Rollup merge of #124460 - long-long-float:show-notice-about-enum-with-debug, r=pnkfelix

Show notice about  "never used" of Debug for enum

Close #123068

If an ADT implements `Debug` trait and it is not used, the compiler says a note that indicates intentionally ignored during dead code analysis as [this note](https://github.com/rust-lang/rust/blob/2207179a591f5f252885a94ab014dafeb6e8e9e8/tests/ui/lint/dead-code/unused-variant.stderr#L9).
However this node is not shown for variants that have fields in enum. This PR fixes to show the note.

1 files changed, 19 insertions, 0 deletions

diff --git a/compiler/rustc_passes/src/dead.rs b/compiler/rustc_passes/src/dead.rs
index 69386c0fbdb..7c7700dd859 100644
--- a/compiler/rustc_passes/src/dead.rs
+++ b/compiler/rustc_passes/src/dead.rs
@@ -1010,6 +1010,22 @@ impl<'tcx> DeadVisitor<'tcx> {
         parent_item: Option<LocalDefId>,
         report_on: ReportOn,
     ) {
+        fn get_parent_if_enum_variant<'tcx>(
+            tcx: TyCtxt<'tcx>,
+            may_variant: LocalDefId,
+        ) -> LocalDefId {
+            if let Node::Variant(_) = tcx.hir_node_by_def_id(may_variant)
+                && let Some(enum_did) = tcx.opt_parent(may_variant.to_def_id())
+                && let Some(enum_local_id) = enum_did.as_local()
+                && let Node::Item(item) = tcx.hir_node_by_def_id(enum_local_id)
+                && let ItemKind::Enum(_, _) = item.kind
+            {
+                enum_local_id
+            } else {
+                may_variant
+            }
+        }
+
         let Some(&first_item) = dead_codes.first() else {
             return;
         };
@@ -1053,6 +1069,9 @@ 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 = get_parent_if_enum_variant(tcx, 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