Add test.

src/test/run-pass/hygiene/issue-47311.rs

@@ -0,0 +1,26 @@
+// Copyright 2018 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.
+
+// ignore-pretty pretty-printing is unhygienic
+
+#![feature(decl_macro)]
+#![allow(unused)]
+
+macro m($S:ident, $x:ident) {
+    $S { $x: 0 }
+}
+
+mod foo {
+    struct S { x: i32 }
+
+    fn f() { ::m!(S, x); }
+}
+
+fn main() {}