Warn on unused #[macro_use] imports.

src/test/compile-fail/imports/unused-macro-use.rs

@@ -0,0 +1,21 @@
+// 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.
+
+#![deny(unused)]
+
+#[macro_use] //~ ERROR unused `#[macro_use]` import
+extern crate core;
+
+#[macro_use(
+    panic //~ ERROR unused `#[macro_use]` import
+)]
+extern crate core as core_2;
+
+fn main() {}

src/test/compile-fail/lint-unused-extern-crate.rs

@@ -26,8 +26,6 @@ extern crate rand; // no error, the use marks it as used
 
 extern crate lint_unused_extern_crate as other; // no error, the use * marks it as used
 
-#[macro_use] extern crate core; // no error, the `#[macro_use]` marks it as used
-
 #[allow(unused_imports)]
 use rand::isaac::IsaacRng;