mark autodiff_into as builtin and permanently unstable

Author: ZuseZ4

compiler/rustc_feature/src/builtin_attrs.rs

@@ -361,13 +361,6 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         large_assignments, experimental!(move_size_limit)
     ),
 
-    // Autodiff
-    ungated!(
-        autodiff_into, Normal,
-        template!(Word, List: r#""...""#),
-        DuplicatesOk,
-    ),
-
     // Entry point:
     gated!(unix_sigpipe, Normal, template!(Word, NameValueStr: "inherit|sig_ign|sig_dfl"), ErrorFollowing, experimental!(unix_sigpipe)),
     ungated!(start, Normal, template!(Word), WarnFollowing),
@@ -595,6 +588,9 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         IMPL_DETAIL,
     ),
     rustc_attr!(rustc_proc_macro_decls, Normal, template!(Word), WarnFollowing, INTERNAL_UNSTABLE),
+    // Autodiff
+    rustc_attr!(autodiff_into, Normal, template!(Word, List: r#""...""#), DuplicatesOk, INTERNAL_UNSTABLE),
+
     rustc_attr!(
         rustc_macro_transparency, Normal,
         template!(NameValueStr: "transparent|semitransparent|opaque"), ErrorFollowing,