Add test.

Author: jseyfried

src/test/compile-fail/macro-shadowing.rs

@@ -0,0 +1,42 @@
+// 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.
+
+// aux-build:two_macros.rs
+
+macro_rules! foo { () => {} }
+macro_rules! macro_one { () => {} }
+
+macro_rules! m1 { () => {
+    macro_rules! foo { () => {} } //~ ERROR `foo` is already in scope
+    //~^ NOTE macro-expanded `macro_rules!`s and `#[macro_use]`s may not shadow existing macros
+
+    #[macro_use] //~ ERROR `macro_one` is already in scope
+    //~^ NOTE macro-expanded `macro_rules!`s and `#[macro_use]`s may not shadow existing macros
+    extern crate two_macros;
+}}
+m1!(); //~ NOTE in this expansion
+       //~| NOTE in this expansion
+       //~| NOTE in this expansion
+       //~| NOTE in this expansion
+
+fn f() { macro_one!(); }
+foo!();
+
+macro_rules! m2 { () => {
+    macro_rules! foo { () => {} }
+    #[macro_use] extern crate two_macros as __;
+
+    fn g() { macro_one!(); }
+    foo!();
+}}
+m2!();
+//^ Since `foo` and `macro_one` are not used outside this expansion, they are not shadowing errors.
+
+fn main() {}