Show notice about "never used" for enum

Author: long-long-float

compiler/rustc_passes/src/dead.rs

@@ -976,6 +976,16 @@ impl<'tcx> DeadVisitor<'tcx> {
         };
 
         let encl_def_id = parent_item.unwrap_or(first_item.def_id);
+        // If parent of encl_def_id is an enum, use the parent ID intead.
+        let encl_def_id = if let Some(enum_did) = self.tcx.opt_parent(encl_def_id.to_def_id())
+            && let Some(enum_local_id) = enum_did.as_local()
+            && let Node::Item(item) = self.tcx.hir_node_by_def_id(enum_local_id)
+            && let ItemKind::Enum(_, _) = item.kind
+        {
+            enum_local_id
+        } else {
+            encl_def_id
+        };
         let ignored_derived_impls =
             if let Some(ign_traits) = self.ignored_derived_traits.get(&encl_def_id) {
                 let trait_list = ign_traits