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

Author: bors

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()));

src/test/parse-fail/match-refactor-to-expr.rs

@@ -0,0 +1,18 @@
+// Copyright 2015 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 main() {
+    let foo =
+        match //~ NOTE did you mean to remove this `match` keyword?
+        Some(4).unwrap_or_else(5)
+        ; //~ ERROR expected one of `.`, `{`, or an operator, found `;`
+
+    println!("{}", foo)
+}