Add test for unused field in union

src/test/compile-fail/union/union-lint-dead-code.rs

@@ -0,0 +1,26 @@
+// Copyright 2016 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(untagged_unions)]
+#![deny(dead_code)]
+
+union Foo {
+    x: usize,
+    b: bool, //~ ERROR: field is never used
+    _unused: u16,
+}
+
+fn field_read(f: Foo) -> usize {
+    unsafe { f.x }
+}
+
+fn main() {
+    let _ = field_read(Foo { x: 2 });
+}