Add a no-op feature gate

Author: tiif

compiler/rustc_feature/src/builtin_attrs.rs

@@ -655,6 +655,11 @@ pub static BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         DuplicatesOk, EncodeCrossCrate::Yes,
         "allow_internal_unstable side-steps feature gating and stability checks",
     ),
+    gated!(
+        impl_stability, Normal, template!(Word, List: "feat1, feat2, ..."),
+        DuplicatesOk, EncodeCrossCrate::Yes,
+        "allow unstable impl",
+    ),
     gated!(
         allow_internal_unsafe, Normal, template!(Word), WarnFollowing,
         EncodeCrossCrate::No, "allow_internal_unsafe side-steps the unsafe_code lint",

compiler/rustc_feature/src/unstable.rs

@@ -213,7 +213,13 @@ declare_features! (
     (internal, custom_mir, "1.65.0", None),
     /// Outputs useful `assert!` messages
     (unstable, generic_assert, "1.63.0", None),
+<<<<<<< HEAD
     /// Allows using the #[rustc_intrinsic] attribute.
+=======
+    /// Allow declaring an impl as unstable.
+    (internal, impl_stability, "CURRENT_RUSTC_VERSION", None),
+    /// Allows using the `rust-intrinsic`'s "ABI".
+>>>>>>> bde3d4141a1 (Add a no-op feature gate)
     (internal, intrinsics, "1.0.0", None),
     /// Allows using `#[lang = ".."]` attribute for linking items to special compiler logic.
     (internal, lang_items, "1.0.0", None),

compiler/rustc_span/src/symbol.rs

@@ -1151,6 +1151,7 @@ symbols! {
         ignore,
         impl_header_lifetime_elision,
         impl_lint_pass,
+        impl_stability,
         impl_trait_in_assoc_type,
         impl_trait_in_bindings,
         impl_trait_in_fn_trait_return,