Correct error message for invalid ref/mut bindings

Closes #15914.

Author: ftxqxd

src/libsyntax/parse/parser.rs

@@ -3402,9 +3402,10 @@ impl<'a> Parser<'a> {
                        binding_mode: ast::BindingMode)
                        -> ast::Pat_ {
         if !is_plain_ident(&self.token) {
-            let last_span = self.last_span;
-            self.span_fatal(last_span,
-                            "expected identifier, found path");
+            let span = self.span;
+            let tok_str = self.this_token_to_string();
+            self.span_fatal(span,
+                            format!("expected identifier, found `{}`", tok_str).as_slice());
         }
         let ident = self.parse_ident();
         let last_span = self.last_span;

src/test/compile-fail/issue-15914.rs

@@ -0,0 +1,14 @@
+// Copyright 2014 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 ref
+        (); //~ ERROR expected identifier, found `(`
+}