Auto merge of #29441 - Ryman:match_refactor_msg, r=alexcrichton

This helps for the case where a match, such as below:
```rust
let foo = match foo {
    Some(x) => x,
    None => 0
};
```
gets refactored to no longer need the match, but the match keyword has been left accidentally: 

```rust
let foo = match foo.unwrap_or(0);
```

This can be hard to spot as the expression grows more complex.

r? @alexcrichton 

1 files changed, 7 insertions, 1 deletions

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 6cee7b86a61..d1fc8ac2002 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -2941,9 +2941,15 @@ impl<'a> Parser<'a> {
     }
 
     fn parse_match_expr(&mut self) -> PResult<P<Expr>> {
+        let match_span = self.last_span;
         let lo = self.last_span.lo;
         let discriminant = try!(self.parse_expr_res(Restrictions::RESTRICTION_NO_STRUCT_LITERAL));
-        try!(self.commit_expr_expecting(&*discriminant, token::OpenDelim(token::Brace)));
+        if let Err(e) = self.commit_expr_expecting(&*discriminant, token::OpenDelim(token::Brace)) {
+            if self.token == token::Token::Semi {
+                self.span_note(match_span, "did you mean to remove this `match` keyword?");
+            }
+            return Err(e)
+        }
         let mut arms: Vec<Arm> = Vec::new();
         while self.token != token::CloseDelim(token::Brace) {
             arms.push(try!(self.parse_arm_nopanic()));