in which ! is suggested for erroneous identifier not

Impressing confused Python users with magical diagnostics is perhaps
worth this not-grossly-unreasonable (only 40ish lines) extra complexity
in the parser?

Thanks to Vadim Petrochenkov for guidance.

This resolves #46836.

src/libsyntax/parse/parser.rs

@@ -2830,7 +2830,48 @@ impl<'a> Parser<'a> {
                 let (span, e) = self.interpolated_or_expr_span(e)?;
                 (lo.to(span), ExprKind::Box(e))
             }
-            _ => return self.parse_dot_or_call_expr(Some(attrs))
+            token::Ident(..) if self.token.is_ident_named("not") => {
+                // `not` is just an ordinary identifier in Rust-the-language,
+                // but as `rustc`-the-compiler, we can issue clever diagnostics
+                // for confused users who really want to say `!`
+                let token_cannot_continue_expr = |t: &token::Token| match *t {
+                    // These tokens can start an expression after `!`, but
+                    // can't continue an expression after an ident
+                    token::Ident(ident, is_raw) => token::ident_can_begin_expr(ident, is_raw),
+                    token::Literal(..) | token::Pound => true,
+                    token::Interpolated(ref nt) => match nt.0 {
+                        token::NtIdent(..) | token::NtExpr(..) |
+                        token::NtBlock(..) | token::NtPath(..) => true,
+                        _ => false,
+                    },
+                    _ => false
+                };
+                let cannot_continue_expr = self.look_ahead(1, token_cannot_continue_expr);
+                if cannot_continue_expr {
+                    self.bump();
+                    // Emit the error ...
+                    let mut err = self.diagnostic()
+                        .struct_span_err(self.span,
+                                         &format!("unexpected {} after identifier",
+                                                  self.this_token_descr()));
+                    // span the `not` plus trailing whitespace to avoid
+                    // trailing whitespace after the `!` in our suggestion
+                    let to_replace = self.sess.codemap()
+                        .span_until_non_whitespace(lo.to(self.span));
+                    err.span_suggestion_short(to_replace,
+                                              "use `!` to perform logical negation",
+                                              "!".to_owned());
+                    err.emit();
+                    // —and recover! (just as if we were in the block
+                    // for the `token::Not` arm)
+                    let e = self.parse_prefix_expr(None);
+                    let (span, e) = self.interpolated_or_expr_span(e)?;
+                    (lo.to(span), self.mk_unary(UnOp::Not, e))
+                } else {
+                    return self.parse_dot_or_call_expr(Some(attrs));
+                }
+            }
+            _ => { return self.parse_dot_or_call_expr(Some(attrs)); }
         };
         return Ok(self.mk_expr(lo.to(hi), ex, attrs));
     }

src/libsyntax/parse/token.rs

@@ -91,7 +91,7 @@ impl Lit {
     }
 }
 
-fn ident_can_begin_expr(ident: ast::Ident, is_raw: bool) -> bool {
+pub(crate) fn ident_can_begin_expr(ident: ast::Ident, is_raw: bool) -> bool {
     let ident_token: Token = Ident(ident, is_raw);
 
     !ident_token.is_reserved_ident() ||
@@ -348,6 +348,15 @@ impl Token {
         self.lifetime().is_some()
     }
 
+    /// Returns `true` if the token is a identifier whose name is the given
+    /// string slice.
+    pub fn is_ident_named(&self, name: &str) -> bool {
+        match self.ident() {
+            Some((ident, _)) => ident.name.as_str() == name,
+            None => false
+        }
+    }
+
     /// Returns `true` if the token is a documentation comment.
     pub fn is_doc_comment(&self) -> bool {
         match *self {

src/test/ui/did_you_mean/issue-46836-identifier-not-instead-of-negation.rs

@@ -0,0 +1,44 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn gratitude() {
+    let for_you = false;
+    if not for_you {
+        //~^ ERROR unexpected `for_you` after identifier
+        println!("I couldn't");
+    }
+}
+
+fn qualification() {
+    let the_worst = true;
+    while not the_worst {
+        //~^ ERROR unexpected `the_worst` after identifier
+        println!("still pretty bad");
+    }
+}
+
+fn should_we() {
+    let not = true;
+    if not  // lack of braces is [sic]
+        println!("Then when?");
+    //~^ ERROR expected `{`, found `;
+    //~| ERROR unexpected `println` after identifier
+}
+
+fn sleepy() {
+    let resource = not 2;
+    //~^ ERROR unexpected `2` after identifier
+}
+
+fn main() {
+    let be_smothered_out_before = true;
+    let young_souls = not be_smothered_out_before;
+    //~^ ERROR unexpected `be_smothered_out_before` after identifier
+}

src/test/ui/did_you_mean/issue-46836-identifier-not-instead-of-negation.stderr

@@ -0,0 +1,50 @@
+error: unexpected `for_you` after identifier
+  --> $DIR/issue-46836-identifier-not-instead-of-negation.rs:13:12
+   |
+LL |     if not for_you {
+   |        ----^^^^^^^
+   |        |
+   |        help: use `!` to perform logical negation
+
+error: unexpected `the_worst` after identifier
+  --> $DIR/issue-46836-identifier-not-instead-of-negation.rs:21:15
+   |
+LL |     while not the_worst {
+   |           ----^^^^^^^^^
+   |           |
+   |           help: use `!` to perform logical negation
+
+error: unexpected `println` after identifier
+  --> $DIR/issue-46836-identifier-not-instead-of-negation.rs:30:9
+   |
+LL |     if not  // lack of braces is [sic]
+   |        ----- help: use `!` to perform logical negation
+LL |         println!("Then when?");
+   |         ^^^^^^^
+
+error: expected `{`, found `;`
+  --> $DIR/issue-46836-identifier-not-instead-of-negation.rs:30:31
+   |
+LL |     if not  // lack of braces is [sic]
+   |     -- this `if` statement has a condition, but no block
+LL |         println!("Then when?");
+   |                               ^
+
+error: unexpected `2` after identifier
+  --> $DIR/issue-46836-identifier-not-instead-of-negation.rs:36:24
+   |
+LL |     let resource = not 2;
+   |                    ----^
+   |                    |
+   |                    help: use `!` to perform logical negation
+
+error: unexpected `be_smothered_out_before` after identifier
+  --> $DIR/issue-46836-identifier-not-instead-of-negation.rs:42:27
+   |
+LL |     let young_souls = not be_smothered_out_before;
+   |                       ----^^^^^^^^^^^^^^^^^^^^^^^
+   |                       |
+   |                       help: use `!` to perform logical negation
+
+error: aborting due to 6 previous errors
+