_maybe_allowed(&expr) => {
+                // Prevent `walk_expr` to happen since we've already done that.
+                return;
+            }
             ExprKind::Closure(_, _, _, ref fn_decl, _, _) => {
                 self.check_fn_decl(fn_decl);
             }
-            ExprKind::IfLet(_, ref expr, _, _) | ExprKind::WhileLet(_, ref expr, _, _) =>
-                self.while_if_let_ambiguity(&expr),
             ExprKind::InlineAsm(..) if !self.session.target.target.options.allow_asm => {
                 span_err!(self.session, expr.span, E0472, "asm! is unsupported on this target");
             }
             _ => {}
         }
 
-        visit::walk_expr(self, expr)
+        self.with_let_allowed(false, |this| visit::walk_expr(this, expr));
     }
 
     fn visit_ty(&mut self, ty: &'a Ty) {
@@ -917,6 +924,7 @@ pub fn check_crate(session: &Session, krate: &Crate) -> (bool, bool) {
         is_assoc_ty_bound_banned: false,
         warning_period_57979_didnt_record_next_impl_trait: false,
         warning_period_57979_impl_trait_in_proj: false,
+        is_let_allowed: false,
     };
     visit::walk_crate(&mut validator, krate);
 
diff --git a/src/librustc_passes/lib.rs b/src/librustc_passes/lib.rs
index bf2f7634e55a1..5f3d7159be6ce 100644
--- a/src/librustc_passes/lib.rs
+++ b/src/librustc_passes/lib.rs
@@ -8,6 +8,7 @@
 
 #![feature(in_band_lifetimes)]
 #![feature(nll)]
+#![feature(bind_by_move_pattern_guards)]
 #![feature(rustc_diagnostic_macros)]
 
 #![recursion_limit="256"]

From c9fb6390859bb1d2aca957528c121c2f99b74f42 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 16:05:05 +0200
Subject: [PATCH 08/35] let_chains: Adjust lowering logic in lieu of ::Let.

---
 src/librustc/hir/lowering.rs | 244 +++++++++++++++--------------------
 1 file changed, 106 insertions(+), 138 deletions(-)

diff --git a/src/librustc/hir/lowering.rs b/src/librustc/hir/lowering.rs
index c87ab68693726..a684816c6defe 100644
--- a/src/librustc/hir/lowering.rs
+++ b/src/librustc/hir/lowering.rs
@@ -4344,53 +4344,120 @@ impl<'a> LoweringContext<'a> {
                 let ohs = P(self.lower_expr(ohs));
                 hir::ExprKind::AddrOf(m, ohs)
             }
-            // More complicated than you might expect because the else branch
-            // might be `if let`.
+            ExprKind::Let(..) => {
+                // This should have been caught `ast_validation`!
+                self.sess.span_err(e.span, "`let` expressions only supported in `if`");
+                // ^-- FIXME(53667): Change to `delay_span_bug` when let_chains handled in lowering.
+                self.sess.abort_if_errors();
+                hir::ExprKind::Err
+            }
+            // FIXME(#53667): handle lowering of && and parens.
             ExprKind::If(ref cond, ref then, ref else_opt) => {
-                // `true => then`:
-                let then_pat = self.pat_bool(e.span, true);
-                let then_blk = self.lower_block(then, false);
-                let then_expr = self.expr_block(then_blk, ThinVec::new());
-                let then_arm = self.arm(hir_vec![then_pat], P(then_expr));
-
                 // `_ => else_block` where `else_block` is `{}` if there's `None`:
                 let else_pat = self.pat_wild(e.span);
-                let else_expr = match else_opt {
-                    None => self.expr_block_empty(e.span),
-                    Some(els) => match els.node {
-                        ExprKind::IfLet(..) => {
-                            // Wrap the `if let` expr in a block.
-                            let els = self.lower_expr(els);
-                            let blk = self.block_all(els.span, hir_vec![], Some(P(els)));
-                            self.expr_block(P(blk), ThinVec::new())
-                        }
-                        _ => self.lower_expr(els),
-                    }
+                let (else_expr, contains_else_clause) = match else_opt {
+                    None => (self.expr_block_empty(e.span), false),
+                    Some(els) => (self.lower_expr(els), true),
                 };
                 let else_arm = self.arm(hir_vec![else_pat], P(else_expr));
 
-                // Lower condition:
-                let span_block = self.mark_span_with_reason(IfTemporary, cond.span, None);
-                let cond = self.lower_expr(cond);
-                // Wrap in a construct equivalent to `{ let _t = $cond; _t }` to preserve drop
-                // semantics since `if cond { ... }` don't let temporaries live outside of `cond`.
-                let cond = self.expr_drop_temps(span_block, P(cond), ThinVec::new());
+                // Handle then + scrutinee:
+                let then_blk = self.lower_block(then, false);
+                let then_expr = self.expr_block(then_blk, ThinVec::new());
+                let (then_pats, scrutinee, desugar) = match cond.node {
+                    // `<pat> => <then>`
+                    ExprKind::Let(ref pats, ref scrutinee) => {
+                        let scrutinee = self.lower_expr(scrutinee);
+                        let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
+                        let desugar = hir::MatchSource::IfLetDesugar { contains_else_clause };
+                        (pats, scrutinee, desugar)
+                    }
+                    // `true => then`:
+                    _ => {
+                        // Lower condition:
+                        let cond = self.lower_expr(cond);
+                        // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
+                        // to preserve drop semantics since `if cond { ... }`
+                        // don't let temporaries live outside of `cond`.
+                        let span_block = self.mark_span_with_reason(IfTemporary, cond.span, None);
+                        // Wrap in a construct equivalent to `{ let _t = $cond; _t }`
+                        // to preserve drop semantics since `if cond { ... }` does not
+                        // let temporaries live outside of `cond`.
+                        let cond = self.expr_drop_temps(span_block, P(cond), ThinVec::new());
+
+                        let desugar = hir::MatchSource::IfDesugar { contains_else_clause };
+                        let pats = hir_vec![self.pat_bool(e.span, true)];
+                        (pats, cond, desugar)
+                    }
+                };
+                let then_arm = self.arm(then_pats, P(then_expr));
 
-                hir::ExprKind::Match(
-                    P(cond),
-                    vec![then_arm, else_arm].into(),
-                    hir::MatchSource::IfDesugar {
-                        contains_else_clause: else_opt.is_some()
-                    },
-                )
+                hir::ExprKind::Match(P(scrutinee), vec![then_arm, else_arm].into(), desugar)
+            }
+            // FIXME(#53667): handle lowering of && and parens.
+            ExprKind::While(ref cond, ref body, opt_label) => {
+                // Desugar `ExprWhileLet`
+                // from: `[opt_ident]: while let <pat> = <sub_expr> <body>`
+                if let ExprKind::Let(ref pats, ref sub_expr) = cond.node {
+                    // to:
+                    //
+                    //   [opt_ident]: loop {
+                    //     match <sub_expr> {
+                    //       <pat> => <body>,
+                    //       _ => break
+                    //     }
+                    //   }
+
+                    // Note that the block AND the condition are evaluated in the loop scope.
+                    // This is done to allow `break` from inside the condition of the loop.
+                    let (body, break_expr, sub_expr) = self.with_loop_scope(e.id, |this| {
+                        (
+                            this.lower_block(body, false),
+                            this.expr_break(e.span, ThinVec::new()),
+                            this.with_loop_condition_scope(|this| P(this.lower_expr(sub_expr))),
+                        )
+                    });
+
+                    // `<pat> => <body>`
+                    let pat_arm = {
+                        let body_expr = P(self.expr_block(body, ThinVec::new()));
+                        let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
+                        self.arm(pats, body_expr)
+                    };
+
+                    // `_ => break`
+                    let break_arm = {
+                        let pat_under = self.pat_wild(e.span);
+                        self.arm(hir_vec![pat_under], break_expr)
+                    };
+
+                    // `match <sub_expr> { ... }`
+                    let arms = hir_vec![pat_arm, break_arm];
+                    let match_expr = self.expr(
+                        sub_expr.span,
+                        hir::ExprKind::Match(sub_expr, arms, hir::MatchSource::WhileLetDesugar),
+                        ThinVec::new(),
+                    );
+
+                    // `[opt_ident]: loop { ... }`
+                    let loop_block = P(self.block_expr(P(match_expr)));
+                    let loop_expr = hir::ExprKind::Loop(
+                        loop_block,
+                        self.lower_label(opt_label),
+                        hir::LoopSource::WhileLet,
+                    );
+                    // Add attributes to the outer returned expr node.
+                    loop_expr
+                } else {
+                    self.with_loop_scope(e.id, |this| {
+                        hir::ExprKind::While(
+                            this.with_loop_condition_scope(|this| P(this.lower_expr(cond))),
+                            this.lower_block(body, false),
+                            this.lower_label(opt_label),
+                        )
+                    })
+                }
             }
-            ExprKind::While(ref cond, ref body, opt_label) => self.with_loop_scope(e.id, |this| {
-                hir::ExprKind::While(
-                    this.with_loop_condition_scope(|this| P(this.lower_expr(cond))),
-                    this.lower_block(body, false),
-                    this.lower_label(opt_label),
-                )
-            }),
             ExprKind::Loop(ref body, opt_label) => self.with_loop_scope(e.id, |this| {
                 hir::ExprKind::Loop(
                     this.lower_block(body, false),
@@ -4703,105 +4770,6 @@ impl<'a> LoweringContext<'a> {
 
             ExprKind::Err => hir::ExprKind::Err,
 
-            // Desugar `ExprIfLet`
-            // from: `if let <pat> = <sub_expr> <body> [<else_opt>]`
-            ExprKind::IfLet(ref pats, ref sub_expr, ref body, ref else_opt) => {
-                // to:
-                //
-                //   match <sub_expr> {
-                //     <pat> => <body>,
-                //     _ => [<else_opt> | ()]
-                //   }
-
-                let mut arms = vec![];
-
-                // `<pat> => <body>`
-                {
-                    let body = self.lower_block(body, false);
-                    let body_expr = P(self.expr_block(body, ThinVec::new()));
-                    let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
-                    arms.push(self.arm(pats, body_expr));
-                }
-
-                // _ => [<else_opt>|{}]
-                {
-                    let wildcard_arm: Option<&Expr> = else_opt.as_ref().map(|p| &**p);
-                    let wildcard_pattern = self.pat_wild(e.span);
-                    let body = if let Some(else_expr) = wildcard_arm {
-                        self.lower_expr(else_expr)
-                    } else {
-                        self.expr_block_empty(e.span)
-                    };
-                    arms.push(self.arm(hir_vec![wildcard_pattern], P(body)));
-                }
-
-                let contains_else_clause = else_opt.is_some();
-
-                let sub_expr = P(self.lower_expr(sub_expr));
-
-                hir::ExprKind::Match(
-                    sub_expr,
-                    arms.into(),
-                    hir::MatchSource::IfLetDesugar {
-                        contains_else_clause,
-                    },
-                )
-            }
-
-            // Desugar `ExprWhileLet`
-            // from: `[opt_ident]: while let <pat> = <sub_expr> <body>`
-            ExprKind::WhileLet(ref pats, ref sub_expr, ref body, opt_label) => {
-                // to:
-                //
-                //   [opt_ident]: loop {
-                //     match <sub_expr> {
-                //       <pat> => <body>,
-                //       _ => break
-                //     }
-                //   }
-
-                // Note that the block AND the condition are evaluated in the loop scope.
-                // This is done to allow `break` from inside the condition of the loop.
-                let (body, break_expr, sub_expr) = self.with_loop_scope(e.id, |this| {
-                    (
-                        this.lower_block(body, false),
-                        this.expr_break(e.span, ThinVec::new()),
-                        this.with_loop_condition_scope(|this| P(this.lower_expr(sub_expr))),
-                    )
-                });
-
-                // `<pat> => <body>`
-                let pat_arm = {
-                    let body_expr = P(self.expr_block(body, ThinVec::new()));
-                    let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
-                    self.arm(pats, body_expr)
-                };
-
-                // `_ => break`
-                let break_arm = {
-                    let pat_under = self.pat_wild(e.span);
-                    self.arm(hir_vec![pat_under], break_expr)
-                };
-
-                // `match <sub_expr> { ... }`
-                let arms = hir_vec![pat_arm, break_arm];
-                let match_expr = self.expr(
-                    sub_expr.span,
-                    hir::ExprKind::Match(sub_expr, arms, hir::MatchSource::WhileLetDesugar),
-                    ThinVec::new(),
-                );
-
-                // `[opt_ident]: loop { ... }`
-                let loop_block = P(self.block_expr(P(match_expr)));
-                let loop_expr = hir::ExprKind::Loop(
-                    loop_block,
-                    self.lower_label(opt_label),
-                    hir::LoopSource::WhileLet,
-                );
-                // Add attributes to the outer returned expr node.
-                loop_expr
-            }
-
             // Desugar `ExprForLoop`
             // from: `[opt_ident]: for <pat> in <head> <body>`
             ExprKind::ForLoop(ref pat, ref head, ref body, opt_label) => {

From ebea1c2cc0088353d92e65d34fcae2799b50a20f Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 11 May 2019 15:11:04 +0200
Subject: [PATCH 09/35] let_chains: Add feature gate.

---
 src/libsyntax/feature_gate.rs | 31 ++++++++++++++++++++++++++++++-
 src/libsyntax_pos/symbol.rs   |  1 +
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 7f80e2099f6ad..1f64f1a68885c 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -560,6 +560,9 @@ declare_features! (
     // Allows calling constructor functions in `const fn`.
     (active, const_constructor, "1.37.0", Some(61456), None),
 
+    // Allows `if/while p && let q = r && ...` chains.
+    (active, let_chains, "1.37.0", Some(53667), None),
+
     // #[repr(transparent)] on enums.
     (active, transparent_enums, "1.37.0", Some(60405), None),
 
@@ -577,7 +580,8 @@ declare_features! (
 const INCOMPLETE_FEATURES: &[Symbol] = &[
     sym::impl_trait_in_bindings,
     sym::generic_associated_types,
-    sym::const_generics
+    sym::const_generics,
+    sym::let_chains,
 ];
 
 declare_features! (
@@ -1936,6 +1940,27 @@ impl<'a> PostExpansionVisitor<'a> {
             Err(mut err) => err.emit(),
         }
     }
+
+    /// Recurse into all places where a `let` expression would be feature gated
+    /// and emit gate post errors for those.
+    fn find_and_gate_lets(&mut self, e: &'a ast::Expr) {
+        match &e.node {
+            ast::ExprKind::Paren(e) => {
+                self.find_and_gate_lets(e);
+            }
+            ast::ExprKind::Binary(op, lhs, rhs) if op.node == ast::BinOpKind::And => {
+                self.find_and_gate_lets(lhs);
+                self.find_and_gate_lets(rhs);
+            }
+            ast::ExprKind::Let(..) => {
+                gate_feature_post!(
+                    &self, let_chains, e.span,
+                    "`let` expressions in this position are experimental"
+                );
+            }
+            _ => {}
+        }
+    }
 }
 
 impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
@@ -2133,6 +2158,10 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 
     fn visit_expr(&mut self, e: &'a ast::Expr) {
         match e.node {
+            ast::ExprKind::If(ref e, ..) | ast::ExprKind::While(ref e, ..) => match e.node {
+                ast::ExprKind::Let(..) => {} // Stable!,
+                _ => self.find_and_gate_lets(e),
+            }
             ast::ExprKind::Box(_) => {
                 gate_feature_post!(&self, box_syntax, e.span, EXPLAIN_BOX_SYNTAX);
             }
diff --git a/src/libsyntax_pos/symbol.rs b/src/libsyntax_pos/symbol.rs
index c25e65eadddc9..756bc8c29d828 100644
--- a/src/libsyntax_pos/symbol.rs
+++ b/src/libsyntax_pos/symbol.rs
@@ -354,6 +354,7 @@ symbols! {
         label_break_value,
         lang,
         lang_items,
+        let_chains,
         lhs,
         lib,
         lifetime,

From 357b49992c5a468b2d737e77e1ef343a2744a689 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 11 May 2019 15:06:36 +0200
Subject: [PATCH 10/35] let_chains: Add tests for places where let expressions
 aren't allowed.

---
 .../disallowed-positions.rs                   | 159 ++++++
 .../disallowed-positions.stderr               | 476 ++++++++++++++++++
 2 files changed, 635 insertions(+)
 create mode 100644 src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs
 create mode 100644 src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr

diff --git a/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs
new file mode 100644
index 0000000000000..d871f2fad2358
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs
@@ -0,0 +1,159 @@
+// Here we test that `ast_validation` behaves correctly wrt. `let $pats = $expr` expressions.
+//
+// We want to make sure that `let` is banned in situations other than:
+//
+// expr =
+//   | ...
+//   | "if" expr_with_let block {"else" block}?
+//   | {label ":"}? while" expr_with_let block
+//   ;
+//
+// expr_with_let =
+//   | "let" top_pats "=" expr
+//   | expr_with_let "&&" expr_with_let
+//   | "(" expr_with_let ")"
+//   | expr
+//   ;
+//
+// To that end, we check some positions which is not part of the language above.
+
+#![allow(irrefutable_let_patterns)]
+
+use std::ops::Range;
+
+fn main() {}
+
+fn nested_within_if_expr() {
+    if &let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR `let` expressions only supported in `if`
+
+    if !let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    if *let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    if -let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+
+    fn _check_try_binds_tighter() -> Result<(), ()> {
+        if let 0 = 0? {}
+        Ok(())
+    }
+    if (let 0 = 0)? {} //~ ERROR `let` expressions are not supported here
+
+    if true || let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    if (true || let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    if true && (true || let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    if true || (true && let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+
+    let mut x = true;
+    if x = let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+
+    if true..(let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    if ..(let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    if (let 0 = 0).. {} //~ ERROR `let` expressions are not supported here
+
+    // Binds as `(let ... = true)..true &&/|| false`.
+    if let Range { start: _, end: _ } = true..true && false {}
+    //~^ ERROR `let` expressions are not supported here
+    if let Range { start: _, end: _ } = true..true || false {}
+    //~^ ERROR `let` expressions are not supported here
+
+    // Binds as `(let Range { start: F, end } = F)..(|| true)`.
+    const F: fn() -> bool = || true;
+    if let Range { start: F, end } = F..|| true {}
+    //~^ ERROR `let` expressions are not supported here
+
+    // Binds as `(let Range { start: true, end } = t)..(&&false)`.
+    let t = &&true;
+    if let Range { start: true, end } = t..&&false {}
+    //~^ ERROR `let` expressions are not supported here
+
+    if let true = let true = true {} //~ ERROR `let` expressions are not supported here
+}
+
+fn nested_within_while_expr() {
+    while &let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+
+    while !let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    while *let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    while -let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+
+    fn _check_try_binds_tighter() -> Result<(), ()> {
+        while let 0 = 0? {}
+        Ok(())
+    }
+    while (let 0 = 0)? {} //~ ERROR `let` expressions are not supported here
+
+    while true || let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    while (true || let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    while true && (true || let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    while true || (true && let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+
+    let mut x = true;
+    while x = let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+
+    while true..(let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    while ..(let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    while (let 0 = 0).. {} //~ ERROR `let` expressions are not supported here
+
+    // Binds as `(let ... = true)..true &&/|| false`.
+    while let Range { start: _, end: _ } = true..true && false {}
+    //~^ ERROR `let` expressions are not supported here
+    while let Range { start: _, end: _ } = true..true || false {}
+    //~^ ERROR `let` expressions are not supported here
+
+    // Binds as `(let Range { start: F, end } = F)..(|| true)`.
+    const F: fn() -> bool = || true;
+    while let Range { start: F, end } = F..|| true {}
+    //~^ ERROR `let` expressions are not supported here
+
+    // Binds as `(let Range { start: true, end } = t)..(&&false)`.
+    let t = &&true;
+    while let Range { start: true, end } = t..&&false {}
+    //~^ ERROR `let` expressions are not supported here
+
+    while let true = let true = true {} //~ ERROR `let` expressions are not supported here
+}
+
+fn not_error_because_clarified_intent() {
+    if let Range { start: _, end: _ } = (true..true || false) { }
+
+    if let Range { start: _, end: _ } = (true..true && false) { }
+
+    while let Range { start: _, end: _ } = (true..true || false) { }
+
+    while let Range { start: _, end: _ } = (true..true && false) { }
+}
+
+fn outside_if_and_while_expr() {
+    &let 0 = 0; //~ ERROR `let` expressions are not supported here
+
+    !let 0 = 0; //~ ERROR `let` expressions are not supported here
+    *let 0 = 0; //~ ERROR `let` expressions are not supported here
+    -let 0 = 0; //~ ERROR `let` expressions are not supported here
+
+    fn _check_try_binds_tighter() -> Result<(), ()> {
+        let 0 = 0?;
+        Ok(())
+    }
+    (let 0 = 0)?; //~ ERROR `let` expressions are not supported here
+
+    true || let 0 = 0; //~ ERROR `let` expressions are not supported here
+    (true || let 0 = 0); //~ ERROR `let` expressions are not supported here
+    true && (true || let 0 = 0); //~ ERROR `let` expressions are not supported here
+
+    let mut x = true;
+    x = let 0 = 0; //~ ERROR `let` expressions are not supported here
+
+    true..(let 0 = 0); //~ ERROR `let` expressions are not supported here
+    ..(let 0 = 0); //~ ERROR `let` expressions are not supported here
+    (let 0 = 0)..; //~ ERROR `let` expressions are not supported here
+
+    (let Range { start: _, end: _ } = true..true || false);
+    //~^ ERROR `let` expressions are not supported here
+
+    (let true = let true = true);
+    //~^ ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
+
+    // Check function tail position.
+    &let 0 = 0
+    //~^ ERROR `let` expressions are not supported here
+}
diff --git a/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr
new file mode 100644
index 0000000000000..18665e7933dd4
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr
@@ -0,0 +1,476 @@
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:27:9
+   |
+LL |     if &let 0 = 0 {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:30:9
+   |
+LL |     if !let 0 = 0 {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:31:9
+   |
+LL |     if *let 0 = 0 {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:32:9
+   |
+LL |     if -let 0 = 0 {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:38:9
+   |
+LL |     if (let 0 = 0)? {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:40:16
+   |
+LL |     if true || let 0 = 0 {}
+   |                ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:41:17
+   |
+LL |     if (true || let 0 = 0) {}
+   |                 ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:42:25
+   |
+LL |     if true && (true || let 0 = 0) {}
+   |                         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:43:25
+   |
+LL |     if true || (true && let 0 = 0) {}
+   |                         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:46:12
+   |
+LL |     if x = let 0 = 0 {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:48:15
+   |
+LL |     if true..(let 0 = 0) {}
+   |               ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:49:11
+   |
+LL |     if ..(let 0 = 0) {}
+   |           ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:50:9
+   |
+LL |     if (let 0 = 0).. {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:53:8
+   |
+LL |     if let Range { start: _, end: _ } = true..true && false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:55:8
+   |
+LL |     if let Range { start: _, end: _ } = true..true || false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:60:8
+   |
+LL |     if let Range { start: F, end } = F..|| true {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:65:8
+   |
+LL |     if let Range { start: true, end } = t..&&false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:68:19
+   |
+LL |     if let true = let true = true {}
+   |                   ^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:72:12
+   |
+LL |     while &let 0 = 0 {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:74:12
+   |
+LL |     while !let 0 = 0 {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:75:12
+   |
+LL |     while *let 0 = 0 {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:76:12
+   |
+LL |     while -let 0 = 0 {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:82:12
+   |
+LL |     while (let 0 = 0)? {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:84:19
+   |
+LL |     while true || let 0 = 0 {}
+   |                   ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:85:20
+   |
+LL |     while (true || let 0 = 0) {}
+   |                    ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:86:28
+   |
+LL |     while true && (true || let 0 = 0) {}
+   |                            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:87:28
+   |
+LL |     while true || (true && let 0 = 0) {}
+   |                            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:90:15
+   |
+LL |     while x = let 0 = 0 {}
+   |               ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:92:18
+   |
+LL |     while true..(let 0 = 0) {}
+   |                  ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:93:14
+   |
+LL |     while ..(let 0 = 0) {}
+   |              ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:94:12
+   |
+LL |     while (let 0 = 0).. {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:97:11
+   |
+LL |     while let Range { start: _, end: _ } = true..true && false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:99:11
+   |
+LL |     while let Range { start: _, end: _ } = true..true || false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:104:11
+   |
+LL |     while let Range { start: F, end } = F..|| true {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:109:11
+   |
+LL |     while let Range { start: true, end } = t..&&false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:112:22
+   |
+LL |     while let true = let true = true {}
+   |                      ^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:126:6
+   |
+LL |     &let 0 = 0;
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:128:6
+   |
+LL |     !let 0 = 0;
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:129:6
+   |
+LL |     *let 0 = 0;
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:130:6
+   |
+LL |     -let 0 = 0;
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:136:6
+   |
+LL |     (let 0 = 0)?;
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:138:13
+   |
+LL |     true || let 0 = 0;
+   |             ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:139:14
+   |
+LL |     (true || let 0 = 0);
+   |              ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:140:22
+   |
+LL |     true && (true || let 0 = 0);
+   |                      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:143:9
+   |
+LL |     x = let 0 = 0;
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:145:12
+   |
+LL |     true..(let 0 = 0);
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:146:8
+   |
+LL |     ..(let 0 = 0);
+   |        ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:147:6
+   |
+LL |     (let 0 = 0)..;
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:149:6
+   |
+LL |     (let Range { start: _, end: _ } = true..true || false);
+   |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:152:6
+   |
+LL |     (let true = let true = true);
+   |      ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:152:17
+   |
+LL |     (let true = let true = true);
+   |                 ^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:157:6
+   |
+LL |     &let 0 = 0
+   |      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions only supported in `if`
+  --> $DIR/disallowed-positions.rs:27:9
+   |
+LL |     if &let 0 = 0 {}
+   |         ^^^^^^^^^
+
+error: aborting due to 53 previous errors
+

