only insert the principal trait in drop_glue

Author: japaric

src/librustc_mir/monomorphize/collector.rs

@@ -1034,14 +1034,12 @@ fn create_mono_items_for_vtable_methods<'a, 'tcx>(
         let drop_glue_instance = visit_drop_use(tcx, impl_ty, false, output);
 
         if let Some(principal) = trait_sty.principal() {
-            let drop_glue = drop_glue.entry(drop_glue_instance).or_default();
-
-            // the principal trait may not have any method but we must consider it in drop glue call
-            // metadata
-            drop_glue.insert(tcx.normalize_erasing_late_bound_regions(
-                ty::ParamEnv::reveal_all(),
-                &principal,
-            ));
+            // we only consider the principal trait for the drop glue call metadata
+            drop_glue.entry(drop_glue_instance).or_default().insert(
+                tcx.normalize_erasing_late_bound_regions(
+                    ty::ParamEnv::reveal_all(),
+                    &principal,
+                ));
 
             let poly_trait_ref = principal.with_self_ty(tcx, impl_ty);
             assert!(!poly_trait_ref.has_escaping_bound_vars());
@@ -1064,8 +1062,7 @@ fn create_mono_items_for_vtable_methods<'a, 'tcx>(
                 .filter(|instance| should_monomorphize_locally(tcx, &instance));
 
             for instance in methods {
-                if let Some((trait_ref, _)) = instance.trait_ref_and_method(tcx) {
-                    drop_glue.insert(trait_ref);
+                if instance.trait_ref_and_method(tcx).is_some() {
                     dynamic_dispatch.insert(instance);
                 }
 
@@ -1354,10 +1351,9 @@ fn collect_miri<'a, 'tcx>(
                 }
             }).next();
 
-            if let Some(drop_glue_instance) = drop_glue_instance {
-                // FIXME(japaric) if the principal trait happens to have no methods the `for` loop
-                // below won't insert it into `drop_glue`; we must include it separately here
-                let drop_glue = drop_glue.entry(drop_glue_instance).or_default();
+            if let Some(_drop_glue_instance) = drop_glue_instance {
+                // FIXME(japaric) we need to insert only the principal trait into `drop_glue`
+                // drop_glue.entry(drop_glue_instance).or_default().insert(???);
 
                 // trait object in const / static context
                 for ((), inner) in alloc.relocations.values() {
@@ -1367,9 +1363,8 @@ fn collect_miri<'a, 'tcx>(
                         if should_monomorphize_locally(tcx, &instance) {
                             trace!("collecting {:?} with {:#?}", alloc_id, instance);
 
-                            if let Some((trait_ref, _)) = instance.trait_ref_and_method(tcx) {
+                            if instance.trait_ref_and_method(tcx).is_some() {
                                 dynamic_dispatch.insert(instance);
-                                drop_glue.insert(trait_ref);
                             }
 
                             output.push(create_fn_mono_item(instance));