Add regression test.

src/test/run-pass/macro-of-higher-order.rs

@@ -16,7 +16,16 @@ macro_rules! higher_order {
     });
 }
 
+macro_rules! outer {
+    ($x:expr; $fragment:ident) => {
+        macro_rules! inner { ($y:$fragment) => { $x + $y } }
+    }
+}
+
 fn main() {
     let val = higher_order!(subst ($x:expr, $y:expr, $foo:expr) => (($x + $y, $foo)));
     assert_eq!(val, (3, "foo"));
+
+    outer!(2; expr);
+    assert_eq!(inner!(3), 5);
 }