From c0c57910490c34517bd8405cd55bf4696cc2fddd Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 11 May 2019 15:11:31 +0200
Subject: [PATCH 11/35] let_chains: Add feature gate tests.

---
 .../ui/rfc-2497-if-let-chains/feature-gate.rs | 101 +++++++
 .../feature-gate.stderr                       | 279 ++++++++++++++++++
 2 files changed, 380 insertions(+)
 create mode 100644 src/test/ui/rfc-2497-if-let-chains/feature-gate.rs
 create mode 100644 src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr

diff --git a/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs b/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs
new file mode 100644
index 0000000000000..cf4a44a9ce773
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs
@@ -0,0 +1,101 @@
+// gate-test-let_chains
+
+// Here we test feature gating for ´let_chains`.
+// See `disallowed-positions.rs` for the grammar
+// defining the language for gated allowed positions.
+
+#![allow(irrefutable_let_patterns)]
+
+use std::ops::Range;
+
+fn _if() {
+    if let 0 = 1 {} // Stable!
+
+    if (let 0 = 1) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions only supported in `if`
+
+    if (((let 0 = 1))) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    if true && let 0 = 1 {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    if let 0 = 1 && true {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    if (let 0 = 1) && true {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    if true && (let 0 = 1) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    if (let 0 = 1) && (let 0 = 1) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+
+    if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+
+    if let Range { start: _, end: _ } = (true..true) && false {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+}
+
+fn _while() {
+    if let 0 = 1 {} // Stable!
+
+    while (let 0 = 1) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    while (((let 0 = 1))) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    while true && let 0 = 1 {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    while let 0 = 1 && true {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    while (let 0 = 1) && true {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    while true && (let 0 = 1) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+
+    while (let 0 = 1) && (let 0 = 1) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+
+    while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions in this position are experimental [E0658]
+
+    while let Range { start: _, end: _ } = (true..true) && false {}
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+}
+
+fn _macros() {
+    macro_rules! noop_expr { ($e:expr) => {}; }
+    noop_expr!((let 0 = 1));
+
+    macro_rules! use_expr {
+        ($e:expr) => {
+            if $e {}
+            while $e {}
+        }
+    }
+    use_expr!(let 0 = 1 && 0 == 0);
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    use_expr!((let 0 = 1));
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    use_expr!(let 0 = 1);
+}
+
+fn main() {}
diff --git a/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr b/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr
new file mode 100644
index 0000000000000..f641429a6bbb9
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr
@@ -0,0 +1,279 @@
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:14:9
+   |
+LL |     if (let 0 = 1) {}
+   |         ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:18:11
+   |
+LL |     if (((let 0 = 1))) {}
+   |           ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:21:16
+   |
+LL |     if true && let 0 = 1 {}
+   |                ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:24:8
+   |
+LL |     if let 0 = 1 && true {}
+   |        ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:27:9
+   |
+LL |     if (let 0 = 1) && true {}
+   |         ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:30:17
+   |
+LL |     if true && (let 0 = 1) {}
+   |                 ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:33:9
+   |
+LL |     if (let 0 = 1) && (let 0 = 1) {}
+   |         ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:33:24
+   |
+LL |     if (let 0 = 1) && (let 0 = 1) {}
+   |                        ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:37:8
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |        ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:37:21
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                     ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:37:35
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                   ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:37:48
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                                ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:37:61
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                                             ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:44:8
+   |
+LL |     if let Range { start: _, end: _ } = (true..true) && false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:51:12
+   |
+LL |     while (let 0 = 1) {}
+   |            ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:54:14
+   |
+LL |     while (((let 0 = 1))) {}
+   |              ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:57:19
+   |
+LL |     while true && let 0 = 1 {}
+   |                   ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:60:11
+   |
+LL |     while let 0 = 1 && true {}
+   |           ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:63:12
+   |
+LL |     while (let 0 = 1) && true {}
+   |            ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:66:20
+   |
+LL |     while true && (let 0 = 1) {}
+   |                    ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:69:12
+   |
+LL |     while (let 0 = 1) && (let 0 = 1) {}
+   |            ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:69:27
+   |
+LL |     while (let 0 = 1) && (let 0 = 1) {}
+   |                           ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:73:11
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |           ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:73:24
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                        ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:73:38
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                      ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:73:51
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                                   ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:73:64
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                                                ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:80:11
+   |
+LL |     while let Range { start: _, end: _ } = (true..true) && false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:94:15
+   |
+LL |     use_expr!(let 0 = 1 && 0 == 0);
+   |               ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:96:16
+   |
+LL |     use_expr!((let 0 = 1));
+   |                ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error: `let` expressions only supported in `if`
+  --> $DIR/feature-gate.rs:14:9
+   |
+LL |     if (let 0 = 1) {}
+   |         ^^^^^^^^^
+
+error: aborting due to 31 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

From 6a6b07906fb063cc9a5ac99fbb176c4ade8c93d8 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 15:33:34 +0200
Subject: [PATCH 12/35] let_chains: Adjust unnecessary parens tests.

---
 src/test/ui/lint/lint-unnecessary-parens.rs     | 4 ++--
 src/test/ui/lint/lint-unnecessary-parens.stderr | 4 ++--
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/test/ui/lint/lint-unnecessary-parens.rs b/src/test/ui/lint/lint-unnecessary-parens.rs
index dc74d69bd8d54..d684616565f8e 100644
--- a/src/test/ui/lint/lint-unnecessary-parens.rs
+++ b/src/test/ui/lint/lint-unnecessary-parens.rs
@@ -22,8 +22,8 @@ fn main() {
     match (true) { //~ ERROR unnecessary parentheses around `match` head expression
         _ => {}
     }
-    if let 1 = (1) {} //~ ERROR unnecessary parentheses around `if let` head expression
-    while let 1 = (2) {} //~ ERROR unnecessary parentheses around `while let` head expression
+    if let 1 = (1) {} //~ ERROR unnecessary parentheses around `let` scrutinee
+    while let 1 = (2) {} //~ ERROR unnecessary parentheses around `let` scrutinee
     let v = X { y: false };
     // struct lits needs parens, so these shouldn't warn.
     if (v == X { y: true }) {}
diff --git a/src/test/ui/lint/lint-unnecessary-parens.stderr b/src/test/ui/lint/lint-unnecessary-parens.stderr
index fe2ee38eb42a3..eb6ead2a274ce 100644
--- a/src/test/ui/lint/lint-unnecessary-parens.stderr
+++ b/src/test/ui/lint/lint-unnecessary-parens.stderr
@@ -40,13 +40,13 @@ error: unnecessary parentheses around `match` head expression
 LL |     match (true) {
    |           ^^^^^^ help: remove these parentheses
 
-error: unnecessary parentheses around `if let` head expression
+error: unnecessary parentheses around `let` scrutinee
   --> $DIR/lint-unnecessary-parens.rs:25:16
    |
 LL |     if let 1 = (1) {}
    |                ^^^ help: remove these parentheses
 
-error: unnecessary parentheses around `while let` head expression
+error: unnecessary parentheses around `let` scrutinee
   --> $DIR/lint-unnecessary-parens.rs:26:19
    |
 LL |     while let 1 = (2) {}

From d45dadd9518f8ab20fb511661d3400192c56dd5c Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 15:16:04 +0200
Subject: [PATCH 13/35] let_chains: Add test protecting the precedence of && in
 relation to other things.

---
 .../protect-precedences.rs                     | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)
 create mode 100644 src/test/ui/rfc-2497-if-let-chains/protect-precedences.rs

diff --git a/src/test/ui/rfc-2497-if-let-chains/protect-precedences.rs b/src/test/ui/rfc-2497-if-let-chains/protect-precedences.rs
new file mode 100644
index 0000000000000..1de4e5bcebee9
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/protect-precedences.rs
@@ -0,0 +1,18 @@
+// run-pass
+
+#![allow(irrefutable_let_patterns)]
+
+use std::ops::Range;
+
+fn main() {
+    let x: bool;
+    // This should associate as: `(x = (true && false));`.
+    x = true && false;
+    assert!(!x);
+
+    fn _f1() -> bool {
+        // Should associate as `(let _ = (return (true && false)))`.
+        if let _ = return true && false {};
+    }
+    assert!(!_f1());
+}

From b425df0be106dfb97ec85031f8b60e33edd983e2 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 11 May 2019 15:11:54 +0200
Subject: [PATCH 14/35] let_chains: Remove redundant tests in
 syntax-ambiguity-*.rs.

---
 .../syntax-ambiguity-2015.rs                  | 38 -------------
 .../syntax-ambiguity-2015.stderr              | 56 -------------------
 .../syntax-ambiguity-2018.rs                  | 38 -------------
 .../syntax-ambiguity-2018.stderr              | 56 -------------------
 4 files changed, 188 deletions(-)
 delete mode 100644 src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.rs
 delete mode 100644 src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.stderr
 delete mode 100644 src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.rs
 delete mode 100644 src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.stderr

diff --git a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.rs b/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.rs
deleted file mode 100644
index d79798d57e820..0000000000000
--- a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.rs
+++ /dev/null
@@ -1,38 +0,0 @@
-// edition:2015
-
-// Enabling `ireffutable_let_patterns` isn't necessary for what this tests, but it makes coming up
-// with examples easier.
-
-#[allow(irrefutable_let_patterns)]
-fn main() {
-    use std::ops::Range;
-
-    if let Range { start: _, end: _ } = true..true && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    if let Range { start: _, end: _ } = true..true || false { }
-    //~^ ERROR ambiguous use of `||`
-
-    while let Range { start: _, end: _ } = true..true && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    while let Range { start: _, end: _ } = true..true || false { }
-    //~^ ERROR ambiguous use of `||`
-
-    if let true = false && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    while let true = (1 == 2) && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    // The following cases are not an error as parenthesis are used to
-    // clarify intent:
-
-    if let Range { start: _, end: _ } = true..(true || false) { }
-
-    if let Range { start: _, end: _ } = true..(true && false) { }
-
-    while let Range { start: _, end: _ } = true..(true || false) { }
-
-    while let Range { start: _, end: _ } = true..(true && false) { }
-}
diff --git a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.stderr b/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.stderr
deleted file mode 100644
index 2cd59fe56cf2d..0000000000000
--- a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2015.stderr
+++ /dev/null
@@ -1,56 +0,0 @@
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2015.rs:10:47
-   |
-LL |     if let Range { start: _, end: _ } = true..true && false { }
-   |                                               ^^^^^^^^^^^^^ help: consider adding parentheses: `(true && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `||`
-  --> $DIR/syntax-ambiguity-2015.rs:13:47
-   |
-LL |     if let Range { start: _, end: _ } = true..true || false { }
-   |                                               ^^^^^^^^^^^^^ help: consider adding parentheses: `(true || false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2015.rs:16:50
-   |
-LL |     while let Range { start: _, end: _ } = true..true && false { }
-   |                                                  ^^^^^^^^^^^^^ help: consider adding parentheses: `(true && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `||`
-  --> $DIR/syntax-ambiguity-2015.rs:19:50
-   |
-LL |     while let Range { start: _, end: _ } = true..true || false { }
-   |                                                  ^^^^^^^^^^^^^ help: consider adding parentheses: `(true || false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2015.rs:22:19
-   |
-LL |     if let true = false && false { }
-   |                   ^^^^^^^^^^^^^^ help: consider adding parentheses: `(false && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2015.rs:25:22
-   |
-LL |     while let true = (1 == 2) && false { }
-   |                      ^^^^^^^^^^^^^^^^^ help: consider adding parentheses: `((1 == 2) && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: aborting due to 6 previous errors
-
diff --git a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.rs b/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.rs
deleted file mode 100644
index 687bf659416ab..0000000000000
--- a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.rs
+++ /dev/null
@@ -1,38 +0,0 @@
-// edition:2018
-
-// Enabling `ireffutable_let_patterns` isn't necessary for what this tests, but it makes coming up
-// with examples easier.
-
-#[allow(irrefutable_let_patterns)]
-fn main() {
-    use std::ops::Range;
-
-    if let Range { start: _, end: _ } = true..true && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    if let Range { start: _, end: _ } = true..true || false { }
-    //~^ ERROR ambiguous use of `||`
-
-    while let Range { start: _, end: _ } = true..true && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    while let Range { start: _, end: _ } = true..true || false { }
-    //~^ ERROR ambiguous use of `||`
-
-    if let true = false && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    while let true = (1 == 2) && false { }
-    //~^ ERROR ambiguous use of `&&`
-
-    // The following cases are not an error as parenthesis are used to
-    // clarify intent:
-
-    if let Range { start: _, end: _ } = true..(true || false) { }
-
-    if let Range { start: _, end: _ } = true..(true && false) { }
-
-    while let Range { start: _, end: _ } = true..(true || false) { }
-
-    while let Range { start: _, end: _ } = true..(true && false) { }
-}
diff --git a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.stderr b/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.stderr
deleted file mode 100644
index cbba2d7473334..0000000000000
--- a/src/test/ui/rfc-2497-if-let-chains/syntax-ambiguity-2018.stderr
+++ /dev/null
@@ -1,56 +0,0 @@
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2018.rs:10:47
-   |
-LL |     if let Range { start: _, end: _ } = true..true && false { }
-   |                                               ^^^^^^^^^^^^^ help: consider adding parentheses: `(true && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `||`
-  --> $DIR/syntax-ambiguity-2018.rs:13:47
-   |
-LL |     if let Range { start: _, end: _ } = true..true || false { }
-   |                                               ^^^^^^^^^^^^^ help: consider adding parentheses: `(true || false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2018.rs:16:50
-   |
-LL |     while let Range { start: _, end: _ } = true..true && false { }
-   |                                                  ^^^^^^^^^^^^^ help: consider adding parentheses: `(true && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `||`
-  --> $DIR/syntax-ambiguity-2018.rs:19:50
-   |
-LL |     while let Range { start: _, end: _ } = true..true || false { }
-   |                                                  ^^^^^^^^^^^^^ help: consider adding parentheses: `(true || false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2018.rs:22:19
-   |
-LL |     if let true = false && false { }
-   |                   ^^^^^^^^^^^^^^ help: consider adding parentheses: `(false && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: ambiguous use of `&&`
-  --> $DIR/syntax-ambiguity-2018.rs:25:22
-   |
-LL |     while let true = (1 == 2) && false { }
-   |                      ^^^^^^^^^^^^^^^^^ help: consider adding parentheses: `((1 == 2) && false)`
-   |
-   = note: this will be a error until the `let_chains` feature is stabilized
-   = note: see rust-lang/rust#53668 for more information
-
-error: aborting due to 6 previous errors
-

From 530f954e92d1e6d3813499e7210d416de0b01693 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Wed, 15 May 2019 21:03:44 +0200
Subject: [PATCH 15/35] let_chains: Test pretty output for simple stable
 if-let.

---
 src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.rs |  6 ++++++
 .../ui/rfc-2497-if-let-chains/ast-pretty-check.stdout  | 10 ++++++++++
 2 files changed, 16 insertions(+)
 create mode 100644 src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.rs
 create mode 100644 src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.stdout

diff --git a/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.rs b/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.rs
new file mode 100644
index 0000000000000..e66d46575664f
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.rs
@@ -0,0 +1,6 @@
+// compile-pass
+// compile-flags: -Z unpretty=expanded
+
+fn main() {
+    if let 0 = 1 {}
+}
diff --git a/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.stdout b/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.stdout
new file mode 100644
index 0000000000000..a6b15f9bbf65d
--- /dev/null
+++ b/src/test/ui/rfc-2497-if-let-chains/ast-pretty-check.stdout
@@ -0,0 +1,10 @@
+#![feature(prelude_import)]
+#![no_std]
+#[prelude_import]
+use ::std::prelude::v1::*;
+#[macro_use]
+extern crate std;
+// compile-pass
+// compile-flags: -Z unpretty=expanded
+
+fn main() { if let 0 = 1 { } }

From 86250a666ed763b151dc0747331bddae5d4f46c4 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Thu, 16 May 2019 14:17:10 +0200
Subject: [PATCH 16/35] let_chains: Comment out Let in ident_can_begin_expr.

---
 src/libsyntax/parse/token.rs                           | 3 ++-
 src/test/ui/rfc-2497-if-let-chains/feature-gate.rs     | 5 +++--
 src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr | 6 +++---
 3 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/src/libsyntax/parse/token.rs b/src/libsyntax/parse/token.rs
index 98e06ad73c94f..eec09c08702d3 100644
--- a/src/libsyntax/parse/token.rs
+++ b/src/libsyntax/parse/token.rs
@@ -134,8 +134,9 @@ pub(crate) fn ident_can_begin_expr(name: ast::Name, span: Span, is_raw: bool) ->
         kw::Continue,
         kw::False,
         kw::For,
-        kw::Let,
         kw::If,
+        // FIXME(53667): Consider whether `Let` can be added here.
+        // kw::Let,
         kw::Loop,
         kw::Match,
         kw::Move,
diff --git a/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs b/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs
index cf4a44a9ce773..b6f70b912acac 100644
--- a/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs
+++ b/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs
@@ -91,11 +91,12 @@ fn _macros() {
             while $e {}
         }
     }
-    use_expr!(let 0 = 1 && 0 == 0);
+    use_expr!((let 0 = 1 && 0 == 0));
     //~^ ERROR `let` expressions in this position are experimental [E0658]
     use_expr!((let 0 = 1));
     //~^ ERROR `let` expressions in this position are experimental [E0658]
-    use_expr!(let 0 = 1);
+    // use_expr!(let 0 = 1);
+    // ^--- FIXME(53667): Consider whether `Let` can be added to `ident_can_begin_expr`.
 }
 
 fn main() {}
diff --git a/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr b/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr
index f641429a6bbb9..fdfbb8fe4d441 100644
--- a/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr
+++ b/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr
@@ -251,10 +251,10 @@ LL |     while let Range { start: _, end: _ } = (true..true) && false {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:94:15
+  --> $DIR/feature-gate.rs:94:16
    |
-LL |     use_expr!(let 0 = 1 && 0 == 0);
-   |               ^^^^^^^^^
+LL |     use_expr!((let 0 = 1 && 0 == 0));
+   |                ^^^^^^^^^
    |
    = note: for more information, see https://github.com/rust-lang/rust/issues/53667
    = help: add #![feature(let_chains)] to the crate attributes to enable

From 8b72e4c8151c176e8f2ec58f95e16c256d2c059b Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 18 May 2019 00:27:12 +0200
Subject: [PATCH 17/35] let_chains: Improve documentation for
 ast::ExprKind::Let(..).

