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/run-pass/rfc-2005-default-binding-mode/tuple-struct.rs

@@ -0,0 +1,29 @@
+// 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)]
+
+enum Foo {
+    Bar(Option<i8>, (), (), Vec<i32>),
+    Baz,
+}
+
+pub fn main() {
+    let foo = Foo::Bar(Some(1), (), (), vec![2, 3]);
+
+    match &foo {
+        Foo::Baz => panic!(),
+        Foo::Bar(None, ..) => panic!(),
+        Foo::Bar(Some(n), .., v) => {
+            assert_eq!((*v).len(), 2);
+            assert_eq!(*n, 1);
+        }
+    }
+}