Fix rustc_ast unit test

Author: matthewjasper

src/librustc_ast/ast/tests.rs

@@ -3,6 +3,9 @@ use super::*;
 // Are ASTs encodable?
 #[test]
 fn check_asts_encodable() {
-    fn assert_encodable<T: rustc_serialize::Encodable>() {}
+    fn assert_encodable<
+        T: for<'a> rustc_serialize::Encodable<rustc_serialize::json::Encoder<'a>>,
+    >() {
+    }
     assert_encodable::<Crate>();
 }