---
 src/libsyntax/ast.rs | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index 4f2fccd20e5f5..b5a432b9d4755 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -1115,8 +1115,11 @@ pub enum ExprKind {
     Cast(P<Expr>, P<Ty>),
     /// A type ascription (e.g., `42: usize`).
     Type(P<Expr>, P<Ty>),
-    /// A `let pat = expr` pseudo-expression that only occurs in the scrutinee
+    /// A `let pats = expr` pseudo-expression that only occurs in the scrutinee
     /// of `if` / `while` expressions. (e.g., `if let 0 = x { .. }`).
+    ///
+    /// The `Vec<P<Pat>>` is for or-patterns at the top level.
+    /// FIXME(54883): Change this to just `P<Pat>`.
     Let(Vec<P<Pat>>, P<Expr>),
     /// An `if` block, with an optional `else` block.
     ///

From 3b7f0cb7e0b5df49c3254471da59a71c2eba934e Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 18 May 2019 00:34:55 +0200
Subject: [PATCH 18/35] let_chains: Fix outdated doc-comment re.
 'parse_if_expr'.

---
 src/libsyntax/parse/parser.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 8cf05059e8a64..56b363439d1e3 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -3154,7 +3154,7 @@ impl<'a> Parser<'a> {
         }
     }
 
-    /// Parses an `if` or `if let` expression (`if` token already eaten).
+    /// Parses an `if` expression (`if` token already eaten).
     fn parse_if_expr(&mut self, attrs: ThinVec<Attribute>) -> PResult<'a, P<Expr>> {
         let lo = self.prev_span;
         let cond = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;

From 1ff947f5967666d9e22162f10ccc572b3e839758 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 18 May 2019 01:22:43 +0200
Subject: [PATCH 19/35] let_chains: scrutinee -> head expression.

---
 src/librustc_lint/unused.rs                     | 2 +-
 src/test/ui/lint/lint-unnecessary-parens.rs     | 4 ++--
 src/test/ui/lint/lint-unnecessary-parens.stderr | 4 ++--
 3 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/librustc_lint/unused.rs b/src/librustc_lint/unused.rs
index 33b2289593f19..b5c5fc0608b95 100644
--- a/src/librustc_lint/unused.rs
+++ b/src/librustc_lint/unused.rs
@@ -343,7 +343,7 @@ impl UnusedParens {
             ast::ExprKind::Let(_, ref expr) => {
                 // FIXME(#60336): Properly handle `let true = (false && true)`
                 // actually needing the parenthesis.
-                self.check_unused_parens_expr(cx, expr, "`let` scrutinee", followed_by_block);
+                self.check_unused_parens_expr(cx, expr, "`let` head expression", followed_by_block);
             }
             _ => {}
         }
diff --git a/src/test/ui/lint/lint-unnecessary-parens.rs b/src/test/ui/lint/lint-unnecessary-parens.rs
index d684616565f8e..c36101043a7f7 100644
--- a/src/test/ui/lint/lint-unnecessary-parens.rs
+++ b/src/test/ui/lint/lint-unnecessary-parens.rs
@@ -22,8 +22,8 @@ fn main() {
     match (true) { //~ ERROR unnecessary parentheses around `match` head expression
         _ => {}
     }
-    if let 1 = (1) {} //~ ERROR unnecessary parentheses around `let` scrutinee
-    while let 1 = (2) {} //~ ERROR unnecessary parentheses around `let` scrutinee
+    if let 1 = (1) {} //~ ERROR unnecessary parentheses around `let` head expression
+    while let 1 = (2) {} //~ ERROR unnecessary parentheses around `let` head expression
     let v = X { y: false };
     // struct lits needs parens, so these shouldn't warn.
     if (v == X { y: true }) {}
diff --git a/src/test/ui/lint/lint-unnecessary-parens.stderr b/src/test/ui/lint/lint-unnecessary-parens.stderr
index eb6ead2a274ce..dfbefd7b03ee5 100644
--- a/src/test/ui/lint/lint-unnecessary-parens.stderr
+++ b/src/test/ui/lint/lint-unnecessary-parens.stderr
@@ -40,13 +40,13 @@ error: unnecessary parentheses around `match` head expression
 LL |     match (true) {
    |           ^^^^^^ help: remove these parentheses
 
-error: unnecessary parentheses around `let` scrutinee
+error: unnecessary parentheses around `let` head expression
   --> $DIR/lint-unnecessary-parens.rs:25:16
    |
 LL |     if let 1 = (1) {}
    |                ^^^ help: remove these parentheses
 
-error: unnecessary parentheses around `let` scrutinee
+error: unnecessary parentheses around `let` head expression
   --> $DIR/lint-unnecessary-parens.rs:26:19
    |
 LL |     while let 1 = (2) {}

From a80aa3495167adf791a46059be78e7b377c98606 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 18 May 2019 11:49:27 +0200
Subject: [PATCH 20/35] let_chains: Change AST validation strategy slightly.

---
 src/librustc_passes/ast_validation.rs | 79 +++++++++++++++------------
 1 file changed, 45 insertions(+), 34 deletions(-)

diff --git a/src/librustc_passes/ast_validation.rs b/src/librustc_passes/ast_validation.rs
index 96f7c9853795e..a168599c9d8b2 100644
--- a/src/librustc_passes/ast_validation.rs
+++ b/src/librustc_passes/ast_validation.rs
@@ -114,8 +114,10 @@ impl<'a> AstValidator<'a> {
         with(self, outer, |this| &mut this.outer_impl_trait, f)
     }
 
-    fn with_let_allowed(&mut self, v: bool, f: impl FnOnce(&mut Self)) {
-        with(self, v, |this| &mut this.is_let_allowed, f)
+    fn with_let_allowed(&mut self, v: bool, f: impl FnOnce(&mut Self, bool)) {
+        let old = mem::replace(&mut self.is_let_allowed, v);
+        f(self, old);
+        self.is_let_allowed = old;
     }
 
     fn visit_assoc_ty_constraint_from_generic_args(&mut self, constraint: &'a AssocTyConstraint) {
@@ -336,27 +338,29 @@ impl<'a> AstValidator<'a> {
     /// Visits the `expr` and adjusts whether `let $pat = $expr` is allowed in decendants.
     /// Returns whether we walked into `expr` or not.
     /// If we did, walking should not happen again.
-    fn visit_expr_with_let_maybe_allowed(&mut self, expr: &'a Expr) -> bool {
+    fn visit_expr_with_let_maybe_allowed(&mut self, expr: &'a Expr, let_allowed: bool) -> bool {
         match &expr.node {
             // Assuming the context permits, `($expr)` does not impose additional constraints.
-            ExprKind::Paren(_) => visit::walk_expr(self, expr),
+            ExprKind::Paren(_) => {
+                self.with_let_allowed(let_allowed, |this, _| visit::walk_expr(this, expr));
+            }
             // Assuming the context permits,
             // l && r` allows decendants in `l` and `r` to be `let` expressions.
-            ExprKind::Binary(op, ..) if op.node == BinOpKind::And => visit::walk_expr(self, expr),
+            ExprKind::Binary(op, ..) if op.node == BinOpKind::And => {
+                self.with_let_allowed(let_allowed, |this, _| visit::walk_expr(this, expr));
+            }
             // However, we do allow it in the condition of the `if` expression.
             // We do not allow `let` in `then` and `opt_else` directly.
-            ExprKind::If(ref cond, ref then, ref opt_else) => {
-                self.with_let_allowed(false, |this| {
-                    this.visit_block(then);
-                    walk_list!(this, visit_expr, opt_else);
-                });
-                self.with_let_allowed(true, |this| this.visit_expr(cond));
+            ExprKind::If(cond, then, opt_else) => {
+                self.visit_block(then);
+                walk_list!(self, visit_expr, opt_else);
+                self.with_let_allowed(true, |this, _| this.visit_expr(cond));
             }
             // The same logic applies to `While`.
-            ExprKind::While(ref cond, ref then, ref opt_label) => {
+            ExprKind::While(cond, then, opt_label) => {
                 walk_list!(self, visit_label, opt_label);
-                self.with_let_allowed(false, |this| this.visit_block(then));
-                self.with_let_allowed(true, |this| this.visit_expr(cond));
+                self.visit_block(then);
+                self.with_let_allowed(true, |this, _| this.visit_expr(cond));
             }
             // Don't walk into `expr` and defer further checks to the caller.
             _ => return false,
@@ -365,6 +369,15 @@ impl<'a> AstValidator<'a> {
         true
     }
 
+    /// Emits an error banning the `let` expression provided.
+    fn ban_let_expr(&self, expr: &'a Expr) {
+        self.err_handler()
+            .struct_span_err(expr.span, "`let` expressions are not supported here")
+            .note("only supported directly in conditions of `if`- and `while`-expressions")
+            .note("as well as when nested within `&&` and parenthesis in those conditions")
+            .emit();
+    }
+
     fn check_fn_decl(&self, fn_decl: &FnDecl) {
         fn_decl
             .inputs
@@ -491,28 +504,26 @@ fn validate_generics_order<'a>(
 
 impl<'a> Visitor<'a> for AstValidator<'a> {
     fn visit_expr(&mut self, expr: &'a Expr) {
-        match expr.node {
-            ExprKind::Let(_, _) if !self.is_let_allowed => {
-                self.err_handler()
-                    .struct_span_err(expr.span, "`let` expressions are not supported here")
-                    .note("only supported directly in conditions of `if`- and `while`-expressions")
-                    .note("as well as when nested within `&&` and parenthesis in those conditions")
-                    .emit();
-            }
-            _ if self.visit_expr_with_let_maybe_allowed(&expr) => {
-                // Prevent `walk_expr` to happen since we've already done that.
-                return;
-            }
-            ExprKind::Closure(_, _, _, ref fn_decl, _, _) => {
-                self.check_fn_decl(fn_decl);
-            }
-            ExprKind::InlineAsm(..) if !self.session.target.target.options.allow_asm => {
-                span_err!(self.session, expr.span, E0472, "asm! is unsupported on this target");
+        self.with_let_allowed(false, |this, let_allowed| {
+            match &expr.node {
+                ExprKind::Let(_, _) if !let_allowed => {
+                    this.ban_let_expr(expr);
+                }
+                _ if this.visit_expr_with_let_maybe_allowed(&expr, let_allowed) => {
+                    // Prevent `walk_expr` to happen since we've already done that.
+                    return;
+                }
+                ExprKind::Closure(_, _, _, fn_decl, _, _) => {
+                    this.check_fn_decl(fn_decl);
+                }
+                ExprKind::InlineAsm(..) if !this.session.target.target.options.allow_asm => {
+                    span_err!(this.session, expr.span, E0472, "asm! is unsupported on this target");
+                }
+                _ => {}
             }
-            _ => {}
-        }
 
-        self.with_let_allowed(false, |this| visit::walk_expr(this, expr));
+            visit::walk_expr(this, expr);
+        });
     }
 
     fn visit_ty(&mut self, ty: &'a Ty) {

From 07f37c88fbad77c37e0043e477e5d9571d48bea9 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Tue, 21 May 2019 06:50:53 +0200
Subject: [PATCH 21/35] let_chains: Account for const generics in validation
 tests.

---
 .../disallowed-positions.rs                   |  30 ++++
 .../disallowed-positions.stderr               | 147 +++++++++++-------
 2 files changed, 123 insertions(+), 54 deletions(-)

diff --git a/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs
index d871f2fad2358..2977b71e5965b 100644
--- a/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs
+++ b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs
@@ -17,6 +17,8 @@
 //
 // To that end, we check some positions which is not part of the language above.
 
+#![feature(const_generics)] //~ WARN the feature `const_generics` is incomplete
+
 #![allow(irrefutable_let_patterns)]
 
 use std::ops::Range;
@@ -157,3 +159,31 @@ fn outside_if_and_while_expr() {
     &let 0 = 0
     //~^ ERROR `let` expressions are not supported here
 }
+
+// Let's make sure that `let` inside const generic arguments are considered.
+fn inside_const_generic_arguments() {
+    struct A<const B: bool>;
+    impl<const B: bool> A<{B}> { const O: u32 = 5; }
+
+    if let A::<{
+        true && let 1 = 1 //~ ERROR `let` expressions are not supported here
+    }>::O = 5 {}
+
+    while let A::<{
+        true && let 1 = 1 //~ ERROR `let` expressions are not supported here
+    }>::O = 5 {}
+
+    if A::<{
+        true && let 1 = 1 //~ ERROR `let` expressions are not supported here
+    }>::O == 5 {}
+
+    // In the cases above we have `ExprKind::Block` to help us out.
+    // Below however, we would not have a block and so an implementation might go
+    // from visiting expressions to types without banning `let` expressions down the tree.
+    // This tests ensures that we are not caught by surprise should the parser
+    // admit non-IDENT expressions in const generic arguments.
+
+    if A::<
+        true && let 1 = 1 //~ ERROR expected one of `,` or `>`, found `&&`
+    >::O == 5 {}
+}
diff --git a/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr
index 18665e7933dd4..bfd55af69c97a 100644
--- a/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr
+++ b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr
@@ -1,5 +1,17 @@
+error: expected one of `,` or `>`, found `&&`
+  --> $DIR/disallowed-positions.rs:187:14
+   |
+LL |         true && let 1 = 1
+   |              ^^ expected one of `,` or `>` here
+
+warning: the feature `const_generics` is incomplete and may cause the compiler to crash
+  --> $DIR/disallowed-positions.rs:20:12
+   |
+LL | #![feature(const_generics)]
+   |            ^^^^^^^^^^^^^^
+
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:27:9
+  --> $DIR/disallowed-positions.rs:29:9
    |
 LL |     if &let 0 = 0 {}
    |         ^^^^^^^^^
@@ -8,7 +20,7 @@ LL |     if &let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:30:9
+  --> $DIR/disallowed-positions.rs:32:9
    |
 LL |     if !let 0 = 0 {}
    |         ^^^^^^^^^
@@ -17,7 +29,7 @@ LL |     if !let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:31:9
+  --> $DIR/disallowed-positions.rs:33:9
    |
 LL |     if *let 0 = 0 {}
    |         ^^^^^^^^^
@@ -26,7 +38,7 @@ LL |     if *let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:32:9
+  --> $DIR/disallowed-positions.rs:34:9
    |
 LL |     if -let 0 = 0 {}
    |         ^^^^^^^^^
@@ -35,7 +47,7 @@ LL |     if -let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:38:9
+  --> $DIR/disallowed-positions.rs:40:9
    |
 LL |     if (let 0 = 0)? {}
    |         ^^^^^^^^^
@@ -44,7 +56,7 @@ LL |     if (let 0 = 0)? {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:40:16
+  --> $DIR/disallowed-positions.rs:42:16
    |
 LL |     if true || let 0 = 0 {}
    |                ^^^^^^^^^
@@ -53,7 +65,7 @@ LL |     if true || let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:41:17
+  --> $DIR/disallowed-positions.rs:43:17
    |
 LL |     if (true || let 0 = 0) {}
    |                 ^^^^^^^^^
@@ -62,7 +74,7 @@ LL |     if (true || let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:42:25
+  --> $DIR/disallowed-positions.rs:44:25
    |
 LL |     if true && (true || let 0 = 0) {}
    |                         ^^^^^^^^^
@@ -71,7 +83,7 @@ LL |     if true && (true || let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:43:25
+  --> $DIR/disallowed-positions.rs:45:25
    |
 LL |     if true || (true && let 0 = 0) {}
    |                         ^^^^^^^^^
@@ -80,7 +92,7 @@ LL |     if true || (true && let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:46:12
+  --> $DIR/disallowed-positions.rs:48:12
    |
 LL |     if x = let 0 = 0 {}
    |            ^^^^^^^^^
@@ -89,7 +101,7 @@ LL |     if x = let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:48:15
+  --> $DIR/disallowed-positions.rs:50:15
    |
 LL |     if true..(let 0 = 0) {}
    |               ^^^^^^^^^
@@ -98,7 +110,7 @@ LL |     if true..(let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:49:11
+  --> $DIR/disallowed-positions.rs:51:11
    |
 LL |     if ..(let 0 = 0) {}
    |           ^^^^^^^^^
@@ -107,7 +119,7 @@ LL |     if ..(let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:50:9
+  --> $DIR/disallowed-positions.rs:52:9
    |
 LL |     if (let 0 = 0).. {}
    |         ^^^^^^^^^
@@ -116,7 +128,7 @@ LL |     if (let 0 = 0).. {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:53:8
+  --> $DIR/disallowed-positions.rs:55:8
    |
 LL |     if let Range { start: _, end: _ } = true..true && false {}
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -125,7 +137,7 @@ LL |     if let Range { start: _, end: _ } = true..true && false {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:55:8
+  --> $DIR/disallowed-positions.rs:57:8
    |
 LL |     if let Range { start: _, end: _ } = true..true || false {}
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -134,7 +146,7 @@ LL |     if let Range { start: _, end: _ } = true..true || false {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:60:8
+  --> $DIR/disallowed-positions.rs:62:8
    |
 LL |     if let Range { start: F, end } = F..|| true {}
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -143,7 +155,7 @@ LL |     if let Range { start: F, end } = F..|| true {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:65:8
+  --> $DIR/disallowed-positions.rs:67:8
    |
 LL |     if let Range { start: true, end } = t..&&false {}
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -152,7 +164,7 @@ LL |     if let Range { start: true, end } = t..&&false {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:68:19
+  --> $DIR/disallowed-positions.rs:70:19
    |
 LL |     if let true = let true = true {}
    |                   ^^^^^^^^^^^^^^^
@@ -161,7 +173,7 @@ LL |     if let true = let true = true {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:72:12
+  --> $DIR/disallowed-positions.rs:74:12
    |
 LL |     while &let 0 = 0 {}
    |            ^^^^^^^^^
@@ -170,7 +182,7 @@ LL |     while &let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:74:12
+  --> $DIR/disallowed-positions.rs:76:12
    |
 LL |     while !let 0 = 0 {}
    |            ^^^^^^^^^
@@ -179,7 +191,7 @@ LL |     while !let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:75:12
+  --> $DIR/disallowed-positions.rs:77:12
    |
 LL |     while *let 0 = 0 {}
    |            ^^^^^^^^^
@@ -188,7 +200,7 @@ LL |     while *let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:76:12
+  --> $DIR/disallowed-positions.rs:78:12
    |
 LL |     while -let 0 = 0 {}
    |            ^^^^^^^^^
@@ -197,7 +209,7 @@ LL |     while -let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:82:12
+  --> $DIR/disallowed-positions.rs:84:12
    |
 LL |     while (let 0 = 0)? {}
    |            ^^^^^^^^^
@@ -206,7 +218,7 @@ LL |     while (let 0 = 0)? {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:84:19
+  --> $DIR/disallowed-positions.rs:86:19
    |
 LL |     while true || let 0 = 0 {}
    |                   ^^^^^^^^^
@@ -215,7 +227,7 @@ LL |     while true || let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:85:20
+  --> $DIR/disallowed-positions.rs:87:20
    |
 LL |     while (true || let 0 = 0) {}
    |                    ^^^^^^^^^
@@ -224,7 +236,7 @@ LL |     while (true || let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:86:28
+  --> $DIR/disallowed-positions.rs:88:28
    |
 LL |     while true && (true || let 0 = 0) {}
    |                            ^^^^^^^^^
@@ -233,7 +245,7 @@ LL |     while true && (true || let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:87:28
+  --> $DIR/disallowed-positions.rs:89:28
    |
 LL |     while true || (true && let 0 = 0) {}
    |                            ^^^^^^^^^
@@ -242,7 +254,7 @@ LL |     while true || (true && let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:90:15
+  --> $DIR/disallowed-positions.rs:92:15
    |
 LL |     while x = let 0 = 0 {}
    |               ^^^^^^^^^
@@ -251,7 +263,7 @@ LL |     while x = let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:92:18
+  --> $DIR/disallowed-positions.rs:94:18
    |
 LL |     while true..(let 0 = 0) {}
    |                  ^^^^^^^^^
@@ -260,7 +272,7 @@ LL |     while true..(let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:93:14
+  --> $DIR/disallowed-positions.rs:95:14
    |
 LL |     while ..(let 0 = 0) {}
    |              ^^^^^^^^^
@@ -269,7 +281,7 @@ LL |     while ..(let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:94:12
+  --> $DIR/disallowed-positions.rs:96:12
    |
 LL |     while (let 0 = 0).. {}
    |            ^^^^^^^^^
@@ -278,7 +290,7 @@ LL |     while (let 0 = 0).. {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:97:11
+  --> $DIR/disallowed-positions.rs:99:11
    |
 LL |     while let Range { start: _, end: _ } = true..true && false {}
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -287,7 +299,7 @@ LL |     while let Range { start: _, end: _ } = true..true && false {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:99:11
+  --> $DIR/disallowed-positions.rs:101:11
    |
 LL |     while let Range { start: _, end: _ } = true..true || false {}
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -296,7 +308,7 @@ LL |     while let Range { start: _, end: _ } = true..true || false {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:104:11
+  --> $DIR/disallowed-positions.rs:106:11
    |
 LL |     while let Range { start: F, end } = F..|| true {}
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -305,7 +317,7 @@ LL |     while let Range { start: F, end } = F..|| true {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:109:11
+  --> $DIR/disallowed-positions.rs:111:11
    |
 LL |     while let Range { start: true, end } = t..&&false {}
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -314,7 +326,7 @@ LL |     while let Range { start: true, end } = t..&&false {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:112:22
+  --> $DIR/disallowed-positions.rs:114:22
    |
 LL |     while let true = let true = true {}
    |                      ^^^^^^^^^^^^^^^
@@ -323,7 +335,7 @@ LL |     while let true = let true = true {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:126:6
+  --> $DIR/disallowed-positions.rs:128:6
    |
 LL |     &let 0 = 0;
    |      ^^^^^^^^^
@@ -332,7 +344,7 @@ LL |     &let 0 = 0;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:128:6
+  --> $DIR/disallowed-positions.rs:130:6
    |
 LL |     !let 0 = 0;
    |      ^^^^^^^^^
@@ -341,7 +353,7 @@ LL |     !let 0 = 0;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:129:6
+  --> $DIR/disallowed-positions.rs:131:6
    |
 LL |     *let 0 = 0;
    |      ^^^^^^^^^
@@ -350,7 +362,7 @@ LL |     *let 0 = 0;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:130:6
+  --> $DIR/disallowed-positions.rs:132:6
    |
 LL |     -let 0 = 0;
    |      ^^^^^^^^^
@@ -359,7 +371,7 @@ LL |     -let 0 = 0;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:136:6
+  --> $DIR/disallowed-positions.rs:138:6
    |
 LL |     (let 0 = 0)?;
    |      ^^^^^^^^^
@@ -368,7 +380,7 @@ LL |     (let 0 = 0)?;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:138:13
+  --> $DIR/disallowed-positions.rs:140:13
    |
 LL |     true || let 0 = 0;
    |             ^^^^^^^^^
@@ -377,7 +389,7 @@ LL |     true || let 0 = 0;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:139:14
+  --> $DIR/disallowed-positions.rs:141:14
    |
 LL |     (true || let 0 = 0);
    |              ^^^^^^^^^
@@ -386,7 +398,7 @@ LL |     (true || let 0 = 0);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:140:22
+  --> $DIR/disallowed-positions.rs:142:22
    |
 LL |     true && (true || let 0 = 0);
    |                      ^^^^^^^^^
@@ -395,7 +407,7 @@ LL |     true && (true || let 0 = 0);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:143:9
+  --> $DIR/disallowed-positions.rs:145:9
    |
 LL |     x = let 0 = 0;
    |         ^^^^^^^^^
@@ -404,7 +416,7 @@ LL |     x = let 0 = 0;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:145:12
+  --> $DIR/disallowed-positions.rs:147:12
    |
 LL |     true..(let 0 = 0);
    |            ^^^^^^^^^
@@ -413,7 +425,7 @@ LL |     true..(let 0 = 0);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:146:8
+  --> $DIR/disallowed-positions.rs:148:8
    |
 LL |     ..(let 0 = 0);
    |        ^^^^^^^^^
@@ -422,7 +434,7 @@ LL |     ..(let 0 = 0);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:147:6
+  --> $DIR/disallowed-positions.rs:149:6
    |
 LL |     (let 0 = 0)..;
    |      ^^^^^^^^^
@@ -431,7 +443,7 @@ LL |     (let 0 = 0)..;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:149:6
+  --> $DIR/disallowed-positions.rs:151:6
    |
 LL |     (let Range { start: _, end: _ } = true..true || false);
    |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -440,7 +452,7 @@ LL |     (let Range { start: _, end: _ } = true..true || false);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:152:6
+  --> $DIR/disallowed-positions.rs:154:6
    |
 LL |     (let true = let true = true);
    |      ^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -449,7 +461,7 @@ LL |     (let true = let true = true);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:152:17
+  --> $DIR/disallowed-positions.rs:154:17
    |
 LL |     (let true = let true = true);
    |                 ^^^^^^^^^^^^^^^
@@ -458,7 +470,7 @@ LL |     (let true = let true = true);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:157:6
+  --> $DIR/disallowed-positions.rs:159:6
    |
 LL |     &let 0 = 0
    |      ^^^^^^^^^
@@ -466,11 +478,38 @@ LL |     &let 0 = 0
    = note: only supported directly in conditions of `if`- and `while`-expressions
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:169:17
+   |
+LL |         true && let 1 = 1
+   |                 ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:173:17
+   |
+LL |         true && let 1 = 1
+   |                 ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/disallowed-positions.rs:177:17
+   |
+LL |         true && let 1 = 1
+   |                 ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
 error: `let` expressions only supported in `if`
-  --> $DIR/disallowed-positions.rs:27:9
+  --> $DIR/disallowed-positions.rs:29:9
    |
 LL |     if &let 0 = 0 {}
    |         ^^^^^^^^^
 
