diff --git a/src/libsyntax/ext/expand.rs b/src/libsyntax/ext/expand.rs
index 71e0d49493b8..b5d7d94c3131 100644
--- a/src/libsyntax/ext/expand.rs
+++ b/src/libsyntax/ext/expand.rs
@@ -531,7 +531,8 @@ pub fn core_macros() -> @str {
                 let expected_val = $expected;
                 // check both directions of equality....
                 if !((given_val == expected_val) && (expected_val == given_val)) {
-                    fail!(\"left: %? does not equal right: %?\", given_val, expected_val);
+                    fail!(\"assertion failed: `(left == right) && (right == \
+                    left)` (left: `%?`, right: `%?`)\", given_val, expected_val);
                 }
             }
         )