Add help in case ref and mut are switched

Author: GuillaumeGomez

src/libsyntax/parse/parser.rs

@@ -640,8 +640,18 @@ impl<'a> Parser<'a> {
         match self.token {
             token::Ident(i) => {
                 if self.token.is_reserved_ident() {
-                    self.span_err(self.span, &format!("expected identifier, found {}",
-                                                      self.this_token_descr()));
+                    let desc = self.this_token_descr();
+                    if self.token.is_keyword(keywords::Ref) {
+                        let mut err = self.span_fatal(self.span,
+                                                      &format!("expected identifier, found {}",
+                                                               desc));
+                        err.span_suggestion(self.span,
+                                            "if you want to make a mutable reference, write:",
+                                            "ref mut".to_owned());
+                        err.emit();
+                    } else {
+                        self.span_err(self.span, &format!("expected identifier, found {}", desc));
+                    }
                 }
                 self.bump();
                 Ok(i)

src/test/parse-fail/expected-identifier.rs

@@ -0,0 +1,17 @@
+// Copyright 2017 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.
+
+// compile-flags: -Z parse-only
+
+fn main() {
+    let mut ref y = &x; //~ ERROR expected identifier, found keyword `ref`
+                        //~| HELP if you want to make a mutable reference, write:
+                        //~^^ ERROR expected one of `:`, `;`, `=`, or `@`, found `y`
+}