Recover on mut $p = $e; and var/auto $p = $e suggesting let instead.

src/libsyntax/parse/parser/stmt.rs

@@ -10,8 +10,9 @@ use crate::ast::{self, DUMMY_NODE_ID, Stmt, StmtKind, Local, Block, BlockCheckMo
 use crate::ast::{Attribute, AttrStyle, VisibilityKind, MacStmtStyle, Mac, MacDelimiter};
 use crate::parse::{classify, DirectoryOwnership};
 use crate::parse::token;
-use crate::source_map::{respan, Span};
+use crate::source_map::respan;
 use crate::symbol::{kw, sym};
+use syntax_pos::Span;
 
 use std::mem;
 use errors::Applicability;
@@ -41,11 +42,29 @@ impl<'a> Parser<'a> {
         let lo = self.token.span;
 
         Ok(Some(if self.eat_keyword(kw::Let) {
-            Stmt {
-                id: DUMMY_NODE_ID,
-                kind: StmtKind::Local(self.parse_local(attrs.into())?),
-                span: lo.to(self.prev_span),
-            }
+            let local = self.parse_local(attrs.into())?;
+            self.mk_dummy_stmt(lo.to(self.prev_span), local)
+
+        } else if self.is_ident_mut() {
+            return self.recover_stmt_local(
+                lo,
+                attrs,
+                "missing `let`",
+            );
+        } else if self.is_var_sym() {
+            self.bump();
+            return self.recover_stmt_local(
+                lo,
+                attrs,
+                "to introduce a variable, write `let` instead of `var`",
+            );
+        } else if self.is_auto_for_let() {
+            self.bump();
+            return self.recover_stmt_local(
+                lo,
+                attrs,
+                "to introduce a variable, write `let` instead of `auto`",
+            );
         } else if let Some(macro_def) = self.eat_macro_def(
             &attrs,
             &respan(lo, VisibilityKind::Inherited),
@@ -473,4 +492,61 @@ impl<'a> Parser<'a> {
             "this was erroneously allowed and will become a hard error in a future release"
         }).emit();
     }
+
+    fn recover_stmt_local(
+        &mut self,
+        span: Span,
+        attrs: Vec<Attribute>,
+        msg: &str,
+    ) -> PResult<'a, Option<Stmt>> {
+        let local = self.parse_local(attrs.into())?;
+        self.struct_span_err(span, "invalid variable declaration")
+            .span_suggestion_short(
+                span,
+                msg,
+                "let".to_string(),
+                Applicability::MachineApplicable
+            ).emit();
+        return Ok(Some(self.mk_dummy_stmt(span, local)));
+    }
+
+    fn mk_dummy_stmt(&mut self, span: Span, kind: P<Local>) -> Stmt {
+        Stmt {
+            id: DUMMY_NODE_ID,
+            kind: StmtKind::Local(kind),
+            span,
+        }
+    }
+
+    /// Returns `true` if the next token is Brace
+    fn is_next_brace(&self) -> bool {
+        self.look_ahead(1, |t|  t.eq(&token::OpenDelim(token::Brace)))
+    }
+
+    /// Returns `true` if the token is kw::Mut and next one is an Ident
+    fn is_ident_mut(&self) -> bool {
+        self.token.is_keyword(kw::Mut) &&
+            self.look_ahead(1, |t| t.is_ident())
+    }
+
+    /// Returns `true` if the token is sym::var.
+    fn is_var_sym(&self) -> bool {
+        self.token.is_non_raw_ident_where(|ident| ident.name == sym::var) &&
+            self.is_malformed_declr()
+    }
+
+    fn has_equals(&self) -> bool {
+        self.look_ahead(2, |t|  t.eq(&token::Eq))
+    }
+
+    fn is_auto_for_let(&self) -> bool {
+        self.token.is_keyword(kw::Auto) && self.is_malformed_declr()
+    }
+
+    fn is_malformed_declr(&self) -> bool {
+        (self.has_equals() && !self.is_next_brace()) ||
+            (self.look_ahead(1, |t| t.is_ident()) &&
+                self.look_ahead(2, |t|  t.eq(&token::Semi))
+            )
+    }
 }

