implement pattern-binding-modes RFC

See the [RFC] and [tracking issue].

[tracking issue]: https://github.com/rust-lang/rust/issues/42640
[RFC]: 491e0af/text/2005-match-ergonomics.md

src/test/ui/rfc-2005-default-binding-mode/for.rs

@@ -0,0 +1,20 @@
+// 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.
+
+#![feature(match_default_bindings)]
+
+struct Foo {}
+
+pub fn main() {
+    let mut tups = vec![(Foo{}, Foo{})];
+    // The below desugars to &(ref n, mut m).
+    for (n, mut m) in &tups {
+    }
+}