Feature gate macro attributes in #[derive] output

compiler/rustc_feature/src/active.rs

@@ -634,6 +634,10 @@ declare_features! (
 
     /// Lessens the requirements for structs to implement `Unsize`.
     (active, relaxed_struct_unsize, "1.51.0", Some(1), None),
+
+    /// Allows macro attributes to observe output of `#[derive]`.
+    (active, macro_attributes_in_derive_output, "1.51.0", Some(81119), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

compiler/rustc_resolve/src/macros.rs

@@ -280,6 +280,36 @@ impl<'a> ResolverExpand for Resolver<'a> {
         if let Res::Def(_, _) = res {
             let normal_module_def_id = self.macro_def_scope(invoc_id).nearest_parent_mod;
             self.definitions.add_parent_module_of_macro_def(invoc_id, normal_module_def_id);
+
+            // Gate macro attributes in `#[derive]` output.
+            if !self.session.features_untracked().macro_attributes_in_derive_output
+                && kind == MacroKind::Attr
+                && ext.builtin_name != Some(sym::derive)
+            {
+                let mut expn_id = parent_scope.expansion;
+                loop {
+                    // Helper attr table is a quick way to determine whether the attr is `derive`.
+                    if self.helper_attrs.contains_key(&expn_id) {
+                        feature_err(
+                            &self.session.parse_sess,
+                            sym::macro_attributes_in_derive_output,
+                            path.span,
+                            "macro attributes in `#[derive]` output are unstable",
+                        )
+                        .emit();
+                        break;
+                    } else {
+                        let expn_data = expn_id.expn_data();
+                        match expn_data.kind {
+                            ExpnKind::Root
+                            | ExpnKind::Macro(MacroKind::Bang | MacroKind::Derive, _) => {
+                                break;
+                            }
+                            _ => expn_id = expn_data.parent,
+                        }
+                    }
+                }
+            }
         }
 
         Ok(ext)

compiler/rustc_span/src/symbol.rs

@@ -679,6 +679,7 @@ symbols! {
         loop_break_value,
         lt,
         macro_at_most_once_rep,
+        macro_attributes_in_derive_output,
         macro_escape,
         macro_export,
         macro_lifetime_matcher,