-error: aborting due to 53 previous errors
+error: aborting due to 57 previous errors
 

From a505d9d20a697d90242158ef1594c611cb23fbfa Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sat, 25 May 2019 12:59:44 +0200
Subject: [PATCH 22/35] let_chains: scrutinee -> condition

---
 src/libsyntax/ast.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index b5a432b9d4755..a837ae167a308 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -1115,7 +1115,7 @@ pub enum ExprKind {
     Cast(P<Expr>, P<Ty>),
     /// A type ascription (e.g., `42: usize`).
     Type(P<Expr>, P<Ty>),
-    /// A `let pats = expr` pseudo-expression that only occurs in the scrutinee
+    /// A `let pats = expr` pseudo-expression that only occurs in the condition
     /// of `if` / `while` expressions. (e.g., `if let 0 = x { .. }`).
     ///
     /// The `Vec<P<Pat>>` is for or-patterns at the top level.

From 61fc727fe07abaeee78475202dd575f0fddfc653 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Mon, 17 Jun 2019 00:16:29 +0200
Subject: [PATCH 23/35] let_chains: Fuse PatternSource::Let & ::LetExpr.

---
 src/librustc_resolve/lib.rs | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/src/librustc_resolve/lib.rs b/src/librustc_resolve/lib.rs
index fdef707e347ca..a9e4ffd5e7f02 100644
--- a/src/librustc_resolve/lib.rs
+++ b/src/librustc_resolve/lib.rs
@@ -485,8 +485,6 @@ type BindingMap = FxHashMap<Ident, BindingInfo>;
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
 enum PatternSource {
     Match,
-    // FIXME(54883): Consider fusing with `Let` below once let-statements support or-patterns.
-    LetExpr,
     Let,
     For,
     FnParam,
@@ -496,7 +494,7 @@ impl PatternSource {
     fn descr(self) -> &'static str {
         match self {
             PatternSource::Match => "match binding",
-            PatternSource::Let | PatternSource::LetExpr => "let binding",
+            PatternSource::Let => "let binding",
             PatternSource::For => "for binding",
             PatternSource::FnParam => "function parameter",
         }
@@ -3153,7 +3151,7 @@ impl<'a> Resolver<'a> {
                 );
             }
             Some(..) if pat_src == PatternSource::Match ||
-                        pat_src == PatternSource::LetExpr => {
+                        pat_src == PatternSource::Let => {
                 // `Variant1(a) | Variant2(a)`, ok
                 // Reuse definition from the first `a`.
                 res = self.ribs[ValueNS].last_mut().unwrap().bindings[&ident];
@@ -4348,7 +4346,7 @@ impl<'a> Resolver<'a> {
 
             ExprKind::Let(ref pats, ref scrutinee) => {
                 self.visit_expr(scrutinee);
-                self.resolve_pats(pats, PatternSource::LetExpr);
+                self.resolve_pats(pats, PatternSource::Let);
             }
 
             ExprKind::If(ref cond, ref then, ref opt_else) => {

From 92587e41d9096bc9eb74f40b93422026a8a57d4e Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Mon, 17 Jun 2019 00:45:09 +0200
Subject: [PATCH 24/35] let_chains: readd kw::let to ident_can_begin_expr.

---
 src/libsyntax/ext/tt/macro_parser.rs | 2 +-
 src/libsyntax/parse/token.rs         | 3 +--
 2 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/src/libsyntax/ext/tt/macro_parser.rs b/src/libsyntax/ext/tt/macro_parser.rs
index d5da4c920bc4f..0db6bd59afc2c 100644
--- a/src/libsyntax/ext/tt/macro_parser.rs
+++ b/src/libsyntax/ext/tt/macro_parser.rs
@@ -825,7 +825,7 @@ fn may_begin_with(token: &Token, name: Name) -> bool {
     }
 
     match name {
-        sym::expr => token.can_begin_expr(),
+        sym::expr => token.can_begin_expr() && !token.is_keyword(kw::Let),
         sym::ty => token.can_begin_type(),
         sym::ident => get_macro_name(token).is_some(),
         sym::literal => token.can_begin_literal_or_bool(),
diff --git a/src/libsyntax/parse/token.rs b/src/libsyntax/parse/token.rs
index eec09c08702d3..ebd0decacb59f 100644
--- a/src/libsyntax/parse/token.rs
+++ b/src/libsyntax/parse/token.rs
@@ -135,8 +135,7 @@ pub(crate) fn ident_can_begin_expr(name: ast::Name, span: Span, is_raw: bool) ->
         kw::False,
         kw::For,
         kw::If,
-        // FIXME(53667): Consider whether `Let` can be added here.
-        // kw::Let,
+        kw::Let,
         kw::Loop,
         kw::Match,
         kw::Move,

From d5518802677d4701185b5cd3b9df4b263fc864b2 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Mon, 17 Jun 2019 01:18:22 +0200
Subject: [PATCH 25/35] let_chains: Inline visit_expr_with_let_maybe_allowed.

---
 src/librustc_passes/ast_validation.rs | 62 +++++++++++----------------
 src/libsyntax/parse/lexer/mod.rs      |  1 +
 2 files changed, 26 insertions(+), 37 deletions(-)

diff --git a/src/librustc_passes/ast_validation.rs b/src/librustc_passes/ast_validation.rs
index a168599c9d8b2..2aa8d5113ff7d 100644
--- a/src/librustc_passes/ast_validation.rs
+++ b/src/librustc_passes/ast_validation.rs
@@ -335,40 +335,6 @@ impl<'a> AstValidator<'a> {
         }
     }
 
-    /// Visits the `expr` and adjusts whether `let $pat = $expr` is allowed in decendants.
-    /// Returns whether we walked into `expr` or not.
-    /// If we did, walking should not happen again.
-    fn visit_expr_with_let_maybe_allowed(&mut self, expr: &'a Expr, let_allowed: bool) -> bool {
-        match &expr.node {
-            // Assuming the context permits, `($expr)` does not impose additional constraints.
-            ExprKind::Paren(_) => {
-                self.with_let_allowed(let_allowed, |this, _| visit::walk_expr(this, expr));
-            }
-            // Assuming the context permits,
-            // l && r` allows decendants in `l` and `r` to be `let` expressions.
-            ExprKind::Binary(op, ..) if op.node == BinOpKind::And => {
-                self.with_let_allowed(let_allowed, |this, _| visit::walk_expr(this, expr));
-            }
-            // However, we do allow it in the condition of the `if` expression.
-            // We do not allow `let` in `then` and `opt_else` directly.
-            ExprKind::If(cond, then, opt_else) => {
-                self.visit_block(then);
-                walk_list!(self, visit_expr, opt_else);
-                self.with_let_allowed(true, |this, _| this.visit_expr(cond));
-            }
-            // The same logic applies to `While`.
-            ExprKind::While(cond, then, opt_label) => {
-                walk_list!(self, visit_label, opt_label);
-                self.visit_block(then);
-                self.with_let_allowed(true, |this, _| this.visit_expr(cond));
-            }
-            // Don't walk into `expr` and defer further checks to the caller.
-            _ => return false,
-        }
-
-        true
-    }
-
     /// Emits an error banning the `let` expression provided.
     fn ban_let_expr(&self, expr: &'a Expr) {
         self.err_handler()
@@ -509,9 +475,31 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                 ExprKind::Let(_, _) if !let_allowed => {
                     this.ban_let_expr(expr);
                 }
-                _ if this.visit_expr_with_let_maybe_allowed(&expr, let_allowed) => {
-                    // Prevent `walk_expr` to happen since we've already done that.
-                    return;
+                // Assuming the context permits, `($expr)` does not impose additional constraints.
+                ExprKind::Paren(_) => {
+                    this.with_let_allowed(let_allowed, |this, _| visit::walk_expr(this, expr));
+                    return; // We've already walked into `expr`.
+                }
+                // Assuming the context permits,
+                // l && r` allows decendants in `l` and `r` to be `let` expressions.
+                ExprKind::Binary(op, ..) if op.node == BinOpKind::And => {
+                    this.with_let_allowed(let_allowed, |this, _| visit::walk_expr(this, expr));
+                    return; // We've already walked into `expr`.
+                }
+                // However, we do allow it in the condition of the `if` expression.
+                // We do not allow `let` in `then` and `opt_else` directly.
+                ExprKind::If(cond, then, opt_else) => {
+                    this.visit_block(then);
+                    walk_list!(this, visit_expr, opt_else);
+                    this.with_let_allowed(true, |this, _| this.visit_expr(cond));
+                    return; // We've already walked into `expr`.
+                }
+                // The same logic applies to `While`.
+                ExprKind::While(cond, then, opt_label) => {
+                    walk_list!(this, visit_label, opt_label);
+                    this.visit_block(then);
+                    this.with_let_allowed(true, |this, _| this.visit_expr(cond));
+                    return; // We've already walked into `expr`.
                 }
                 ExprKind::Closure(_, _, _, fn_decl, _, _) => {
                     this.check_fn_decl(fn_decl);
diff --git a/src/libsyntax/parse/lexer/mod.rs b/src/libsyntax/parse/lexer/mod.rs
index 1c44155150c41..ead5d543bec7d 100644
--- a/src/libsyntax/parse/lexer/mod.rs
+++ b/src/libsyntax/parse/lexer/mod.rs
@@ -1491,6 +1491,7 @@ mod tests {
             edition: Edition::from_session(),
             ambiguous_block_expr_parse: Lock::new(FxHashMap::default()),
             param_attr_spans: Lock::new(Vec::new()),
+            let_chains_spans: Lock::new(Vec::new()),
         }
     }
 

From eb4f54a58d0247dbd830165fd2ff214e07890964 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Mon, 17 Jun 2019 05:41:21 +0200
Subject: [PATCH 26/35] let_chains: Move feature gating to pre-expansion.

---
 src/libsyntax/feature_gate.rs | 36 +++++++++++------------------------
 src/libsyntax/parse/mod.rs    |  5 ++++-
 src/libsyntax/parse/parser.rs | 15 +++++++++++++--
 3 files changed, 28 insertions(+), 28 deletions(-)

diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 1f64f1a68885c..8ec07de5fab73 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -1940,27 +1940,6 @@ impl<'a> PostExpansionVisitor<'a> {
             Err(mut err) => err.emit(),
         }
     }
-
-    /// Recurse into all places where a `let` expression would be feature gated
-    /// and emit gate post errors for those.
-    fn find_and_gate_lets(&mut self, e: &'a ast::Expr) {
-        match &e.node {
-            ast::ExprKind::Paren(e) => {
-                self.find_and_gate_lets(e);
-            }
-            ast::ExprKind::Binary(op, lhs, rhs) if op.node == ast::BinOpKind::And => {
-                self.find_and_gate_lets(lhs);
-                self.find_and_gate_lets(rhs);
-            }
-            ast::ExprKind::Let(..) => {
-                gate_feature_post!(
-                    &self, let_chains, e.span,
-                    "`let` expressions in this position are experimental"
-                );
-            }
-            _ => {}
-        }
-    }
 }
 
 impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
@@ -2158,10 +2137,6 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 
     fn visit_expr(&mut self, e: &'a ast::Expr) {
         match e.node {
-            ast::ExprKind::If(ref e, ..) | ast::ExprKind::While(ref e, ..) => match e.node {
-                ast::ExprKind::Let(..) => {} // Stable!,
-                _ => self.find_and_gate_lets(e),
-            }
             ast::ExprKind::Box(_) => {
                 gate_feature_post!(&self, box_syntax, e.span, EXPLAIN_BOX_SYNTAX);
             }
@@ -2546,6 +2521,17 @@ pub fn check_crate(krate: &ast::Crate,
             "attributes on function parameters are unstable"
         ));
 
+    sess
+        .let_chains_spans
+        .borrow()
+        .iter()
+        .for_each(|span| gate_feature!(
+            &ctx,
+            let_chains,
+            *span,
+            "`let` expressions in this position are experimental"
+        ));
+
     let visitor = &mut PostExpansionVisitor {
         context: &ctx,
         builtin_attributes: &*BUILTIN_ATTRIBUTE_MAP,
diff --git a/src/libsyntax/parse/mod.rs b/src/libsyntax/parse/mod.rs
index ba5d1d009d15f..e19eab371f44e 100644
--- a/src/libsyntax/parse/mod.rs
+++ b/src/libsyntax/parse/mod.rs
@@ -54,7 +54,9 @@ pub struct ParseSess {
     /// operation token that followed it, but that the parser cannot identify without further
     /// analysis.
     pub ambiguous_block_expr_parse: Lock<FxHashMap<Span, Span>>,
-    pub param_attr_spans: Lock<Vec<Span>>
+    pub param_attr_spans: Lock<Vec<Span>>,
+    // Places where `let` exprs were used and should be feature gated according to `let_chains`.
+    pub let_chains_spans: Lock<Vec<Span>>,
 }
 
 impl ParseSess {
@@ -81,6 +83,7 @@ impl ParseSess {
             edition: Edition::from_session(),
             ambiguous_block_expr_parse: Lock::new(FxHashMap::default()),
             param_attr_spans: Lock::new(Vec::new()),
+            let_chains_spans: Lock::new(Vec::new()),
         }
     }
 
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 56b363439d1e3..1e420855b676b 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -3158,6 +3158,7 @@ impl<'a> Parser<'a> {
     fn parse_if_expr(&mut self, attrs: ThinVec<Attribute>) -> PResult<'a, P<Expr>> {
         let lo = self.prev_span;
         let cond = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
+        self.ungate_prev_let_expr(&cond);
 
         // Verify that the parsed `if` condition makes sense as a condition. If it is a block, then
         // verify that the last statement is either an implicit return (no `;`) or an explicit
@@ -3187,18 +3188,27 @@ impl<'a> Parser<'a> {
         Ok(self.mk_expr(lo.to(hi), ExprKind::If(cond, thn, els), attrs))
     }
 
+    /// Remove the last feature gating of a `let` expression that must the one provided.
+    fn ungate_prev_let_expr(&mut self, expr: &Expr) {
+        if let ExprKind::Let(..) = expr.node {
+            let last = self.sess.let_chains_spans.borrow_mut().pop();
+            debug_assert_eq!(expr.span, last.unwrap());
+        }
+    }
+
     /// Parses a `let $pats = $expr` pseudo-expression.
     /// The `let` token has already been eaten.
     fn parse_let_expr(&mut self, attrs: ThinVec<Attribute>) -> PResult<'a, P<Expr>> {
         let lo = self.prev_span;
         let pats = self.parse_pats()?;
         self.expect(&token::Eq)?;
-
         let expr = self.with_res(
             Restrictions::NO_STRUCT_LITERAL,
             |this| this.parse_assoc_expr_with(1 + AssocOp::LAnd.precedence(), None.into())
         )?;
-        Ok(self.mk_expr(lo.to(expr.span), ExprKind::Let(pats, expr), attrs))
+        let span = lo.to(expr.span);
+        self.sess.let_chains_spans.borrow_mut().push(span);
+        Ok(self.mk_expr(span, ExprKind::Let(pats, expr), attrs))
     }
 
     /// Parses `move |args| expr`.
@@ -3286,6 +3296,7 @@ impl<'a> Parser<'a> {
                             span_lo: Span,
                             mut attrs: ThinVec<Attribute>) -> PResult<'a, P<Expr>> {
         let cond = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
+        self.ungate_prev_let_expr(&cond);
         let (iattrs, body) = self.parse_inner_attrs_and_block()?;
         attrs.extend(iattrs);
         let span = span_lo.to(body.span);

From 10234d286ac0b74a3bfe1581df91137a21f66033 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Mon, 17 Jun 2019 05:41:45 +0200
Subject: [PATCH 27/35] let_chains: Adjust tests for pre-expansion gating.

---
 .../disallowed-positions.rs                   |   5 +-
 .../disallowed-positions.stderr               | 120 +++++++++---------
 .../ui/rfc-2497-if-let-chains/feature-gate.rs |   7 +-
 .../feature-gate.stderr                       |  33 ++++-
 4 files changed, 103 insertions(+), 62 deletions(-)

diff --git a/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs
index 2977b71e5965b..43e0ee8a86cf9 100644
--- a/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs
+++ b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs
@@ -17,7 +17,10 @@
 //
 // To that end, we check some positions which is not part of the language above.
 