src/libsyntax/parse/token.rs

@@ -538,7 +538,7 @@ impl Token {
     }
 
     /// Returns `true` if the token is a non-raw identifier for which `pred` holds.
-    fn is_non_raw_ident_where(&self, pred: impl FnOnce(ast::Ident) -> bool) -> bool {
+    pub fn is_non_raw_ident_where(&self, pred: impl FnOnce(ast::Ident) -> bool) -> bool {
         match self.ident() {
             Some((id, false)) => pred(id),
             _ => false,

src/libsyntax_pos/symbol.rs

@@ -725,6 +725,7 @@ symbols! {
         usize,
         v1,
         val,
+        var,
         vec,
         Vec,
         vis,

src/test/ui/parser/issue-65257--auto-instead-of-let-recovery.rs

@@ -0,0 +1,8 @@
+#[allow(dead_code)]
+fn main() {
+    auto n = 0;//~ ERROR invalid variable declaration
+    //~^ HELP to introduce a variable, write `let` instead of `auto`
+    auto m;//~ ERROR invalid variable declaration
+    //~^ HELP to introduce a variable, write `let` instead of `auto`
+    m = 0;
+}

src/test/ui/parser/issue-65257--auto-instead-of-let-recovery.stderr

@@ -0,0 +1,14 @@
+error: invalid variable declaration
+  --> $DIR/issue-65257--auto-instead-of-let-recovery.rs:4:5
+   |
+LL |     auto n = 0;
+   |     ^^^^ help: to introduce a variable, write `let` instead of `auto`
+
+error: invalid variable declaration
+  --> $DIR/issue-65257--auto-instead-of-let-recovery.rs:6:5
+   |
+LL |     auto m;
+   |     ^^^^ help: to introduce a variable, write `let` instead of `auto`
+
+error: aborting due to 2 previous errors
+

src/test/ui/parser/issue-65257--var-instead-of-let-recovery.rs

@@ -0,0 +1,8 @@
+#[allow(dead_code)]
+fn main() {
+    var n = 0;//~ ERROR invalid variable declaration
+    //~^ HELP to introduce a variable, write `let` instead of `var`
+    var m;//~ ERROR invalid variable declaration
+    //~^ HELP to introduce a variable, write `let` instead of `var`
+    m = 0;
+}

src/test/ui/parser/issue-65257--var-instead-of-let-recovery.stderr

@@ -0,0 +1,14 @@
+error: invalid variable declaration
+  --> $DIR/issue-65257--var-instead-of-let-recovery.rs:4:5
+   |
+LL |     var n = 0;
+   |     ^^^ help: to introduce a variable, write `let` instead of `var`
+
+error: invalid variable declaration
+  --> $DIR/issue-65257--var-instead-of-let-recovery.rs:6:5
+   |
+LL |     var m;
+   |     ^^^ help: to introduce a variable, write `let` instead of `var`
+
+error: aborting due to 2 previous errors
+

src/test/ui/parser/issue-65257-mut-instead-of-let-recovery.rs

@@ -0,0 +1,8 @@
+#[allow(dead_code)]
+fn main() {
+    mut n = 0;//~ ERROR invalid variable declaration
+    //~^ HELP missing `let`
+    mut var;//~ ERROR invalid variable declaration
+    //~^ HELP missing `let`
+    var = 0;
+}

src/test/ui/parser/issue-65257-mut-instead-of-let-recovery.stderr

@@ -0,0 +1,14 @@
+error: invalid variable declaration
+  --> $DIR/issue-65257-mut-instead-of-let-recovery.rs:4:5
+   |
+LL |     mut n = 0;
+   |     ^^^ help: missing `let`
+
+error: invalid variable declaration
+  --> $DIR/issue-65257-mut-instead-of-let-recovery.rs:6:5
+   |
+LL |     mut var;
+   |     ^^^ help: missing `let`
+
+error: aborting due to 2 previous errors
+