Setup feature gate and attribute

Author: tiif

compiler/rustc_attr_data_structures/src/attributes.rs

@@ -162,6 +162,7 @@ pub enum AttributeKind {
     // tidy-alphabetical-start
     AllowConstFnUnstable(ThinVec<Symbol>),
     AllowInternalUnstable(ThinVec<(Symbol, Span)>),
+    AllowUnstableFeature(ThinVec<(Symbol, Span)>),
     BodyStability {
         stability: DefaultBodyStability,
         /// Span of the `#[rustc_default_body_unstable(...)]` attribute

compiler/rustc_attr_parsing/src/attributes/allow_unstable.rs

@@ -22,6 +22,20 @@ impl CombineAttributeParser for AllowInternalUnstableParser {
     }
 }
 
+pub(crate) struct AllowUnstableFeatureParser;
+impl CombineAttributeParser for AllowUnstableFeatureParser {
+    const PATH: &'static [rustc_span::Symbol] = &[sym::unstable_feature_bound];
+    type Item = (Symbol, Span);
+    const CONVERT: ConvertFn<Self::Item> = AttributeKind::AllowUnstableFeature;
+
+    fn extend<'a>(
+        cx: &'a AcceptContext<'a>,
+        args: &'a ArgParser<'a>,
+    ) -> impl IntoIterator<Item = Self::Item> + 'a {
+        parse_unstable(cx, args, Self::PATH[0]).into_iter().zip(iter::repeat(cx.attr_span))
+    }
+}
+
 pub(crate) struct AllowConstFnUnstableParser;
 impl CombineAttributeParser for AllowConstFnUnstableParser {
     const PATH: &'static [Symbol] = &[sym::rustc_allow_const_fn_unstable];

compiler/rustc_attr_parsing/src/context.rs

@@ -11,7 +11,9 @@ use rustc_hir::{AttrArgs, AttrItem, AttrPath, Attribute, HashIgnoredAttrId};
 use rustc_session::Session;
 use rustc_span::{DUMMY_SP, ErrorGuaranteed, Span, Symbol, sym};
 
-use crate::attributes::allow_unstable::{AllowConstFnUnstableParser, AllowInternalUnstableParser};
+use crate::attributes::allow_unstable::{
+    AllowConstFnUnstableParser, AllowInternalUnstableParser, AllowUnstableFeatureParser,
+};
 use crate::attributes::confusables::ConfusablesParser;
 use crate::attributes::deprecation::DeprecationParser;
 use crate::attributes::repr::ReprParser;
@@ -75,6 +77,7 @@ attribute_groups!(
         // tidy-alphabetical-start
         Combine<AllowConstFnUnstableParser>,
         Combine<AllowInternalUnstableParser>,
+        Combine<AllowUnstableFeatureParser>,
         Combine<ReprParser>,
         // tidy-alphabetical-end
 

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!(
+        unstable_feature_bound, Normal, template!(Word, List: "feat1, feat2, ..."),
+        DuplicatesOk, EncodeCrossCrate::No, impl_stability,
+        "used internally to mark impl as unstable",
+    ),
     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

@@ -213,6 +213,8 @@ declare_features! (
     (internal, custom_mir, "1.65.0", None),
     /// Outputs useful `assert!` messages
     (unstable, generic_assert, "1.63.0", None),
+    /// Allow declaring an impl as unstable.
+    (internal, impl_stability, "CURRENT_RUSTC_VERSION", None),
     /// Allows using the #[rustc_intrinsic] attribute.
     (internal, intrinsics, "1.0.0", None),
     /// Allows using `#[lang = ".."]` attribute for linking items to special compiler logic.

tests/ui/feature-gates/feature-gate-impl-stability.rs

@@ -0,0 +1,11 @@
+/// Test for impl_stability gate.
+
+trait Foo{}
+struct Bar;
+
+#[unstable_feature_bound(feat_foo)]
+//~^ ERROR: used internally to mark impl as unstable
+impl Foo for Bar{}
+
+fn main() {
+}

tests/ui/feature-gates/feature-gate-impl-stability.stderr

@@ -0,0 +1,12 @@
+error[E0658]: used internally to mark impl as unstable
+  --> $DIR/feature-gate-impl-stability.rs:6:1
+   |
+LL | #[unstable_feature_bound(feat_foo)]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add `#![feature(impl_stability)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.