-#![feature(const_generics)] //~ WARN the feature `const_generics` is incomplete
+#![feature(const_generics)]
+//~^ WARN the feature `const_generics` is incomplete
+#![feature(let_chains)] // Avoid inflating `.stderr` with overzealous gates in this test.
+//~^ WARN the feature `let_chains` is incomplete
 
 #![allow(irrefutable_let_patterns)]
 
diff --git a/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr
index bfd55af69c97a..86baa038c0b98 100644
--- a/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr
+++ b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr
@@ -1,5 +1,5 @@
 error: expected one of `,` or `>`, found `&&`
-  --> $DIR/disallowed-positions.rs:187:14
+  --> $DIR/disallowed-positions.rs:190:14
    |
 LL |         true && let 1 = 1
    |              ^^ expected one of `,` or `>` here
@@ -10,8 +10,14 @@ warning: the feature `const_generics` is incomplete and may cause the compiler t
 LL | #![feature(const_generics)]
    |            ^^^^^^^^^^^^^^
 
+warning: the feature `let_chains` is incomplete and may cause the compiler to crash
+  --> $DIR/disallowed-positions.rs:22:12
+   |
+LL | #![feature(let_chains)] // Avoid inflating `.stderr` with overzealous gates in this test.
+   |            ^^^^^^^^^^
+
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:29:9
+  --> $DIR/disallowed-positions.rs:32:9
    |
 LL |     if &let 0 = 0 {}
    |         ^^^^^^^^^
@@ -20,7 +26,7 @@ LL |     if &let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:32:9
+  --> $DIR/disallowed-positions.rs:35:9
    |
 LL |     if !let 0 = 0 {}
    |         ^^^^^^^^^
@@ -29,7 +35,7 @@ LL |     if !let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:33:9
+  --> $DIR/disallowed-positions.rs:36:9
    |
 LL |     if *let 0 = 0 {}
    |         ^^^^^^^^^
@@ -38,7 +44,7 @@ LL |     if *let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:34:9
+  --> $DIR/disallowed-positions.rs:37:9
    |
 LL |     if -let 0 = 0 {}
    |         ^^^^^^^^^
@@ -47,7 +53,7 @@ LL |     if -let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:40:9
+  --> $DIR/disallowed-positions.rs:43:9
    |
 LL |     if (let 0 = 0)? {}
    |         ^^^^^^^^^
@@ -56,7 +62,7 @@ LL |     if (let 0 = 0)? {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:42:16
+  --> $DIR/disallowed-positions.rs:45:16
    |
 LL |     if true || let 0 = 0 {}
    |                ^^^^^^^^^
@@ -65,7 +71,7 @@ LL |     if true || let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:43:17
+  --> $DIR/disallowed-positions.rs:46:17
    |
 LL |     if (true || let 0 = 0) {}
    |                 ^^^^^^^^^
@@ -74,7 +80,7 @@ LL |     if (true || let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:44:25
+  --> $DIR/disallowed-positions.rs:47:25
    |
 LL |     if true && (true || let 0 = 0) {}
    |                         ^^^^^^^^^
@@ -83,7 +89,7 @@ LL |     if true && (true || let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:45:25
+  --> $DIR/disallowed-positions.rs:48:25
    |
 LL |     if true || (true && let 0 = 0) {}
    |                         ^^^^^^^^^
@@ -92,7 +98,7 @@ LL |     if true || (true && let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:48:12
+  --> $DIR/disallowed-positions.rs:51:12
    |
 LL |     if x = let 0 = 0 {}
    |            ^^^^^^^^^
@@ -101,7 +107,7 @@ LL |     if x = let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:50:15
+  --> $DIR/disallowed-positions.rs:53:15
    |
 LL |     if true..(let 0 = 0) {}
    |               ^^^^^^^^^
@@ -110,7 +116,7 @@ LL |     if true..(let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:51:11
+  --> $DIR/disallowed-positions.rs:54:11
    |
 LL |     if ..(let 0 = 0) {}
    |           ^^^^^^^^^
@@ -119,7 +125,7 @@ LL |     if ..(let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:52:9
+  --> $DIR/disallowed-positions.rs:55:9
    |
 LL |     if (let 0 = 0).. {}
    |         ^^^^^^^^^
@@ -128,7 +134,7 @@ LL |     if (let 0 = 0).. {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:55:8
+  --> $DIR/disallowed-positions.rs:58:8
    |
 LL |     if let Range { start: _, end: _ } = true..true && false {}
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -137,7 +143,7 @@ LL |     if let Range { start: _, end: _ } = true..true && false {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:57:8
+  --> $DIR/disallowed-positions.rs:60:8
    |
 LL |     if let Range { start: _, end: _ } = true..true || false {}
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -146,7 +152,7 @@ LL |     if let Range { start: _, end: _ } = true..true || false {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:62:8
+  --> $DIR/disallowed-positions.rs:65:8
    |
 LL |     if let Range { start: F, end } = F..|| true {}
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -155,7 +161,7 @@ LL |     if let Range { start: F, end } = F..|| true {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:67:8
+  --> $DIR/disallowed-positions.rs:70:8
    |
 LL |     if let Range { start: true, end } = t..&&false {}
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -164,7 +170,7 @@ LL |     if let Range { start: true, end } = t..&&false {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:70:19
+  --> $DIR/disallowed-positions.rs:73:19
    |
 LL |     if let true = let true = true {}
    |                   ^^^^^^^^^^^^^^^
@@ -173,7 +179,7 @@ LL |     if let true = let true = true {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:74:12
+  --> $DIR/disallowed-positions.rs:77:12
    |
 LL |     while &let 0 = 0 {}
    |            ^^^^^^^^^
@@ -182,7 +188,7 @@ LL |     while &let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:76:12
+  --> $DIR/disallowed-positions.rs:79:12
    |
 LL |     while !let 0 = 0 {}
    |            ^^^^^^^^^
@@ -191,7 +197,7 @@ LL |     while !let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:77:12
+  --> $DIR/disallowed-positions.rs:80:12
    |
 LL |     while *let 0 = 0 {}
    |            ^^^^^^^^^
@@ -200,7 +206,7 @@ LL |     while *let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:78:12
+  --> $DIR/disallowed-positions.rs:81:12
    |
 LL |     while -let 0 = 0 {}
    |            ^^^^^^^^^
@@ -209,7 +215,7 @@ LL |     while -let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:84:12
+  --> $DIR/disallowed-positions.rs:87:12
    |
 LL |     while (let 0 = 0)? {}
    |            ^^^^^^^^^
@@ -218,7 +224,7 @@ LL |     while (let 0 = 0)? {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:86:19
+  --> $DIR/disallowed-positions.rs:89:19
    |
 LL |     while true || let 0 = 0 {}
    |                   ^^^^^^^^^
@@ -227,7 +233,7 @@ LL |     while true || let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:87:20
+  --> $DIR/disallowed-positions.rs:90:20
    |
 LL |     while (true || let 0 = 0) {}
    |                    ^^^^^^^^^
@@ -236,7 +242,7 @@ LL |     while (true || let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:88:28
+  --> $DIR/disallowed-positions.rs:91:28
    |
 LL |     while true && (true || let 0 = 0) {}
    |                            ^^^^^^^^^
@@ -245,7 +251,7 @@ LL |     while true && (true || let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:89:28
+  --> $DIR/disallowed-positions.rs:92:28
    |
 LL |     while true || (true && let 0 = 0) {}
    |                            ^^^^^^^^^
@@ -254,7 +260,7 @@ LL |     while true || (true && let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:92:15
+  --> $DIR/disallowed-positions.rs:95:15
    |
 LL |     while x = let 0 = 0 {}
    |               ^^^^^^^^^
@@ -263,7 +269,7 @@ LL |     while x = let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:94:18
+  --> $DIR/disallowed-positions.rs:97:18
    |
 LL |     while true..(let 0 = 0) {}
    |                  ^^^^^^^^^
@@ -272,7 +278,7 @@ LL |     while true..(let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:95:14
+  --> $DIR/disallowed-positions.rs:98:14
    |
 LL |     while ..(let 0 = 0) {}
    |              ^^^^^^^^^
@@ -281,7 +287,7 @@ LL |     while ..(let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:96:12
+  --> $DIR/disallowed-positions.rs:99:12
    |
 LL |     while (let 0 = 0).. {}
    |            ^^^^^^^^^
@@ -290,7 +296,7 @@ LL |     while (let 0 = 0).. {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:99:11
+  --> $DIR/disallowed-positions.rs:102:11
    |
 LL |     while let Range { start: _, end: _ } = true..true && false {}
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -299,7 +305,7 @@ LL |     while let Range { start: _, end: _ } = true..true && false {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:101:11
+  --> $DIR/disallowed-positions.rs:104:11
    |
 LL |     while let Range { start: _, end: _ } = true..true || false {}
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -308,7 +314,7 @@ LL |     while let Range { start: _, end: _ } = true..true || false {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:106:11
+  --> $DIR/disallowed-positions.rs:109:11
    |
 LL |     while let Range { start: F, end } = F..|| true {}
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -317,7 +323,7 @@ LL |     while let Range { start: F, end } = F..|| true {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:111:11
+  --> $DIR/disallowed-positions.rs:114:11
    |
 LL |     while let Range { start: true, end } = t..&&false {}
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -326,7 +332,7 @@ LL |     while let Range { start: true, end } = t..&&false {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:114:22
+  --> $DIR/disallowed-positions.rs:117:22
    |
 LL |     while let true = let true = true {}
    |                      ^^^^^^^^^^^^^^^
@@ -335,7 +341,7 @@ LL |     while let true = let true = true {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:128:6
+  --> $DIR/disallowed-positions.rs:131:6
    |
 LL |     &let 0 = 0;
    |      ^^^^^^^^^
@@ -344,7 +350,7 @@ LL |     &let 0 = 0;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:130:6
+  --> $DIR/disallowed-positions.rs:133:6
    |
 LL |     !let 0 = 0;
    |      ^^^^^^^^^
@@ -353,7 +359,7 @@ LL |     !let 0 = 0;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:131:6
+  --> $DIR/disallowed-positions.rs:134:6
    |
 LL |     *let 0 = 0;
    |      ^^^^^^^^^
@@ -362,7 +368,7 @@ LL |     *let 0 = 0;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:132:6
+  --> $DIR/disallowed-positions.rs:135:6
    |
 LL |     -let 0 = 0;
    |      ^^^^^^^^^
@@ -371,7 +377,7 @@ LL |     -let 0 = 0;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:138:6
+  --> $DIR/disallowed-positions.rs:141:6
    |
 LL |     (let 0 = 0)?;
    |      ^^^^^^^^^
@@ -380,7 +386,7 @@ LL |     (let 0 = 0)?;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:140:13
+  --> $DIR/disallowed-positions.rs:143:13
    |
 LL |     true || let 0 = 0;
    |             ^^^^^^^^^
@@ -389,7 +395,7 @@ LL |     true || let 0 = 0;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:141:14
+  --> $DIR/disallowed-positions.rs:144:14
    |
 LL |     (true || let 0 = 0);
    |              ^^^^^^^^^
@@ -398,7 +404,7 @@ LL |     (true || let 0 = 0);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:142:22
+  --> $DIR/disallowed-positions.rs:145:22
    |
 LL |     true && (true || let 0 = 0);
    |                      ^^^^^^^^^
@@ -407,7 +413,7 @@ LL |     true && (true || let 0 = 0);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:145:9
+  --> $DIR/disallowed-positions.rs:148:9
    |
 LL |     x = let 0 = 0;
    |         ^^^^^^^^^
@@ -416,7 +422,7 @@ LL |     x = let 0 = 0;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:147:12
+  --> $DIR/disallowed-positions.rs:150:12
    |
 LL |     true..(let 0 = 0);
    |            ^^^^^^^^^
@@ -425,7 +431,7 @@ LL |     true..(let 0 = 0);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:148:8
+  --> $DIR/disallowed-positions.rs:151:8
    |
 LL |     ..(let 0 = 0);
    |        ^^^^^^^^^
@@ -434,7 +440,7 @@ LL |     ..(let 0 = 0);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:149:6
+  --> $DIR/disallowed-positions.rs:152:6
    |
 LL |     (let 0 = 0)..;
    |      ^^^^^^^^^
@@ -443,7 +449,7 @@ LL |     (let 0 = 0)..;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:151:6
+  --> $DIR/disallowed-positions.rs:154:6
    |
 LL |     (let Range { start: _, end: _ } = true..true || false);
    |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -452,7 +458,7 @@ LL |     (let Range { start: _, end: _ } = true..true || false);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:154:6
+  --> $DIR/disallowed-positions.rs:157:6
    |
 LL |     (let true = let true = true);
    |      ^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -461,7 +467,7 @@ LL |     (let true = let true = true);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:154:17
+  --> $DIR/disallowed-positions.rs:157:17
    |
 LL |     (let true = let true = true);
    |                 ^^^^^^^^^^^^^^^
@@ -470,7 +476,7 @@ LL |     (let true = let true = true);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:159:6
+  --> $DIR/disallowed-positions.rs:162:6
    |
 LL |     &let 0 = 0
    |      ^^^^^^^^^
@@ -479,7 +485,7 @@ LL |     &let 0 = 0
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:169:17
+  --> $DIR/disallowed-positions.rs:172:17
    |
 LL |         true && let 1 = 1
    |                 ^^^^^^^^^
@@ -488,7 +494,7 @@ LL |         true && let 1 = 1
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:173:17
+  --> $DIR/disallowed-positions.rs:176:17
    |
 LL |         true && let 1 = 1
    |                 ^^^^^^^^^
@@ -497,7 +503,7 @@ LL |         true && let 1 = 1
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:177:17
+  --> $DIR/disallowed-positions.rs:180:17
    |
 LL |         true && let 1 = 1
    |                 ^^^^^^^^^
@@ -506,7 +512,7 @@ LL |         true && let 1 = 1
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions only supported in `if`
-  --> $DIR/disallowed-positions.rs:29:9
+  --> $DIR/disallowed-positions.rs:32:9
    |
 LL |     if &let 0 = 0 {}
    |         ^^^^^^^^^
diff --git a/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs b/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs
index b6f70b912acac..c125ec89cb574 100644
--- a/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs
+++ b/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs
@@ -83,7 +83,9 @@ fn _while() {
 
 fn _macros() {
     macro_rules! noop_expr { ($e:expr) => {}; }
+
     noop_expr!((let 0 = 1));
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
 
     macro_rules! use_expr {
         ($e:expr) => {
@@ -95,7 +97,10 @@ fn _macros() {
     //~^ ERROR `let` expressions in this position are experimental [E0658]
     use_expr!((let 0 = 1));
     //~^ ERROR `let` expressions in this position are experimental [E0658]
-    // use_expr!(let 0 = 1);
+    #[cfg(FALSE)] (let 0 = 1);
+    //~^ ERROR `let` expressions in this position are experimental [E0658]
+    use_expr!(let 0 = 1);
+    //~^ ERROR no rules expected the token `let`
     // ^--- FIXME(53667): Consider whether `Let` can be added to `ident_can_begin_expr`.
 }
 
diff --git a/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr b/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr
index fdfbb8fe4d441..5ed0dd3cab68e 100644
--- a/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr
+++ b/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr
@@ -1,3 +1,12 @@
+error: no rules expected the token `let`
+  --> $DIR/feature-gate.rs:102:15
+   |
+LL |     macro_rules! use_expr {
+   |     --------------------- when calling this macro
+...
+LL |     use_expr!(let 0 = 1);
+   |               ^^^ no rules expected this token in macro call
+
 error[E0658]: `let` expressions in this position are experimental
   --> $DIR/feature-gate.rs:14:9
    |
@@ -251,7 +260,25 @@ LL |     while let Range { start: _, end: _ } = (true..true) && false {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:94:16
+  --> $DIR/feature-gate.rs:100:20
+   |
+LL |     #[cfg(FALSE)] (let 0 = 1);
+   |                    ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:87:17
+   |
+LL |     noop_expr!((let 0 = 1));
+   |                 ^^^^^^^^^
+   |
+   = note: for more information, see https://github.com/rust-lang/rust/issues/53667
+   = help: add #![feature(let_chains)] to the crate attributes to enable
+
+error[E0658]: `let` expressions in this position are experimental
+  --> $DIR/feature-gate.rs:96:16
    |
 LL |     use_expr!((let 0 = 1 && 0 == 0));
    |                ^^^^^^^^^
@@ -260,7 +287,7 @@ LL |     use_expr!((let 0 = 1 && 0 == 0));
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:96:16
+  --> $DIR/feature-gate.rs:98:16
    |
 LL |     use_expr!((let 0 = 1));
    |                ^^^^^^^^^
@@ -274,6 +301,6 @@ error: `let` expressions only supported in `if`
 LL |     if (let 0 = 1) {}
    |         ^^^^^^^^^
 
-error: aborting due to 31 previous errors
+error: aborting due to 34 previous errors
 
 For more information about this error, try `rustc --explain E0658`.

From 2017be4ef657d1a7d03a9dac80d868645c6673dd Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Mon, 17 Jun 2019 07:36:45 +0200
Subject: [PATCH 28/35] let_chains: Remove ast_validation logic in favor of
 lowering with recovery.

---
 src/librustc/hir/lowering.rs          | 48 +++++++++++++++----
 src/librustc_passes/ast_validation.rs | 68 ++++-----------------------
 2 files changed, 49 insertions(+), 67 deletions(-)

diff --git a/src/librustc/hir/lowering.rs b/src/librustc/hir/lowering.rs
index a684816c6defe..36b29ae6e6076 100644
--- a/src/librustc/hir/lowering.rs
+++ b/src/librustc/hir/lowering.rs
@@ -4344,12 +4344,39 @@ impl<'a> LoweringContext<'a> {
                 let ohs = P(self.lower_expr(ohs));
                 hir::ExprKind::AddrOf(m, ohs)
             }
-            ExprKind::Let(..) => {
-                // This should have been caught `ast_validation`!
-                self.sess.span_err(e.span, "`let` expressions only supported in `if`");
-                // ^-- FIXME(53667): Change to `delay_span_bug` when let_chains handled in lowering.
-                self.sess.abort_if_errors();
-                hir::ExprKind::Err
+            ExprKind::Let(ref pats, ref scrutinee) => {
+                // If we got here, the `let` expression is not allowed.
+                self.sess
+                    .struct_span_err(e.span, "`let` expressions are not supported here")
+                    .note("only supported directly in conditions of `if`- and `while`-expressions")
+                    .note("as well as when nested within `&&` and parenthesis in those conditions")
+                    .emit();
+
+                // For better recovery, we emit:
+                // ```
+                // match scrutinee { pats => true, _ => false }
+                // ```
+                // While this doesn't fully match the user's intent, it has key advantages:
+                // 1. We can avoid using `abort_if_errors`.
+                // 2. We can typeck both `pats` and `scrutinee`.
+                // 3. `pats` is allowed to be refutable.
+                // 4. The return type of the block is `bool` which seems like what the user wanted.
+                let scrutinee = self.lower_expr(scrutinee);
+                let then_arm = {
+                    let pats = pats.iter().map(|pat| self.lower_pat(pat)).collect();
+                    let expr = self.expr_bool(e.span, true);
+                    self.arm(pats, P(expr))
+                };
+                let else_arm = {
+                    let pats = hir_vec![self.pat_wild(e.span)];
+                    let expr = self.expr_bool(e.span, false);
+                    self.arm(pats, P(expr))
+                };
+                hir::ExprKind::Match(
+                    P(scrutinee),
+                    vec![then_arm, else_arm].into(),
+                    hir::MatchSource::Normal,
+                )
             }
             // FIXME(#53667): handle lowering of && and parens.
             ExprKind::If(ref cond, ref then, ref else_opt) => {
@@ -5431,10 +5458,15 @@ impl<'a> LoweringContext<'a> {
         )
     }
 
+    /// Constructs a `true` or `false` literal expression.
+    fn expr_bool(&mut self, span: Span, val: bool) -> hir::Expr {
+        let lit = Spanned { span, node: LitKind::Bool(val) };
+        self.expr(span, hir::ExprKind::Lit(lit), ThinVec::new())
+    }
+
     /// Constructs a `true` or `false` literal pattern.
     fn pat_bool(&mut self, span: Span, val: bool) -> P<hir::Pat> {
-        let lit = Spanned { span, node: LitKind::Bool(val) };
-        let expr = self.expr(span, hir::ExprKind::Lit(lit), ThinVec::new());
+        let expr = self.expr_bool(span, val);
         self.pat(span, hir::PatKind::Lit(P(expr)))
     }
 
diff --git a/src/librustc_passes/ast_validation.rs b/src/librustc_passes/ast_validation.rs
index 2aa8d5113ff7d..c6bc4f2f49e3e 100644
--- a/src/librustc_passes/ast_validation.rs
+++ b/src/librustc_passes/ast_validation.rs
@@ -74,9 +74,6 @@ struct AstValidator<'a> {
     /// these booleans.
     warning_period_57979_didnt_record_next_impl_trait: bool,
     warning_period_57979_impl_trait_in_proj: bool,
-
-    /// Used to ban `let` expressions in inappropriate places.
-    is_let_allowed: bool,
 }
 
 /// With the `new` value in `store`,
@@ -114,12 +111,6 @@ impl<'a> AstValidator<'a> {
         with(self, outer, |this| &mut this.outer_impl_trait, f)
     }
 
-    fn with_let_allowed(&mut self, v: bool, f: impl FnOnce(&mut Self, bool)) {
-        let old = mem::replace(&mut self.is_let_allowed, v);
-        f(self, old);
-        self.is_let_allowed = old;
-    }
-
     fn visit_assoc_ty_constraint_from_generic_args(&mut self, constraint: &'a AssocTyConstraint) {
         match constraint.kind {
             AssocTyConstraintKind::Equality { ref ty } => {
@@ -335,15 +326,6 @@ impl<'a> AstValidator<'a> {
         }
     }
 
-    /// Emits an error banning the `let` expression provided.
-    fn ban_let_expr(&self, expr: &'a Expr) {
-        self.err_handler()
-            .struct_span_err(expr.span, "`let` expressions are not supported here")
-            .note("only supported directly in conditions of `if`- and `while`-expressions")
-            .note("as well as when nested within `&&` and parenthesis in those conditions")
-            .emit();
-    }
-
     fn check_fn_decl(&self, fn_decl: &FnDecl) {
         fn_decl
             .inputs
@@ -470,48 +452,17 @@ fn validate_generics_order<'a>(
 
 impl<'a> Visitor<'a> for AstValidator<'a> {
     fn visit_expr(&mut self, expr: &'a Expr) {
-        self.with_let_allowed(false, |this, let_allowed| {
-            match &expr.node {
-                ExprKind::Let(_, _) if !let_allowed => {
-                    this.ban_let_expr(expr);
-                }
-                // Assuming the context permits, `($expr)` does not impose additional constraints.
-                ExprKind::Paren(_) => {
-                    this.with_let_allowed(let_allowed, |this, _| visit::walk_expr(this, expr));
-                    return; // We've already walked into `expr`.
-                }
-                // Assuming the context permits,
-                // l && r` allows decendants in `l` and `r` to be `let` expressions.
-                ExprKind::Binary(op, ..) if op.node == BinOpKind::And => {
-                    this.with_let_allowed(let_allowed, |this, _| visit::walk_expr(this, expr));
-                    return; // We've already walked into `expr`.
-                }
-                // However, we do allow it in the condition of the `if` expression.
-                // We do not allow `let` in `then` and `opt_else` directly.
-                ExprKind::If(cond, then, opt_else) => {
-                    this.visit_block(then);
-                    walk_list!(this, visit_expr, opt_else);
-                    this.with_let_allowed(true, |this, _| this.visit_expr(cond));
-                    return; // We've already walked into `expr`.
-                }
-                // The same logic applies to `While`.
-                ExprKind::While(cond, then, opt_label) => {
-                    walk_list!(this, visit_label, opt_label);
-                    this.visit_block(then);
-                    this.with_let_allowed(true, |this, _| this.visit_expr(cond));
-                    return; // We've already walked into `expr`.
-                }
-                ExprKind::Closure(_, _, _, fn_decl, _, _) => {
-                    this.check_fn_decl(fn_decl);
-                }
-                ExprKind::InlineAsm(..) if !this.session.target.target.options.allow_asm => {
-                    span_err!(this.session, expr.span, E0472, "asm! is unsupported on this target");
-                }
-                _ => {}
+        match &expr.node {
+            ExprKind::Closure(_, _, _, fn_decl, _, _) => {
+                self.check_fn_decl(fn_decl);
             }
+            ExprKind::InlineAsm(..) if !self.session.target.target.options.allow_asm => {
+                span_err!(self.session, expr.span, E0472, "asm! is unsupported on this target");
+            }
+            _ => {}
+        }
 
-            visit::walk_expr(this, expr);
-        });
+        visit::walk_expr(self, expr);
     }
 
     fn visit_ty(&mut self, ty: &'a Ty) {
@@ -923,7 +874,6 @@ pub fn check_crate(session: &Session, krate: &Crate) -> (bool, bool) {
         is_assoc_ty_bound_banned: false,
         warning_period_57979_didnt_record_next_impl_trait: false,
         warning_period_57979_impl_trait_in_proj: false,
-        is_let_allowed: false,
     };
     visit::walk_crate(&mut validator, krate);
 

From 5ae5086cc59a19bb35b53df0580610e0df03d812 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Mon, 17 Jun 2019 07:38:26 +0200
Subject: [PATCH 29/35] let_chains: --bless tests due to recovery in lowering.

