Add an experimental unsafe(force_target_feature) attribute.

This uses the feature gate for
https://github.com/rust-lang/rust/issues/143352, but is described in
https://github.com/rust-lang/rfcs/pull/3820 which is strongly tied to
the experiment.

2 files changed, 81 insertions, 42 deletions

diff --git a/compiler/rustc_attr_parsing/src/attributes/codegen_attrs.rs b/compiler/rustc_attr_parsing/src/attributes/codegen_attrs.rs
index 6ea073896c2..706d51e182d 100644
--- a/compiler/rustc_attr_parsing/src/attributes/codegen_attrs.rs
+++ b/compiler/rustc_attr_parsing/src/attributes/codegen_attrs.rs
@@ -385,57 +385,68 @@ impl<S: Stage> AttributeParser<S> for UsedParser {
     }
 }
 
+fn parse_tf_attribute<'c, S: Stage>(
+    cx: &'c mut AcceptContext<'_, '_, S>,
+    args: &'c ArgParser<'_>,
+) -> impl IntoIterator<Item = (Symbol, Span)> + 'c {
+    let mut features = Vec::new();
+    let ArgParser::List(list) = args else {
+        cx.expected_list(cx.attr_span);
+        return features;
+    };
+    if list.is_empty() {
+        cx.warn_empty_attribute(cx.attr_span);
+        return features;
+    }
+    for item in list.mixed() {
+        let Some(name_value) = item.meta_item() else {
+            cx.expected_name_value(item.span(), Some(sym::enable));
+            return features;
+        };
+
+        // Validate name
+        let Some(name) = name_value.path().word_sym() else {
+            cx.expected_name_value(name_value.path().span(), Some(sym::enable));
+            return features;
+        };
+        if name != sym::enable {
+            cx.expected_name_value(name_value.path().span(), Some(sym::enable));
+            return features;
+        }
+
+        // Use value
+        let Some(name_value) = name_value.args().name_value() else {
+            cx.expected_name_value(item.span(), Some(sym::enable));
+            return features;
+        };
+        let Some(value_str) = name_value.value_as_str() else {
+            cx.expected_string_literal(name_value.value_span, Some(name_value.value_as_lit()));
+            return features;
+        };
+        for feature in value_str.as_str().split(",") {
+            features.push((Symbol::intern(feature), item.span()));
+        }
+    }
+    features
+}
+
 pub(crate) struct TargetFeatureParser;
 
 impl<S: Stage> CombineAttributeParser<S> for TargetFeatureParser {
     type Item = (Symbol, Span);
     const PATH: &[Symbol] = &[sym::target_feature];
-    const CONVERT: ConvertFn<Self::Item> = |items, span| AttributeKind::TargetFeature(items, span);
+    const CONVERT: ConvertFn<Self::Item> = |items, span| AttributeKind::TargetFeature {
+        features: items,
+        attr_span: span,
+        was_forced: false,
+    };
     const TEMPLATE: AttributeTemplate = template!(List: &["enable = \"feat1, feat2\""]);
 
     fn extend<'c>(
         cx: &'c mut AcceptContext<'_, '_, S>,
         args: &'c ArgParser<'_>,
     ) -> impl IntoIterator<Item = Self::Item> + 'c {
-        let mut features = Vec::new();
-        let ArgParser::List(list) = args else {
-            cx.expected_list(cx.attr_span);
-            return features;
-        };
-        if list.is_empty() {
-            cx.warn_empty_attribute(cx.attr_span);
-            return features;
-        }
-        for item in list.mixed() {
-            let Some(name_value) = item.meta_item() else {
-                cx.expected_name_value(item.span(), Some(sym::enable));
-                return features;
-            };
-
-            // Validate name
-            let Some(name) = name_value.path().word_sym() else {
-                cx.expected_name_value(name_value.path().span(), Some(sym::enable));
-                return features;
-            };
-            if name != sym::enable {
-                cx.expected_name_value(name_value.path().span(), Some(sym::enable));
-                return features;
-            }
-
-            // Use value
-            let Some(name_value) = name_value.args().name_value() else {
-                cx.expected_name_value(item.span(), Some(sym::enable));
-                return features;
-            };
-            let Some(value_str) = name_value.value_as_str() else {
-                cx.expected_string_literal(name_value.value_span, Some(name_value.value_as_lit()));
-                return features;
-            };
-            for feature in value_str.as_str().split(",") {
-                features.push((Symbol::intern(feature), item.span()));
-            }
-        }
-        features
+        parse_tf_attribute(cx, args)
     }
 
     const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(&[
@@ -449,3 +460,30 @@ impl<S: Stage> CombineAttributeParser<S> for TargetFeatureParser {
         Warn(Target::MacroDef),
     ]);
 }
+
+pub(crate) struct ForceTargetFeatureParser;
+
+impl<S: Stage> CombineAttributeParser<S> for ForceTargetFeatureParser {
+    type Item = (Symbol, Span);
+    const PATH: &[Symbol] = &[sym::force_target_feature];
+    const CONVERT: ConvertFn<Self::Item> = |items, span| AttributeKind::TargetFeature {
+        features: items,
+        attr_span: span,
+        was_forced: true,
+    };
+    const TEMPLATE: AttributeTemplate = template!(List: &["enable = \"feat1, feat2\""]);
+
+    fn extend<'c>(
+        cx: &'c mut AcceptContext<'_, '_, S>,
+        args: &'c ArgParser<'_>,
+    ) -> impl IntoIterator<Item = Self::Item> + 'c {
+        parse_tf_attribute(cx, args)
+    }
+
+    const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(&[
+        Allow(Target::Fn),
+        Allow(Target::Method(MethodKind::Inherent)),
+        Allow(Target::Method(MethodKind::Trait { body: true })),
+        Allow(Target::Method(MethodKind::TraitImpl)),
+    ]);
+}
diff --git a/compiler/rustc_attr_parsing/src/context.rs b/compiler/rustc_attr_parsing/src/context.rs
index c0d3bc99ba9..0791d6eb778 100644
--- a/compiler/rustc_attr_parsing/src/context.rs
+++ b/compiler/rustc_attr_parsing/src/context.rs
@@ -21,8 +21,8 @@ use crate::attributes::allow_unstable::{
 };
 use crate::attributes::body::CoroutineParser;
 use crate::attributes::codegen_attrs::{
-    ColdParser, CoverageParser, ExportNameParser, NakedParser, NoMangleParser, OptimizeParser,
-    TargetFeatureParser, TrackCallerParser, UsedParser,
+    ColdParser, CoverageParser, ExportNameParser, ForceTargetFeatureParser, NakedParser,
+    NoMangleParser, OptimizeParser, TargetFeatureParser, TrackCallerParser, UsedParser,
 };
 use crate::attributes::confusables::ConfusablesParser;
 use crate::attributes::deprecation::DeprecationParser;
@@ -161,6 +161,7 @@ attribute_parsers!(
         // tidy-alphabetical-start
         Combine<AllowConstFnUnstableParser>,
         Combine<AllowInternalUnstableParser>,
+        Combine<ForceTargetFeatureParser>,
         Combine<ReprParser>,
         Combine<TargetFeatureParser>,
         Combine<UnstableFeatureBoundParser>,