Support parentheses in patterns under feature gate

Improve recovery for trailing comma after `..`

src/test/ui/feature-gate-pattern_parentheses.rs

@@ -0,0 +1,15 @@
+// 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.
+
+fn main() {
+    match 0 {
+        (pat) => {} //~ ERROR parentheses in patterns are unstable
+    }
+}

src/test/ui/feature-gate-pattern_parentheses.stderr

@@ -0,0 +1,11 @@
+error[E0658]: parentheses in patterns are unstable
+  --> $DIR/feature-gate-pattern_parentheses.rs:13:9
+   |
+LL |         (pat) => {} //~ ERROR parentheses in patterns are unstable
+   |         ^^^^^
+   |
+   = help: add #![feature(pattern_parentheses)] to the crate attributes to enable
+
+error: aborting due to previous error
+
+If you want more information on this error, try using "rustc --explain E0658"