---
 .../disallowed-positions.rs                   |  56 +-
 .../disallowed-positions.stderr               | 580 ++++++++++++++++--
 .../ui/rfc-2497-if-let-chains/feature-gate.rs |  33 +-
 .../feature-gate.stderr                       | 330 +++++++++-
 4 files changed, 905 insertions(+), 94 deletions(-)

diff --git a/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs
index 43e0ee8a86cf9..7d1e5c3d64df3 100644
--- a/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs
+++ b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.rs
@@ -1,4 +1,4 @@
-// Here we test that `ast_validation` behaves correctly wrt. `let $pats = $expr` expressions.
+// Here we test that `lowering` behaves correctly wrt. `let $pats = $expr` expressions.
 //
 // We want to make sure that `let` is banned in situations other than:
 //
@@ -30,17 +30,22 @@ fn main() {}
 
 fn nested_within_if_expr() {
     if &let 0 = 0 {} //~ ERROR `let` expressions are not supported here
-    //~^ ERROR `let` expressions only supported in `if`
+    //~^ ERROR mismatched types
 
     if !let 0 = 0 {} //~ ERROR `let` expressions are not supported here
     if *let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR type `bool` cannot be dereferenced
     if -let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR cannot apply unary operator `-` to type `bool`
 
     fn _check_try_binds_tighter() -> Result<(), ()> {
         if let 0 = 0? {}
+        //~^ ERROR the `?` operator can only be applied to values that implement `std::ops::Try`
         Ok(())
     }
     if (let 0 = 0)? {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR the `?` operator can only be applied to values that implement `std::ops::Try`
+    //~| ERROR the `?` operator can only be used in a function that returns `Result`
 
     if true || let 0 = 0 {} //~ ERROR `let` expressions are not supported here
     if (true || let 0 = 0) {} //~ ERROR `let` expressions are not supported here
@@ -49,42 +54,62 @@ fn nested_within_if_expr() {
 
     let mut x = true;
     if x = let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR mismatched types
 
     if true..(let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR mismatched types
     if ..(let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR mismatched types
     if (let 0 = 0).. {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR mismatched types
 
     // Binds as `(let ... = true)..true &&/|| false`.
     if let Range { start: _, end: _ } = true..true && false {}
     //~^ ERROR `let` expressions are not supported here
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
     if let Range { start: _, end: _ } = true..true || false {}
     //~^ ERROR `let` expressions are not supported here
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
 
     // Binds as `(let Range { start: F, end } = F)..(|| true)`.
     const F: fn() -> bool = || true;
     if let Range { start: F, end } = F..|| true {}
     //~^ ERROR `let` expressions are not supported here
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
 
     // Binds as `(let Range { start: true, end } = t)..(&&false)`.
     let t = &&true;
     if let Range { start: true, end } = t..&&false {}
     //~^ ERROR `let` expressions are not supported here
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
 
     if let true = let true = true {} //~ ERROR `let` expressions are not supported here
 }
 
 fn nested_within_while_expr() {
     while &let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR mismatched types
 
     while !let 0 = 0 {} //~ ERROR `let` expressions are not supported here
     while *let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR type `bool` cannot be dereferenced
     while -let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR cannot apply unary operator `-` to type `bool`
 
     fn _check_try_binds_tighter() -> Result<(), ()> {
         while let 0 = 0? {}
+        //~^ ERROR the `?` operator can only be applied to values that implement `std::ops::Try`
         Ok(())
     }
     while (let 0 = 0)? {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR the `?` operator can only be applied to values that implement `std::ops::Try`
+    //~| ERROR the `?` operator can only be used in a function that returns `Result`
 
     while true || let 0 = 0 {} //~ ERROR `let` expressions are not supported here
     while (true || let 0 = 0) {} //~ ERROR `let` expressions are not supported here
@@ -93,26 +118,40 @@ fn nested_within_while_expr() {
 
     let mut x = true;
     while x = let 0 = 0 {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR mismatched types
 
     while true..(let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR mismatched types
     while ..(let 0 = 0) {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR mismatched types
     while (let 0 = 0).. {} //~ ERROR `let` expressions are not supported here
+    //~^ ERROR mismatched types
 
     // Binds as `(let ... = true)..true &&/|| false`.
     while let Range { start: _, end: _ } = true..true && false {}
     //~^ ERROR `let` expressions are not supported here
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
     while let Range { start: _, end: _ } = true..true || false {}
     //~^ ERROR `let` expressions are not supported here
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
 
     // Binds as `(let Range { start: F, end } = F)..(|| true)`.
     const F: fn() -> bool = || true;
     while let Range { start: F, end } = F..|| true {}
     //~^ ERROR `let` expressions are not supported here
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
 
     // Binds as `(let Range { start: true, end } = t)..(&&false)`.
     let t = &&true;
     while let Range { start: true, end } = t..&&false {}
     //~^ ERROR `let` expressions are not supported here
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
+    //~| ERROR mismatched types
 
     while let true = let true = true {} //~ ERROR `let` expressions are not supported here
 }
@@ -132,13 +171,18 @@ fn outside_if_and_while_expr() {
 
     !let 0 = 0; //~ ERROR `let` expressions are not supported here
     *let 0 = 0; //~ ERROR `let` expressions are not supported here
+    //~^ ERROR type `bool` cannot be dereferenced
     -let 0 = 0; //~ ERROR `let` expressions are not supported here
+    //~^ ERROR cannot apply unary operator `-` to type `bool`
 
     fn _check_try_binds_tighter() -> Result<(), ()> {
         let 0 = 0?;
+        //~^ ERROR the `?` operator can only be applied to values that implement `std::ops::Try`
         Ok(())
     }
     (let 0 = 0)?; //~ ERROR `let` expressions are not supported here
+    //~^ ERROR the `?` operator can only be used in a function that returns `Result`
+    //~| ERROR the `?` operator can only be applied to values that implement `std::ops::Try`
 
     true || let 0 = 0; //~ ERROR `let` expressions are not supported here
     (true || let 0 = 0); //~ ERROR `let` expressions are not supported here
@@ -153,6 +197,7 @@ fn outside_if_and_while_expr() {
 
     (let Range { start: _, end: _ } = true..true || false);
     //~^ ERROR `let` expressions are not supported here
+    //~| ERROR mismatched types
 
     (let true = let true = true);
     //~^ ERROR `let` expressions are not supported here
@@ -161,6 +206,7 @@ fn outside_if_and_while_expr() {
     // Check function tail position.
     &let 0 = 0
     //~^ ERROR `let` expressions are not supported here
+    //~| ERROR mismatched types
 }
 
 // Let's make sure that `let` inside const generic arguments are considered.
@@ -170,14 +216,20 @@ fn inside_const_generic_arguments() {
 
     if let A::<{
         true && let 1 = 1 //~ ERROR `let` expressions are not supported here
+        //~^ ERROR constant contains unimplemented expression type
+        //~| ERROR constant contains unimplemented expression type
     }>::O = 5 {}
 
     while let A::<{
         true && let 1 = 1 //~ ERROR `let` expressions are not supported here
+        //~^ ERROR constant contains unimplemented expression type
+        //~| ERROR constant contains unimplemented expression type
     }>::O = 5 {}
 
     if A::<{
         true && let 1 = 1 //~ ERROR `let` expressions are not supported here
+        //~^ ERROR constant contains unimplemented expression type
+        //~| ERROR constant contains unimplemented expression type
     }>::O == 5 {}
 
     // In the cases above we have `ExprKind::Block` to help us out.
diff --git a/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr
index 86baa038c0b98..207d0d6d6b84a 100644
--- a/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr
+++ b/src/test/ui/rfc-2497-if-let-chains/disallowed-positions.stderr
@@ -1,5 +1,5 @@
 error: expected one of `,` or `>`, found `&&`
-  --> $DIR/disallowed-positions.rs:190:14
+  --> $DIR/disallowed-positions.rs:242:14
    |
 LL |         true && let 1 = 1
    |              ^^ expected one of `,` or `>` here
@@ -44,7 +44,7 @@ LL |     if *let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:37:9
+  --> $DIR/disallowed-positions.rs:38:9
    |
 LL |     if -let 0 = 0 {}
    |         ^^^^^^^^^
@@ -53,7 +53,7 @@ LL |     if -let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:43:9
+  --> $DIR/disallowed-positions.rs:46:9
    |
 LL |     if (let 0 = 0)? {}
    |         ^^^^^^^^^
@@ -62,7 +62,7 @@ LL |     if (let 0 = 0)? {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:45:16
+  --> $DIR/disallowed-positions.rs:50:16
    |
 LL |     if true || let 0 = 0 {}
    |                ^^^^^^^^^
@@ -71,7 +71,7 @@ LL |     if true || let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:46:17
+  --> $DIR/disallowed-positions.rs:51:17
    |
 LL |     if (true || let 0 = 0) {}
    |                 ^^^^^^^^^
@@ -80,7 +80,7 @@ LL |     if (true || let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:47:25
+  --> $DIR/disallowed-positions.rs:52:25
    |
 LL |     if true && (true || let 0 = 0) {}
    |                         ^^^^^^^^^
@@ -89,7 +89,7 @@ LL |     if true && (true || let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:48:25
+  --> $DIR/disallowed-positions.rs:53:25
    |
 LL |     if true || (true && let 0 = 0) {}
    |                         ^^^^^^^^^
@@ -98,7 +98,7 @@ LL |     if true || (true && let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:51:12
+  --> $DIR/disallowed-positions.rs:56:12
    |
 LL |     if x = let 0 = 0 {}
    |            ^^^^^^^^^
@@ -107,7 +107,7 @@ LL |     if x = let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:53:15
+  --> $DIR/disallowed-positions.rs:59:15
    |
 LL |     if true..(let 0 = 0) {}
    |               ^^^^^^^^^
@@ -116,7 +116,7 @@ LL |     if true..(let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:54:11
+  --> $DIR/disallowed-positions.rs:61:11
    |
 LL |     if ..(let 0 = 0) {}
    |           ^^^^^^^^^
@@ -125,7 +125,7 @@ LL |     if ..(let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:55:9
+  --> $DIR/disallowed-positions.rs:63:9
    |
 LL |     if (let 0 = 0).. {}
    |         ^^^^^^^^^
@@ -134,7 +134,7 @@ LL |     if (let 0 = 0).. {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:58:8
+  --> $DIR/disallowed-positions.rs:67:8
    |
 LL |     if let Range { start: _, end: _ } = true..true && false {}
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -143,7 +143,7 @@ LL |     if let Range { start: _, end: _ } = true..true && false {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:60:8
+  --> $DIR/disallowed-positions.rs:71:8
    |
 LL |     if let Range { start: _, end: _ } = true..true || false {}
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -152,7 +152,7 @@ LL |     if let Range { start: _, end: _ } = true..true || false {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:65:8
+  --> $DIR/disallowed-positions.rs:78:8
    |
 LL |     if let Range { start: F, end } = F..|| true {}
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -161,7 +161,7 @@ LL |     if let Range { start: F, end } = F..|| true {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:70:8
+  --> $DIR/disallowed-positions.rs:86:8
    |
 LL |     if let Range { start: true, end } = t..&&false {}
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -170,7 +170,7 @@ LL |     if let Range { start: true, end } = t..&&false {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:73:19
+  --> $DIR/disallowed-positions.rs:92:19
    |
 LL |     if let true = let true = true {}
    |                   ^^^^^^^^^^^^^^^
@@ -179,7 +179,7 @@ LL |     if let true = let true = true {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:77:12
+  --> $DIR/disallowed-positions.rs:96:12
    |
 LL |     while &let 0 = 0 {}
    |            ^^^^^^^^^
@@ -188,7 +188,7 @@ LL |     while &let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:79:12
+  --> $DIR/disallowed-positions.rs:99:12
    |
 LL |     while !let 0 = 0 {}
    |            ^^^^^^^^^
@@ -197,7 +197,7 @@ LL |     while !let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:80:12
+  --> $DIR/disallowed-positions.rs:100:12
    |
 LL |     while *let 0 = 0 {}
    |            ^^^^^^^^^
@@ -206,7 +206,7 @@ LL |     while *let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:81:12
+  --> $DIR/disallowed-positions.rs:102:12
    |
 LL |     while -let 0 = 0 {}
    |            ^^^^^^^^^
@@ -215,7 +215,7 @@ LL |     while -let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:87:12
+  --> $DIR/disallowed-positions.rs:110:12
    |
 LL |     while (let 0 = 0)? {}
    |            ^^^^^^^^^
@@ -224,7 +224,7 @@ LL |     while (let 0 = 0)? {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:89:19
+  --> $DIR/disallowed-positions.rs:114:19
    |
 LL |     while true || let 0 = 0 {}
    |                   ^^^^^^^^^
@@ -233,7 +233,7 @@ LL |     while true || let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:90:20
+  --> $DIR/disallowed-positions.rs:115:20
    |
 LL |     while (true || let 0 = 0) {}
    |                    ^^^^^^^^^
@@ -242,7 +242,7 @@ LL |     while (true || let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:91:28
+  --> $DIR/disallowed-positions.rs:116:28
    |
 LL |     while true && (true || let 0 = 0) {}
    |                            ^^^^^^^^^
@@ -251,7 +251,7 @@ LL |     while true && (true || let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:92:28
+  --> $DIR/disallowed-positions.rs:117:28
    |
 LL |     while true || (true && let 0 = 0) {}
    |                            ^^^^^^^^^
@@ -260,7 +260,7 @@ LL |     while true || (true && let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:95:15
+  --> $DIR/disallowed-positions.rs:120:15
    |
 LL |     while x = let 0 = 0 {}
    |               ^^^^^^^^^
@@ -269,7 +269,7 @@ LL |     while x = let 0 = 0 {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:97:18
+  --> $DIR/disallowed-positions.rs:123:18
    |
 LL |     while true..(let 0 = 0) {}
    |                  ^^^^^^^^^
@@ -278,7 +278,7 @@ LL |     while true..(let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:98:14
+  --> $DIR/disallowed-positions.rs:125:14
    |
 LL |     while ..(let 0 = 0) {}
    |              ^^^^^^^^^
@@ -287,7 +287,7 @@ LL |     while ..(let 0 = 0) {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:99:12
+  --> $DIR/disallowed-positions.rs:127:12
    |
 LL |     while (let 0 = 0).. {}
    |            ^^^^^^^^^
@@ -296,7 +296,7 @@ LL |     while (let 0 = 0).. {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:102:11
+  --> $DIR/disallowed-positions.rs:131:11
    |
 LL |     while let Range { start: _, end: _ } = true..true && false {}
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -305,7 +305,7 @@ LL |     while let Range { start: _, end: _ } = true..true && false {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:104:11
+  --> $DIR/disallowed-positions.rs:135:11
    |
 LL |     while let Range { start: _, end: _ } = true..true || false {}
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -314,7 +314,7 @@ LL |     while let Range { start: _, end: _ } = true..true || false {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:109:11
+  --> $DIR/disallowed-positions.rs:142:11
    |
 LL |     while let Range { start: F, end } = F..|| true {}
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -323,7 +323,7 @@ LL |     while let Range { start: F, end } = F..|| true {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:114:11
+  --> $DIR/disallowed-positions.rs:150:11
    |
 LL |     while let Range { start: true, end } = t..&&false {}
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -332,7 +332,7 @@ LL |     while let Range { start: true, end } = t..&&false {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:117:22
+  --> $DIR/disallowed-positions.rs:156:22
    |
 LL |     while let true = let true = true {}
    |                      ^^^^^^^^^^^^^^^
@@ -341,7 +341,7 @@ LL |     while let true = let true = true {}
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:131:6
+  --> $DIR/disallowed-positions.rs:170:6
    |
 LL |     &let 0 = 0;
    |      ^^^^^^^^^
@@ -350,7 +350,7 @@ LL |     &let 0 = 0;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:133:6
+  --> $DIR/disallowed-positions.rs:172:6
    |
 LL |     !let 0 = 0;
    |      ^^^^^^^^^
@@ -359,7 +359,7 @@ LL |     !let 0 = 0;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:134:6
+  --> $DIR/disallowed-positions.rs:173:6
    |
 LL |     *let 0 = 0;
    |      ^^^^^^^^^
@@ -368,7 +368,7 @@ LL |     *let 0 = 0;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:135:6
+  --> $DIR/disallowed-positions.rs:175:6
    |
 LL |     -let 0 = 0;
    |      ^^^^^^^^^
@@ -377,7 +377,7 @@ LL |     -let 0 = 0;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:141:6
+  --> $DIR/disallowed-positions.rs:183:6
    |
 LL |     (let 0 = 0)?;
    |      ^^^^^^^^^
@@ -386,7 +386,7 @@ LL |     (let 0 = 0)?;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:143:13
+  --> $DIR/disallowed-positions.rs:187:13
    |
 LL |     true || let 0 = 0;
    |             ^^^^^^^^^
@@ -395,7 +395,7 @@ LL |     true || let 0 = 0;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:144:14
+  --> $DIR/disallowed-positions.rs:188:14
    |
 LL |     (true || let 0 = 0);
    |              ^^^^^^^^^
@@ -404,7 +404,7 @@ LL |     (true || let 0 = 0);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:145:22
+  --> $DIR/disallowed-positions.rs:189:22
    |
 LL |     true && (true || let 0 = 0);
    |                      ^^^^^^^^^
@@ -413,7 +413,7 @@ LL |     true && (true || let 0 = 0);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:148:9
+  --> $DIR/disallowed-positions.rs:192:9
    |
 LL |     x = let 0 = 0;
    |         ^^^^^^^^^
@@ -422,7 +422,7 @@ LL |     x = let 0 = 0;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:150:12
+  --> $DIR/disallowed-positions.rs:194:12
    |
 LL |     true..(let 0 = 0);
    |            ^^^^^^^^^
@@ -431,7 +431,7 @@ LL |     true..(let 0 = 0);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:151:8
+  --> $DIR/disallowed-positions.rs:195:8
    |
 LL |     ..(let 0 = 0);
    |        ^^^^^^^^^
@@ -440,7 +440,7 @@ LL |     ..(let 0 = 0);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:152:6
+  --> $DIR/disallowed-positions.rs:196:6
    |
 LL |     (let 0 = 0)..;
    |      ^^^^^^^^^
@@ -449,7 +449,7 @@ LL |     (let 0 = 0)..;
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:154:6
+  --> $DIR/disallowed-positions.rs:198:6
    |
 LL |     (let Range { start: _, end: _ } = true..true || false);
    |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -458,7 +458,7 @@ LL |     (let Range { start: _, end: _ } = true..true || false);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:157:6
+  --> $DIR/disallowed-positions.rs:202:6
    |
 LL |     (let true = let true = true);
    |      ^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -467,7 +467,7 @@ LL |     (let true = let true = true);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:157:17
+  --> $DIR/disallowed-positions.rs:202:17
    |
 LL |     (let true = let true = true);
    |                 ^^^^^^^^^^^^^^^
@@ -476,7 +476,7 @@ LL |     (let true = let true = true);
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:162:6
+  --> $DIR/disallowed-positions.rs:207:6
    |
 LL |     &let 0 = 0
    |      ^^^^^^^^^
@@ -485,7 +485,7 @@ LL |     &let 0 = 0
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:172:17
+  --> $DIR/disallowed-positions.rs:218:17
    |
 LL |         true && let 1 = 1
    |                 ^^^^^^^^^
@@ -494,7 +494,7 @@ LL |         true && let 1 = 1
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:176:17
+  --> $DIR/disallowed-positions.rs:224:17
    |
 LL |         true && let 1 = 1
    |                 ^^^^^^^^^
@@ -503,7 +503,7 @@ LL |         true && let 1 = 1
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
 error: `let` expressions are not supported here
-  --> $DIR/disallowed-positions.rs:180:17
+  --> $DIR/disallowed-positions.rs:230:17
    |
 LL |         true && let 1 = 1
    |                 ^^^^^^^^^
@@ -511,11 +511,477 @@ LL |         true && let 1 = 1
    = note: only supported directly in conditions of `if`- and `while`-expressions
    = note: as well as when nested within `&&` and parenthesis in those conditions
 
