Improve selection errors for ~const trait bounds

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -659,7 +659,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             _ => bug!("closure candidate for non-closure {:?}", obligation),
         };
 
-        let obligation_predicate = obligation.predicate.to_poly_trait_ref();
+        let obligation_predicate = obligation.predicate;
         let Normalized { value: obligation_predicate, mut obligations } =
             ensure_sufficient_stack(|| {
                 normalize_with_depth(
@@ -689,7 +689,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         obligations.extend(self.confirm_poly_trait_refs(
             obligation.cause.clone(),
             obligation.param_env,
-            obligation_predicate,
+            obligation_predicate.to_poly_trait_ref(),
             trait_ref,
         )?);
 

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -2413,7 +2413,7 @@ impl<'tcx> TraitObligationExt<'tcx> for TraitObligation<'tcx> {
         // chain. Ideally, we should have a way to configure this either
         // by using -Z verbose or just a CLI argument.
         let derived_cause = DerivedObligationCause {
-            parent_trait_ref: obligation.predicate.to_poly_trait_ref(),
+            parent_trait_pred: obligation.predicate,
             parent_code: obligation.cause.clone_code(),
         };
         let derived_code = variant(derived_cause);