give a more informative failure in this case

diff --git a/src/librustc/traits/query/evaluate_obligation.rs b/src/librustc/traits/query/evaluate_obligation.rs
index 6bd92678362544ac62522f02dcfbddd4ef7d8ffb..1a906b5da6f322220a6271d0f4fdf041b4926b51 100644 (file)
--- a/src/librustc/traits/query/evaluate_obligation.rs
+++ b/src/librustc/traits/query/evaluate_obligation.rs
@@ -51,7 +51,14 @@ fn evaluate_obligation(
                 let mut selcx =
                     SelectionContext::with_query_mode(&self, TraitQueryMode::Standard);
                 selcx.evaluate_obligation_recursively(obligation)
-                     .expect("Overflow should be caught earlier in standard query mode")
+                    .unwrap_or_else(|r| {
+                        span_bug!(
+                            obligation.cause.span,
+                            "Overflow should be caught earlier in standard query mode: {:?}, {:?}",
+                            obligation,
+                            r,
+                        )
+                    })
             }
         }
     }