Merge attrs, better validation

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1586,6 +1586,10 @@ fn assemble_candidates_from_object_ty<'cx, 'tcx>(
 
     let tcx = selcx.tcx();
 
+    if !tcx.trait_def(obligation.predicate.trait_def_id(tcx)).implement_via_object {
+        return;
+    }
+
     let self_ty = obligation.predicate.self_ty();
     let object_ty = selcx.infcx.shallow_resolve(self_ty);
     let data = match object_ty.kind() {

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

@@ -554,6 +554,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             "assemble_candidates_from_object_ty",
         );
 
+        if !self.tcx().trait_def(obligation.predicate.def_id()).implement_via_object {
+            return;
+        }
+
         self.infcx.probe(|_snapshot| {
             if obligation.has_non_region_late_bound() {
                 return;