Add regression test

Author: jseyfried

src/test/compile-fail/cfg_attr_path.rs

@@ -0,0 +1,12 @@
+// 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.
+
+#[cfg_attr(all(), path = "nonexistent_file.rs")] mod foo;
+//~^ ERROR nonexistent_file.rs