Extend dead code lint to detect more unused enum variants

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

@@ -0,0 +1,22 @@
+// Copyright 2015 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.
+
+#![deny(dead_code)]
+
+#[derive(Clone)]
+enum Enum {
+    Variant1, //~ ERROR: variant is never used
+    Variant2,
+}
+
+fn main() {
+    let e = Enum::Variant2;
+    e.clone();
+}