Switch DefineOpaqueTypes::No to Yes where the self type can't be an opaque anyway

Author: oli-obk

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

@@ -894,10 +894,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                             return;
                         }
                     }
-                    ty::Infer(ty::TyVar(_)) => {
-                        debug!("assemble_candidates_from_object_ty: ambiguous");
-                        candidates.ambiguous = true; // could wind up being an object type
-                        return;
+                    ty::Infer(ty::TyVar(_)) => bug!("assemble_candidates_from_object_ty is already gated behind an infer var check"),
+                    ty::Alias(ty::Opaque, alias) if self.infcx.can_define_opaque_ty(alias.def_id) => {
+                        bug!("assemble_candidates_from_object_ty is already gated behind an opaque type ");
                     }
                     _ => return,
                 };

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

@@ -1719,7 +1719,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         });
         self.infcx
             .at(&obligation.cause, obligation.param_env)
-            .eq(DefineOpaqueTypes::No, placeholder_trait_ref, trait_bound)
+            .eq(DefineOpaqueTypes::Yes, placeholder_trait_ref, trait_bound)
             .map(|InferOk { obligations: _, value: () }| {
                 // This method is called within a probe, so we can't have
                 // inference variables and placeholders escape.