-error: `let` expressions only supported in `if`
-  --> $DIR/disallowed-positions.rs:32:9
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:32:8
    |
 LL |     if &let 0 = 0 {}
-   |         ^^^^^^^^^
+   |        ^^^^^^^^^^ expected bool, found &bool
+   |
+   = note: expected type `bool`
+              found type `&bool`
+
+error[E0614]: type `bool` cannot be dereferenced
+  --> $DIR/disallowed-positions.rs:36:8
+   |
+LL |     if *let 0 = 0 {}
+   |        ^^^^^^^^^^
+
+error[E0600]: cannot apply unary operator `-` to type `bool`
+  --> $DIR/disallowed-positions.rs:38:8
+   |
+LL |     if -let 0 = 0 {}
+   |        ^^^^^^^^^^ cannot apply unary operator `-`
+   |
+   = note: an implementation of `std::ops::Neg` might be missing for `bool`
+
+error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try`
+  --> $DIR/disallowed-positions.rs:46:8
+   |
+LL |     if (let 0 = 0)? {}
+   |        ^^^^^^^^^^^^ the `?` operator cannot be applied to type `bool`
+   |
+   = help: the trait `std::ops::Try` is not implemented for `bool`
+   = note: required by `std::ops::Try::into_result`
+
+error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
+  --> $DIR/disallowed-positions.rs:46:8
+   |
+LL |     if (let 0 = 0)? {}
+   |        ^^^^^^^^^^^^ cannot use the `?` operator in a function that returns `()`
+   |
+   = help: the trait `std::ops::Try` is not implemented for `()`
+   = note: required by `std::ops::Try::from_error`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:56:8
+   |
+LL |     if x = let 0 = 0 {}
+   |        ^^^^^^^^^^^^^
+   |        |
+   |        expected bool, found ()
+   |        help: try comparing for equality: `x == let 0 = 0`
+   |
+   = note: expected type `bool`
+              found type `()`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:59:8
+   |
+LL |     if true..(let 0 = 0) {}
+   |        ^^^^^^^^^^^^^^^^^ expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:61:8
+   |
+LL |     if ..(let 0 = 0) {}
+   |        ^^^^^^^^^^^^^ expected bool, found struct `std::ops::RangeTo`
+   |
+   = note: expected type `bool`
+              found type `std::ops::RangeTo<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:63:8
+   |
+LL |     if (let 0 = 0).. {}
+   |        ^^^^^^^^^^^^^ expected bool, found struct `std::ops::RangeFrom`
+   |
+   = note: expected type `bool`
+              found type `std::ops::RangeFrom<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:67:12
+   |
+LL |     if let Range { start: _, end: _ } = true..true && false {}
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^   ---- this match expression has type `bool`
+   |            |
+   |            expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<_>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:67:8
+   |
+LL |     if let Range { start: _, end: _ } = true..true && false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:71:12
+   |
+LL |     if let Range { start: _, end: _ } = true..true || false {}
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^   ---- this match expression has type `bool`
+   |            |
+   |            expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<_>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:71:8
+   |
+LL |     if let Range { start: _, end: _ } = true..true || false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:78:12
+   |
+LL |     if let Range { start: F, end } = F..|| true {}
+   |            ^^^^^^^^^^^^^^^^^^^^^^^ expected fn pointer, found struct `std::ops::Range`
+   |
+   = note: expected type `fn() -> bool`
+              found type `std::ops::Range<_>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:78:41
+   |
+LL |     if let Range { start: F, end } = F..|| true {}
+   |                                         ^^^^^^^ expected bool, found closure
+   |
+   = note: expected type `bool`
+              found type `[closure@$DIR/disallowed-positions.rs:78:41: 78:48]`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:78:8
+   |
+LL |     if let Range { start: F, end } = F..|| true {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:86:12
+   |
+LL |     if let Range { start: true, end } = t..&&false {}
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^   - this match expression has type `bool`
+   |            |
+   |            expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<_>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:86:44
+   |
+LL |     if let Range { start: true, end } = t..&&false {}
+   |                                            ^^^^^^^ expected bool, found &&bool
+   |
+   = note: expected type `bool`
+              found type `&&bool`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:86:8
+   |
+LL |     if let Range { start: true, end } = t..&&false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<bool>`
+
+error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try`
+  --> $DIR/disallowed-positions.rs:42:20
+   |
+LL |         if let 0 = 0? {}
+   |                    ^^ the `?` operator cannot be applied to type `{integer}`
+   |
+   = help: the trait `std::ops::Try` is not implemented for `{integer}`
+   = note: required by `std::ops::Try::into_result`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:96:11
+   |
+LL |     while &let 0 = 0 {}
+   |           ^^^^^^^^^^ expected bool, found &bool
+   |
+   = note: expected type `bool`
+              found type `&bool`
+
+error[E0614]: type `bool` cannot be dereferenced
+  --> $DIR/disallowed-positions.rs:100:11
+   |
+LL |     while *let 0 = 0 {}
+   |           ^^^^^^^^^^
+
+error[E0600]: cannot apply unary operator `-` to type `bool`
+  --> $DIR/disallowed-positions.rs:102:11
+   |
+LL |     while -let 0 = 0 {}
+   |           ^^^^^^^^^^ cannot apply unary operator `-`
+   |
+   = note: an implementation of `std::ops::Neg` might be missing for `bool`
+
+error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try`
+  --> $DIR/disallowed-positions.rs:110:11
+   |
+LL |     while (let 0 = 0)? {}
+   |           ^^^^^^^^^^^^ the `?` operator cannot be applied to type `bool`
+   |
+   = help: the trait `std::ops::Try` is not implemented for `bool`
+   = note: required by `std::ops::Try::into_result`
+
+error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
+  --> $DIR/disallowed-positions.rs:110:11
+   |
+LL |     while (let 0 = 0)? {}
+   |           ^^^^^^^^^^^^ cannot use the `?` operator in a function that returns `()`
+   |
+   = help: the trait `std::ops::Try` is not implemented for `()`
+   = note: required by `std::ops::Try::from_error`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:120:11
+   |
+LL |     while x = let 0 = 0 {}
+   |           ^^^^^^^^^^^^^
+   |           |
+   |           expected bool, found ()
+   |           help: try comparing for equality: `x == let 0 = 0`
+   |
+   = note: expected type `bool`
+              found type `()`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:123:11
+   |
+LL |     while true..(let 0 = 0) {}
+   |           ^^^^^^^^^^^^^^^^^ expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:125:11
+   |
+LL |     while ..(let 0 = 0) {}
+   |           ^^^^^^^^^^^^^ expected bool, found struct `std::ops::RangeTo`
+   |
+   = note: expected type `bool`
+              found type `std::ops::RangeTo<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:127:11
+   |
+LL |     while (let 0 = 0).. {}
+   |           ^^^^^^^^^^^^^ expected bool, found struct `std::ops::RangeFrom`
+   |
+   = note: expected type `bool`
+              found type `std::ops::RangeFrom<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:131:15
+   |
+LL |     while let Range { start: _, end: _ } = true..true && false {}
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^   ---- this match expression has type `bool`
+   |               |
+   |               expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<_>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:131:11
+   |
+LL |     while let Range { start: _, end: _ } = true..true && false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:135:15
+   |
+LL |     while let Range { start: _, end: _ } = true..true || false {}
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^   ---- this match expression has type `bool`
+   |               |
+   |               expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<_>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:135:11
+   |
+LL |     while let Range { start: _, end: _ } = true..true || false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:142:15
+   |
+LL |     while let Range { start: F, end } = F..|| true {}
+   |               ^^^^^^^^^^^^^^^^^^^^^^^ expected fn pointer, found struct `std::ops::Range`
+   |
+   = note: expected type `fn() -> bool`
+              found type `std::ops::Range<_>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:142:44
+   |
+LL |     while let Range { start: F, end } = F..|| true {}
+   |                                            ^^^^^^^ expected bool, found closure
+   |
+   = note: expected type `bool`
+              found type `[closure@$DIR/disallowed-positions.rs:142:44: 142:51]`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:142:11
+   |
+LL |     while let Range { start: F, end } = F..|| true {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<bool>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:150:15
+   |
+LL |     while let Range { start: true, end } = t..&&false {}
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^   - this match expression has type `bool`
+   |               |
+   |               expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<_>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:150:47
+   |
+LL |     while let Range { start: true, end } = t..&&false {}
+   |                                               ^^^^^^^ expected bool, found &&bool
+   |
+   = note: expected type `bool`
+              found type `&&bool`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:150:11
+   |
+LL |     while let Range { start: true, end } = t..&&false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<bool>`
+
+error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try`
+  --> $DIR/disallowed-positions.rs:106:23
+   |
+LL |         while let 0 = 0? {}
+   |                       ^^ the `?` operator cannot be applied to type `{integer}`
+   |
+   = help: the trait `std::ops::Try` is not implemented for `{integer}`
+   = note: required by `std::ops::Try::into_result`
+
+error[E0614]: type `bool` cannot be dereferenced
+  --> $DIR/disallowed-positions.rs:173:5
+   |
+LL |     *let 0 = 0;
+   |     ^^^^^^^^^^
+
+error[E0600]: cannot apply unary operator `-` to type `bool`
+  --> $DIR/disallowed-positions.rs:175:5
+   |
+LL |     -let 0 = 0;
+   |     ^^^^^^^^^^ cannot apply unary operator `-`
+   |
+   = note: an implementation of `std::ops::Neg` might be missing for `bool`
+
+error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try`
+  --> $DIR/disallowed-positions.rs:183:5
+   |
+LL |     (let 0 = 0)?;
+   |     ^^^^^^^^^^^^ the `?` operator cannot be applied to type `bool`
+   |
+   = help: the trait `std::ops::Try` is not implemented for `bool`
+   = note: required by `std::ops::Try::into_result`
+
+error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `std::ops::Try`)
+  --> $DIR/disallowed-positions.rs:183:5
+   |
+LL |     (let 0 = 0)?;
+   |     ^^^^^^^^^^^^ cannot use the `?` operator in a function that returns `()`
+   |
+   = help: the trait `std::ops::Try` is not implemented for `()`
+   = note: required by `std::ops::Try::from_error`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:198:10
+   |
+LL |     (let Range { start: _, end: _ } = true..true || false);
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^   ---- this match expression has type `bool`
+   |          |
+   |          expected bool, found struct `std::ops::Range`
+   |
+   = note: expected type `bool`
+              found type `std::ops::Range<_>`
+
+error[E0308]: mismatched types
+  --> $DIR/disallowed-positions.rs:207:5
+   |
+LL | fn outside_if_and_while_expr() {
+   |                                - help: try adding a return type: `-> &bool`
+...
+LL |     &let 0 = 0
+   |     ^^^^^^^^^^ expected (), found &bool
+   |
+   = note: expected type `()`
+              found type `&bool`
+
+error[E0277]: the `?` operator can only be applied to values that implement `std::ops::Try`
+  --> $DIR/disallowed-positions.rs:179:17
+   |
+LL |         let 0 = 0?;
+   |                 ^^ the `?` operator cannot be applied to type `{integer}`
+   |
+   = help: the trait `std::ops::Try` is not implemented for `{integer}`
+   = note: required by `std::ops::Try::into_result`
+
+error[E0019]: constant contains unimplemented expression type
+  --> $DIR/disallowed-positions.rs:218:25
+   |
+LL |         true && let 1 = 1
+   |                         ^
+
+error[E0019]: constant contains unimplemented expression type
+  --> $DIR/disallowed-positions.rs:218:21
+   |
+LL |         true && let 1 = 1
+   |                     ^
+
+error[E0019]: constant contains unimplemented expression type
+  --> $DIR/disallowed-positions.rs:224:25
+   |
+LL |         true && let 1 = 1
+   |                         ^
+
+error[E0019]: constant contains unimplemented expression type
+  --> $DIR/disallowed-positions.rs:224:21
+   |
+LL |         true && let 1 = 1
+   |                     ^
+
+error[E0019]: constant contains unimplemented expression type
+  --> $DIR/disallowed-positions.rs:230:25
+   |
+LL |         true && let 1 = 1
+   |                         ^
+
+error[E0019]: constant contains unimplemented expression type
+  --> $DIR/disallowed-positions.rs:230:21
+   |
+LL |         true && let 1 = 1
+   |                     ^
 
-error: aborting due to 57 previous errors
+error: aborting due to 109 previous errors
 
+Some errors have detailed explanations: E0019, E0277, E0308, E0600, E0614.
+For more information about an error, try `rustc --explain E0019`.
diff --git a/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs b/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs
index c125ec89cb574..64987663adb90 100644
--- a/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs
+++ b/src/test/ui/rfc-2497-if-let-chains/feature-gate.rs
@@ -13,26 +13,33 @@ fn _if() {
 
     if (let 0 = 1) {}
     //~^ ERROR `let` expressions in this position are experimental [E0658]
-    //~| ERROR `let` expressions only supported in `if`
+    //~| ERROR `let` expressions are not supported here
 
     if (((let 0 = 1))) {}
     //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
 
     if true && let 0 = 1 {}
     //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
 
     if let 0 = 1 && true {}
     //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
 
     if (let 0 = 1) && true {}
     //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
 
     if true && (let 0 = 1) {}
     //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
 
     if (let 0 = 1) && (let 0 = 1) {}
     //~^ ERROR `let` expressions in this position are experimental [E0658]
     //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
 
     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
     //~^ ERROR `let` expressions in this position are experimental [E0658]
@@ -40,35 +47,49 @@ fn _if() {
     //~| ERROR `let` expressions in this position are experimental [E0658]
     //~| ERROR `let` expressions in this position are experimental [E0658]
     //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
 
     if let Range { start: _, end: _ } = (true..true) && false {}
     //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
 }
 
 fn _while() {
-    if let 0 = 1 {} // Stable!
+    while let 0 = 1 {} // Stable!
 
     while (let 0 = 1) {}
     //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
 
     while (((let 0 = 1))) {}
     //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
 
     while true && let 0 = 1 {}
     //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
 
     while let 0 = 1 && true {}
     //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
 
     while (let 0 = 1) && true {}
     //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
 
     while true && (let 0 = 1) {}
     //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
 
     while (let 0 = 1) && (let 0 = 1) {}
     //~^ ERROR `let` expressions in this position are experimental [E0658]
     //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
 
     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
     //~^ ERROR `let` expressions in this position are experimental [E0658]
@@ -76,9 +97,15 @@ fn _while() {
     //~| ERROR `let` expressions in this position are experimental [E0658]
     //~| ERROR `let` expressions in this position are experimental [E0658]
     //~| ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
+    //~| ERROR `let` expressions are not supported here
 
     while let Range { start: _, end: _ } = (true..true) && false {}
     //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
 }
 
 fn _macros() {
@@ -95,8 +122,10 @@ fn _macros() {
     }
     use_expr!((let 0 = 1 && 0 == 0));
     //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
     use_expr!((let 0 = 1));
     //~^ ERROR `let` expressions in this position are experimental [E0658]
+    //~| ERROR `let` expressions are not supported here
     #[cfg(FALSE)] (let 0 = 1);
     //~^ ERROR `let` expressions in this position are experimental [E0658]
     use_expr!(let 0 = 1);
diff --git a/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr b/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr
index 5ed0dd3cab68e..6167427fa9fdc 100644
--- a/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr
+++ b/src/test/ui/rfc-2497-if-let-chains/feature-gate.stderr
@@ -1,5 +1,5 @@
 error: no rules expected the token `let`
-  --> $DIR/feature-gate.rs:102:15
+  --> $DIR/feature-gate.rs:131:15
    |
 LL |     macro_rules! use_expr {
    |     --------------------- when calling this macro
@@ -26,7 +26,7 @@ LL |     if (((let 0 = 1))) {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:21:16
+  --> $DIR/feature-gate.rs:22:16
    |
 LL |     if true && let 0 = 1 {}
    |                ^^^^^^^^^
@@ -35,7 +35,7 @@ LL |     if true && let 0 = 1 {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:24:8
+  --> $DIR/feature-gate.rs:26:8
    |
 LL |     if let 0 = 1 && true {}
    |        ^^^^^^^^^
@@ -44,7 +44,7 @@ LL |     if let 0 = 1 && true {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:27:9
+  --> $DIR/feature-gate.rs:30:9
    |
 LL |     if (let 0 = 1) && true {}
    |         ^^^^^^^^^
@@ -53,7 +53,7 @@ LL |     if (let 0 = 1) && true {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:30:17
+  --> $DIR/feature-gate.rs:34:17
    |
 LL |     if true && (let 0 = 1) {}
    |                 ^^^^^^^^^
@@ -62,7 +62,7 @@ LL |     if true && (let 0 = 1) {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:33:9
+  --> $DIR/feature-gate.rs:38:9
    |
 LL |     if (let 0 = 1) && (let 0 = 1) {}
    |         ^^^^^^^^^
@@ -71,7 +71,7 @@ LL |     if (let 0 = 1) && (let 0 = 1) {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:33:24
+  --> $DIR/feature-gate.rs:38:24
    |
 LL |     if (let 0 = 1) && (let 0 = 1) {}
    |                        ^^^^^^^^^
@@ -80,7 +80,7 @@ LL |     if (let 0 = 1) && (let 0 = 1) {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:37:8
+  --> $DIR/feature-gate.rs:44:8
    |
 LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
    |        ^^^^^^^^^
@@ -89,7 +89,7 @@ LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:37:21
+  --> $DIR/feature-gate.rs:44:21
    |
 LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
    |                     ^^^^^^^^^
@@ -98,7 +98,7 @@ LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:37:35
+  --> $DIR/feature-gate.rs:44:35
    |
 LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
    |                                   ^^^^^^^^^
@@ -107,7 +107,7 @@ LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:37:48
+  --> $DIR/feature-gate.rs:44:48
    |
 LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
    |                                                ^^^^^^^^^
@@ -116,7 +116,7 @@ LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:37:61
+  --> $DIR/feature-gate.rs:44:61
    |
 LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
    |                                                             ^^^^^^^^^
@@ -125,7 +125,7 @@ LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:44:8
+  --> $DIR/feature-gate.rs:56:8
    |
 LL |     if let Range { start: _, end: _ } = (true..true) && false {}
    |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -134,7 +134,7 @@ LL |     if let Range { start: _, end: _ } = (true..true) && false {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:51:12
+  --> $DIR/feature-gate.rs:64:12
    |
 LL |     while (let 0 = 1) {}
    |            ^^^^^^^^^
@@ -143,7 +143,7 @@ LL |     while (let 0 = 1) {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:54:14
+  --> $DIR/feature-gate.rs:68:14
    |
 LL |     while (((let 0 = 1))) {}
    |              ^^^^^^^^^
@@ -152,7 +152,7 @@ LL |     while (((let 0 = 1))) {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:57:19
+  --> $DIR/feature-gate.rs:72:19
    |
 LL |     while true && let 0 = 1 {}
    |                   ^^^^^^^^^
@@ -161,7 +161,7 @@ LL |     while true && let 0 = 1 {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:60:11
+  --> $DIR/feature-gate.rs:76:11
    |
 LL |     while let 0 = 1 && true {}
    |           ^^^^^^^^^
@@ -170,7 +170,7 @@ LL |     while let 0 = 1 && true {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:63:12
+  --> $DIR/feature-gate.rs:80:12
    |
 LL |     while (let 0 = 1) && true {}
    |            ^^^^^^^^^
@@ -179,7 +179,7 @@ LL |     while (let 0 = 1) && true {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:66:20
+  --> $DIR/feature-gate.rs:84:20
    |
 LL |     while true && (let 0 = 1) {}
    |                    ^^^^^^^^^
@@ -188,7 +188,7 @@ LL |     while true && (let 0 = 1) {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:69:12
+  --> $DIR/feature-gate.rs:88:12
    |
 LL |     while (let 0 = 1) && (let 0 = 1) {}
    |            ^^^^^^^^^
@@ -197,7 +197,7 @@ LL |     while (let 0 = 1) && (let 0 = 1) {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:69:27
+  --> $DIR/feature-gate.rs:88:27
    |
 LL |     while (let 0 = 1) && (let 0 = 1) {}
    |                           ^^^^^^^^^
@@ -206,7 +206,7 @@ LL |     while (let 0 = 1) && (let 0 = 1) {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:73:11
+  --> $DIR/feature-gate.rs:94:11
    |
 LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
    |           ^^^^^^^^^
@@ -215,7 +215,7 @@ LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:73:24
+  --> $DIR/feature-gate.rs:94:24
    |
 LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
    |                        ^^^^^^^^^
@@ -224,7 +224,7 @@ LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:73:38
+  --> $DIR/feature-gate.rs:94:38
    |
 LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
    |                                      ^^^^^^^^^
@@ -233,7 +233,7 @@ LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:73:51
+  --> $DIR/feature-gate.rs:94:51
    |
 LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
    |                                                   ^^^^^^^^^
@@ -242,7 +242,7 @@ LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:73:64
+  --> $DIR/feature-gate.rs:94:64
    |
 LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
    |                                                                ^^^^^^^^^
@@ -251,7 +251,7 @@ LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:80:11
+  --> $DIR/feature-gate.rs:106:11
    |
 LL |     while let Range { start: _, end: _ } = (true..true) && false {}
    |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@@ -260,7 +260,7 @@ LL |     while let Range { start: _, end: _ } = (true..true) && false {}
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:100:20
+  --> $DIR/feature-gate.rs:129:20
    |
 LL |     #[cfg(FALSE)] (let 0 = 1);
    |                    ^^^^^^^^^
@@ -269,7 +269,7 @@ LL |     #[cfg(FALSE)] (let 0 = 1);
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:87:17
+  --> $DIR/feature-gate.rs:114:17
    |
 LL |     noop_expr!((let 0 = 1));
    |                 ^^^^^^^^^
@@ -278,7 +278,7 @@ LL |     noop_expr!((let 0 = 1));
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:96:16
+  --> $DIR/feature-gate.rs:123:16
    |
 LL |     use_expr!((let 0 = 1 && 0 == 0));
    |                ^^^^^^^^^
@@ -287,7 +287,7 @@ LL |     use_expr!((let 0 = 1 && 0 == 0));
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
 error[E0658]: `let` expressions in this position are experimental
-  --> $DIR/feature-gate.rs:98:16
+  --> $DIR/feature-gate.rs:126:16
    |
 LL |     use_expr!((let 0 = 1));
    |                ^^^^^^^^^
@@ -295,12 +295,276 @@ LL |     use_expr!((let 0 = 1));
    = note: for more information, see https://github.com/rust-lang/rust/issues/53667
    = help: add #![feature(let_chains)] to the crate attributes to enable
 
-error: `let` expressions only supported in `if`
+error: `let` expressions are not supported here
   --> $DIR/feature-gate.rs:14:9
    |
 LL |     if (let 0 = 1) {}
    |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:18:11
+   |
+LL |     if (((let 0 = 1))) {}
+   |           ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:22:16
+   |
+LL |     if true && let 0 = 1 {}
+   |                ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:26:8
+   |
+LL |     if let 0 = 1 && true {}
+   |        ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:30:9
+   |
+LL |     if (let 0 = 1) && true {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:34:17
+   |
+LL |     if true && (let 0 = 1) {}
+   |                 ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:38:9
+   |
+LL |     if (let 0 = 1) && (let 0 = 1) {}
+   |         ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:38:24
+   |
+LL |     if (let 0 = 1) && (let 0 = 1) {}
+   |                        ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:44:8
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |        ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:44:21
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                     ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:44:35
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                   ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:44:48
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                                ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:44:61
+   |
+LL |     if let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                                             ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:56:8
+   |
+LL |     if let Range { start: _, end: _ } = (true..true) && false {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:64:12
+   |
+LL |     while (let 0 = 1) {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:68:14
+   |
+LL |     while (((let 0 = 1))) {}
+   |              ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:72:19
+   |
+LL |     while true && let 0 = 1 {}
+   |                   ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:76:11
+   |
+LL |     while let 0 = 1 && true {}
+   |           ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:80:12
+   |
+LL |     while (let 0 = 1) && true {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:84:20
+   |
+LL |     while true && (let 0 = 1) {}
+   |                    ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:88:12
+   |
+LL |     while (let 0 = 1) && (let 0 = 1) {}
+   |            ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:88:27
+   |
+LL |     while (let 0 = 1) && (let 0 = 1) {}
+   |                           ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:94:11
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |           ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:94:24
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                        ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:94:38
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                      ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:94:51
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                                   ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:94:64
+   |
+LL |     while let 0 = 1 && let 1 = 2 && (let 2 = 3 && let 3 = 4 && let 4 = 5) {}
+   |                                                                ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:106:11
+   |
+LL |     while let Range { start: _, end: _ } = (true..true) && false {}
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:123:16
+   |
+LL |     use_expr!((let 0 = 1 && 0 == 0));
+   |                ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
+
+error: `let` expressions are not supported here
+  --> $DIR/feature-gate.rs:126:16
+   |
+LL |     use_expr!((let 0 = 1));
+   |                ^^^^^^^^^
+   |
+   = note: only supported directly in conditions of `if`- and `while`-expressions
+   = note: as well as when nested within `&&` and parenthesis in those conditions
 
-error: aborting due to 34 previous errors
+error: aborting due to 63 previous errors
 
 For more information about this error, try `rustc --explain E0658`.

