test: Add a test for distinguishing attributes from extensions

Issue #487

src/test/run-pass/item-attributes.rs

@@ -161,6 +161,18 @@ mod test_attr_inner_then_outer_multi {
     }
 }
 
+mod test_distinguish_syntax_ext {
+
+    use std;
+
+    fn f() {
+        #fmt("test%s", "s");
+        #[attr = "val"]
+        fn g() {
+        }
+    }
+}
+
 fn main() {
 }