auto merge of #6686 : cmr/rust/fix-6596, r=catamorphism

The error message is extremely unideal.

src/test/compile-fail/issue-6596.rs

@@ -0,0 +1,9 @@
+macro_rules! e( //~ ERROR unknown macro variable `nonexistent`
+    ($inp:ident) => (
+        $nonexistent
+    );
+)
+
+fn main() {
+    e!(foo);
+}