use Display impl query config

Author: nikomatsakis

src/librustc/ty/maps/config.rs

@@ -75,7 +75,7 @@ impl<'tcx> QueryDescription<'tcx> for queries::normalize_ty_after_erasing_region
 
 impl<'tcx> QueryDescription<'tcx> for queries::evaluate_obligation<'tcx> {
     fn describe(_tcx: TyCtxt, goal: CanonicalPredicateGoal<'tcx>) -> String {
-        format!("evaluating trait selection obligation `{:?}`", goal)
+        format!("evaluating trait selection obligation `{}`", goal.value.value)
     }
 }
 

src/test/ui/impl-trait/auto-trait-leak.stderr

@@ -39,6 +39,7 @@ note: the cycle begins when processing `cycle1`...
    |
 LL | fn cycle1() -> impl Clone {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which then requires evaluating trait selection obligation `impl std::clone::Clone: std::marker::Send`...
 note: ...which then requires processing `cycle2::{{impl-Trait}}`...
   --> $DIR/auto-trait-leak.rs:52:16
    |
@@ -49,6 +50,7 @@ note: ...which then requires processing `cycle2`...
    |
 LL | fn cycle2() -> impl Clone {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^
+note: ...which then requires evaluating trait selection obligation `impl std::clone::Clone: std::marker::Send`...
 note: ...which then requires processing `cycle1::{{impl-Trait}}`...
   --> $DIR/auto-trait-leak.rs:44:16
    |