From 7465eb44f0288da9b48c86881b9e88e73d8f27dd Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Mon, 17 Jun 2019 22:03:23 +0200
Subject: [PATCH 30/35] let_chains: Refactor parse_{if,while}_expr a bit.

---
 src/libsyntax/parse/parser.rs | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 1e420855b676b..65936345fe1ac 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -3157,8 +3157,7 @@ impl<'a> Parser<'a> {
     /// Parses an `if` expression (`if` token already eaten).
     fn parse_if_expr(&mut self, attrs: ThinVec<Attribute>) -> PResult<'a, P<Expr>> {
         let lo = self.prev_span;
-        let cond = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
-        self.ungate_prev_let_expr(&cond);
+        let cond = self.parse_cond_expr()?;
 
         // Verify that the parsed `if` condition makes sense as a condition. If it is a block, then
         // verify that the last statement is either an implicit return (no `;`) or an explicit
@@ -3188,12 +3187,17 @@ impl<'a> Parser<'a> {
         Ok(self.mk_expr(lo.to(hi), ExprKind::If(cond, thn, els), attrs))
     }
 
-    /// Remove the last feature gating of a `let` expression that must the one provided.
-    fn ungate_prev_let_expr(&mut self, expr: &Expr) {
-        if let ExprKind::Let(..) = expr.node {
+    /// Parse the condition of a `if`- or `while`-expression
+    fn parse_cond_expr(&mut self) -> PResult<'a, P<Expr>> {
+        let cond = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
+
+        if let ExprKind::Let(..) = cond.node {
+            // Remove the last feature gating of a `let` expression since it's stable.
             let last = self.sess.let_chains_spans.borrow_mut().pop();
-            debug_assert_eq!(expr.span, last.unwrap());
+            debug_assert_eq!(cond.span, last.unwrap());
         }
+
+        Ok(cond)
     }
 
     /// Parses a `let $pats = $expr` pseudo-expression.
@@ -3295,12 +3299,11 @@ impl<'a> Parser<'a> {
     fn parse_while_expr(&mut self, opt_label: Option<Label>,
                             span_lo: Span,
                             mut attrs: ThinVec<Attribute>) -> PResult<'a, P<Expr>> {
-        let cond = self.parse_expr_res(Restrictions::NO_STRUCT_LITERAL, None)?;
-        self.ungate_prev_let_expr(&cond);
+        let cond = self.parse_cond_expr()?;
         let (iattrs, body) = self.parse_inner_attrs_and_block()?;
         attrs.extend(iattrs);
         let span = span_lo.to(body.span);
-        return Ok(self.mk_expr(span, ExprKind::While(cond, body, opt_label), attrs));
+        Ok(self.mk_expr(span, ExprKind::While(cond, body, opt_label), attrs))
     }
 
     // parse `loop {...}`, `loop` token already eaten

From 851066f57e19b645e2dd33392ee7f822cbb2e374 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Tue, 18 Jun 2019 01:28:20 +0200
Subject: [PATCH 31/35] let_chains: Fix bugs in pretty printing.

---
 src/libsyntax/parse/parser.rs |  4 ++--
 src/libsyntax/print/pprust.rs | 40 ++++++++++++++++++++++++-----------
 src/libsyntax/util/parser.rs  | 16 ++++++++++++++
 3 files changed, 46 insertions(+), 14 deletions(-)

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 65936345fe1ac..b2003e2d6bd66 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -41,7 +41,7 @@ use crate::parse::lexer::UnmatchedBrace;
 use crate::parse::lexer::comments::{doc_comment_style, strip_doc_comment_decoration};
 use crate::parse::token::{Token, TokenKind, DelimToken};
 use crate::parse::{new_sub_parser_from_file, ParseSess, Directory, DirectoryOwnership};
-use crate::util::parser::{AssocOp, Fixity};
+use crate::util::parser::{AssocOp, Fixity, prec_let_scrutinee_needs_par};
 use crate::print::pprust;
 use crate::ptr::P;
 use crate::parse::PResult;
@@ -3208,7 +3208,7 @@ impl<'a> Parser<'a> {
         self.expect(&token::Eq)?;
         let expr = self.with_res(
             Restrictions::NO_STRUCT_LITERAL,
-            |this| this.parse_assoc_expr_with(1 + AssocOp::LAnd.precedence(), None.into())
+            |this| this.parse_assoc_expr_with(1 + prec_let_scrutinee_needs_par(), None.into())
         )?;
         let span = lo.to(expr.span);
         self.sess.let_chains_spans.borrow_mut().push(span);
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index c525f2efb49fb..164fe2f36e1da 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -1715,6 +1715,7 @@ impl<'a> State<'a> {
         self.ann.post(self, AnnNode::Block(blk))
     }
 
+    /// Print a `let pats = scrutinee` expression.
     pub fn print_let(&mut self, pats: &[P<ast::Pat>], scrutinee: &ast::Expr) -> io::Result<()> {
         self.s.word("let ")?;
 
@@ -1722,7 +1723,11 @@ impl<'a> State<'a> {
         self.s.space()?;
 
         self.word_space("=")?;
-        self.print_expr_as_cond(scrutinee)
+        self.print_expr_cond_paren(
+            scrutinee,
+            Self::cond_needs_par(scrutinee)
+            || parser::needs_par_as_let_scrutinee(scrutinee.precedence().order())
+        )
     }
 
     fn print_else(&mut self, els: Option<&ast::Expr>) -> io::Result<()> {
@@ -1794,21 +1799,18 @@ impl<'a> State<'a> {
     }
 
     pub fn print_expr_maybe_paren(&mut self, expr: &ast::Expr, prec: i8) -> io::Result<()> {
-        let needs_par = expr.precedence().order() < prec;
-        if needs_par {
-            self.popen()?;
-        }
-        self.print_expr(expr)?;
-        if needs_par {
-            self.pclose()?;
-        }
-        Ok(())
+        self.print_expr_cond_paren(expr, expr.precedence().order() < prec)
     }
 
     /// Print an expr using syntax that's acceptable in a condition position, such as the `cond` in
     /// `if cond { ... }`.
     pub fn print_expr_as_cond(&mut self, expr: &ast::Expr) -> io::Result<()> {
-        let needs_par = match expr.node {
+        self.print_expr_cond_paren(expr, Self::cond_needs_par(expr))
+    }
+
+    /// Does `expr` need parenthesis when printed in a condition position?
+    fn cond_needs_par(expr: &ast::Expr) -> bool {
+        match expr.node {
             // These cases need parens due to the parse error observed in #26461: `if return {}`
             // parses as the erroneous construct `if (return {})`, not `if (return) {}`.
             ast::ExprKind::Closure(..) |
@@ -1816,8 +1818,11 @@ impl<'a> State<'a> {
             ast::ExprKind::Break(..) => true,
 
             _ => parser::contains_exterior_struct_lit(expr),
-        };
+        }
+    }
 
+    /// Print `expr` or `(expr)` when `needs_par` holds.
+    fn print_expr_cond_paren(&mut self, expr: &ast::Expr, needs_par: bool) -> io::Result<()> {
         if needs_par {
             self.popen()?;
         }
@@ -1949,6 +1954,17 @@ impl<'a> State<'a> {
             // of `(x as i32) < ...`. We need to convince it _not_ to do that.
             (&ast::ExprKind::Cast { .. }, ast::BinOpKind::Lt) |
             (&ast::ExprKind::Cast { .. }, ast::BinOpKind::Shl) => parser::PREC_FORCE_PAREN,
+            // We are given `(let _ = a) OP b`.
+            //
+            // - When `OP <= LAnd` we should print `let _ = a OP b` to avoid redundant parens
+            //   as the parser will interpret this as `(let _ = a) OP b`.
+            //
+            // - Otherwise, e.g. when we have `(let a = b) < c` in AST,
+            //   parens are required since the parser would interpret `let a = b < c` as
+            //   `let a = (b < c)`. To achieve this, we force parens.
+            (&ast::ExprKind::Let { .. }, _) if !parser::needs_par_as_let_scrutinee(prec) => {
+                parser::PREC_FORCE_PAREN
+            }
             _ => left_prec,
         };
 
diff --git a/src/libsyntax/util/parser.rs b/src/libsyntax/util/parser.rs
index 81b5b08593726..1e52186a106c1 100644
--- a/src/libsyntax/util/parser.rs
+++ b/src/libsyntax/util/parser.rs
@@ -318,6 +318,9 @@ impl ExprPrecedence {
             ExprPrecedence::Box |
             ExprPrecedence::AddrOf |
             // Here `let pats = expr` has `let pats =` as a "unary" prefix of `expr`.
+            // However, this is not exactly right. When `let _ = a` is the LHS of a binop we
+            // need parens sometimes. E.g. we can print `(let _ = a) && b` as `let _ = a && b`
+            // but we need to print `(let _ = a) < b` as-is with parens.
             ExprPrecedence::Let |
             ExprPrecedence::Unary => PREC_PREFIX,
 
@@ -352,6 +355,19 @@ impl ExprPrecedence {
     }
 }
 
+/// In `let p = e`, operators with precedence `<=` this one requires parenthesis in `e`.
+crate fn prec_let_scrutinee_needs_par() -> usize {
+    AssocOp::LAnd.precedence()
+}
+
+/// Suppose we have `let _ = e` and the `order` of `e`.
+/// Is the `order` such that `e` in `let _ = e` needs parenthesis when it is on the RHS?
+///
+/// Conversely, suppose that we have `(let _ = a) OP b` and `order` is that of `OP`.
+/// Can we print this as `let _ = a OP b`?
+crate fn needs_par_as_let_scrutinee(order: i8) -> bool {
+    order <= prec_let_scrutinee_needs_par() as i8
+}
 
 /// Expressions that syntactically contain an "exterior" struct literal i.e., not surrounded by any
 /// parens or other delimiters, e.g., `X { y: 1 }`, `X { y: 1 }.method()`, `foo == X { y: 1 }` and

From bc72ce66a18fdd153aa719937a0a5028642110c6 Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Tue, 18 Jun 2019 01:28:51 +0200
Subject: [PATCH 32/35] let_chains: Add test cases to pprust-expr-roundtrip.

---
 .../pprust-expr-roundtrip.rs                  | 55 +++++++++++--------
 1 file changed, 32 insertions(+), 23 deletions(-)

diff --git a/src/test/run-pass-fulldeps/pprust-expr-roundtrip.rs b/src/test/run-pass-fulldeps/pprust-expr-roundtrip.rs
index 7e819e2b34e38..5716e6d45a0b6 100644
--- a/src/test/run-pass-fulldeps/pprust-expr-roundtrip.rs
+++ b/src/test/run-pass-fulldeps/pprust-expr-roundtrip.rs
@@ -68,7 +68,7 @@ fn iter_exprs(depth: usize, f: &mut dyn FnMut(P<Expr>)) {
 
     let mut g = |e| f(expr(e));
 
-    for kind in 0 .. 16 {
+    for kind in 0..=19 {
         match kind {
             0 => iter_exprs(depth - 1, &mut |e| g(ExprKind::Box(e))),
             1 => iter_exprs(depth - 1, &mut |e| g(ExprKind::Call(e, vec![]))),
@@ -79,25 +79,26 @@ fn iter_exprs(depth: usize, f: &mut dyn FnMut(P<Expr>)) {
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::MethodCall(
                             seg.clone(), vec![make_x(), e])));
             },
-            3 => {
-                let op = Spanned { span: DUMMY_SP, node: BinOpKind::Add };
-                iter_exprs(depth - 1, &mut |e| g(ExprKind::Binary(op, e, make_x())));
-                iter_exprs(depth - 1, &mut |e| g(ExprKind::Binary(op, make_x(), e)));
-            },
-            4 => {
-                let op = Spanned { span: DUMMY_SP, node: BinOpKind::Mul };
-                iter_exprs(depth - 1, &mut |e| g(ExprKind::Binary(op, e, make_x())));
-                iter_exprs(depth - 1, &mut |e| g(ExprKind::Binary(op, make_x(), e)));
-            },
-            5 => {
-                let op = Spanned { span: DUMMY_SP, node: BinOpKind::Shl };
+            3..=8 => {
+                let op = Spanned {
+                    span: DUMMY_SP,
+                    node: match kind {
+                        3 => BinOpKind::Add,
+                        4 => BinOpKind::Mul,
+                        5 => BinOpKind::Shl,
+                        6 => BinOpKind::And,
+                        7 => BinOpKind::Or,
+                        8 => BinOpKind::Lt,
+                        _ => unreachable!(),
+                    }
+                };
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Binary(op, e, make_x())));
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Binary(op, make_x(), e)));
             },
-            6 => {
+            9 => {
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Unary(UnOp::Deref, e)));
             },
-            7 => {
+            10 => {
                 let block = P(Block {
                     stmts: Vec::new(),
                     id: DUMMY_NODE_ID,
@@ -106,7 +107,7 @@ fn iter_exprs(depth: usize, f: &mut dyn FnMut(P<Expr>)) {
                 });
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::If(e, block.clone(), None)));
             },
-            8 => {
+            11 => {
                 let decl = P(FnDecl {
                     inputs: vec![],
                     output: FunctionRetTy::Default(DUMMY_SP),
@@ -120,33 +121,41 @@ fn iter_exprs(depth: usize, f: &mut dyn FnMut(P<Expr>)) {
                                           e,
                                           DUMMY_SP)));
             },
-            9 => {
+            12 => {
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Assign(e, make_x())));
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Assign(make_x(), e)));
             },
-            10 => {
+            13 => {
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Field(e, Ident::from_str("f"))));
             },
-            11 => {
+            14 => {
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Range(
                             Some(e), Some(make_x()), RangeLimits::HalfOpen)));
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Range(
                             Some(make_x()), Some(e), RangeLimits::HalfOpen)));
             },
-            12 => {
+            15 => {
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::AddrOf(Mutability::Immutable, e)));
             },
-            13 => {
+            16 => {
                 g(ExprKind::Ret(None));
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Ret(Some(e))));
             },
-            14 => {
+            17 => {
                 let path = Path::from_ident(Ident::from_str("S"));
                 g(ExprKind::Struct(path, vec![], Some(make_x())));
             },
-            15 => {
+            18 => {
                 iter_exprs(depth - 1, &mut |e| g(ExprKind::Try(e)));
             },
+            19 => {
+                let ps = vec![P(Pat {
+                    id: DUMMY_NODE_ID,
+                    node: PatKind::Wild,
+                    span: DUMMY_SP,
+                })];
+                iter_exprs(depth - 1, &mut |e| g(ExprKind::Let(ps.clone(), e)))
+            },
             _ => panic!("bad counter value in iter_exprs"),
         }
     }

From 90b9e96cc343b81d8f3e4105b222c573e95ee71c Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 23 Jun 2019 01:17:11 +0200
Subject: [PATCH 33/35] let_chains: More accurately describe
 `ast::ExprKind::Let`.

Co-Authored-By: Vadim Petrochenkov <vadim.petrochenkov@gmail.com>
---
 src/libsyntax/ast.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index a837ae167a308..21704206cbf86 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -1115,7 +1115,7 @@ pub enum ExprKind {
     Cast(P<Expr>, P<Ty>),
     /// A type ascription (e.g., `42: usize`).
     Type(P<Expr>, P<Ty>),
-    /// A `let pats = expr` pseudo-expression that only occurs in the condition
+    /// A `let pats = expr` expression that is only semantically allowed in the condition
     /// of `if` / `while` expressions. (e.g., `if let 0 = x { .. }`).
     ///
     /// The `Vec<P<Pat>>` is for or-patterns at the top level.

From 7abb2350df361effc0f2f4bad938c42547fbb8ed Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 23 Jun 2019 01:39:13 +0200
Subject: [PATCH 34/35] let_chains: Revert 'fn with' in ast_validation.

---
 src/librustc_passes/ast_validation.rs | 29 +++++++++------------------
 1 file changed, 10 insertions(+), 19 deletions(-)

diff --git a/src/librustc_passes/ast_validation.rs b/src/librustc_passes/ast_validation.rs
index c6bc4f2f49e3e..2da9c5adf9baf 100644
--- a/src/librustc_passes/ast_validation.rs
+++ b/src/librustc_passes/ast_validation.rs
@@ -76,29 +76,18 @@ struct AstValidator<'a> {
     warning_period_57979_impl_trait_in_proj: bool,
 }
 
-/// With the `new` value in `store`,
-/// runs and returns the `scoped` computation,
-/// resetting the old value of `store` after,
-/// and returning the result of `scoped`.
-fn with<C, T, S>(
-    this: &mut C,
-    new: S,
-    store: impl Fn(&mut C) -> &mut S,
-    scoped: impl FnOnce(&mut C) -> T
-) -> T {
-    let old = mem::replace(store(this), new);
-    let ret = scoped(this);
-    *store(this) = old;
-    ret
-}
-
 impl<'a> AstValidator<'a> {
     fn with_impl_trait_in_proj_warning<T>(&mut self, v: bool, f: impl FnOnce(&mut Self) -> T) -> T {
-        with(self, v, |this| &mut this.warning_period_57979_impl_trait_in_proj, f)
+        let old = mem::replace(&mut self.warning_period_57979_impl_trait_in_proj, v);
+        let ret = f(self);
+        self.warning_period_57979_impl_trait_in_proj = old;
+        ret
     }
 
     fn with_banned_impl_trait(&mut self, f: impl FnOnce(&mut Self)) {
-        with(self, true, |this| &mut this.is_impl_trait_banned, f)
+        let old = mem::replace(&mut self.is_impl_trait_banned, true);
+        f(self);
+        self.is_impl_trait_banned = old;
     }
 
     fn with_banned_assoc_ty_bound(&mut self, f: impl FnOnce(&mut Self)) {
@@ -108,7 +97,9 @@ impl<'a> AstValidator<'a> {
     }
 
     fn with_impl_trait(&mut self, outer: Option<OuterImplTrait>, f: impl FnOnce(&mut Self)) {
-        with(self, outer, |this| &mut this.outer_impl_trait, f)
+        let old = mem::replace(&mut self.outer_impl_trait, outer);
+        f(self);
+        self.outer_impl_trait = old;
     }
 
     fn visit_assoc_ty_constraint_from_generic_args(&mut self, constraint: &'a AssocTyConstraint) {

From c75f7ecaee508c568c0bc01c102965ce8b2246ef Mon Sep 17 00:00:00 2001
From: Mazdak Farrokhzad <twingoow@gmail.com>
Date: Sun, 23 Jun 2019 01:41:09 +0200
Subject: [PATCH 35/35] let_chains: note re. back-compat wrt. expr beginning.

---
 src/libsyntax/ext/tt/macro_parser.rs | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/libsyntax/ext/tt/macro_parser.rs b/src/libsyntax/ext/tt/macro_parser.rs
index 0db6bd59afc2c..92ce3779a3c81 100644
--- a/src/libsyntax/ext/tt/macro_parser.rs
+++ b/src/libsyntax/ext/tt/macro_parser.rs
@@ -825,7 +825,9 @@ fn may_begin_with(token: &Token, name: Name) -> bool {
     }
 
     match name {
-        sym::expr => token.can_begin_expr() && !token.is_keyword(kw::Let),
+        sym::expr => token.can_begin_expr()
+            // This exception is here for backwards compatibility.
+            && !token.is_keyword(kw::Let),
         sym::ty => token.can_begin_type(),
         sym::ident => get_macro_name(token).is_some(),
         sym::literal => token.can_begin_literal_or_bool(),