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.

compiler/rustc_ast_lowering/src/item.rs

@@ -1596,7 +1596,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let safety = self.lower_safety(h.safety, default_safety);
 
         // Treat safe `#[target_feature]` functions as unsafe, but also remember that we did so.
-        let safety = if find_attr!(attrs, AttributeKind::TargetFeature { .. })
+        let safety = if find_attr!(attrs, AttributeKind::TargetFeature { was_forced: false, .. })
             && safety.is_safe()
             && !self.tcx.sess.target.is_like_wasm
         {

compiler/rustc_attr_parsing/src/attributes/codegen_attrs.rs

@@ -385,18 +385,10 @@ impl<S: Stage> AttributeParser<S> for UsedParser {
     }
 }
 
-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 TEMPLATE: AttributeTemplate = template!(List: &["enable = \"feat1, feat2\""]);
-
-    fn extend<'c>(
+fn parse_tf_attribute<'c, S: Stage>(
     cx: &'c mut AcceptContext<'_, '_, S>,
     args: &'c ArgParser<'_>,
-    ) -> impl IntoIterator<Item = Self::Item> + 'c {
+) -> impl IntoIterator<Item = (Symbol, Span)> + 'c {
     let mut features = Vec::new();
     let ArgParser::List(list) = args else {
         cx.expected_list(cx.attr_span);
@@ -436,6 +428,25 @@ impl<S: Stage> CombineAttributeParser<S> for TargetFeatureParser {
         }
     }
     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 {
+        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 {
+        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)),
+    ]);
+}

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>,

compiler/rustc_codegen_ssa/src/codegen_attrs.rs

@@ -193,7 +193,7 @@ fn process_builtin_attrs(
                     }
                 }
                 AttributeKind::Optimize(optimize, _) => codegen_fn_attrs.optimize = *optimize,
-                AttributeKind::TargetFeature(features, attr_span) => {
+                AttributeKind::TargetFeature { features, attr_span, was_forced } => {
                     let Some(sig) = tcx.hir_node_by_def_id(did).fn_sig() else {
                         tcx.dcx().span_delayed_bug(*attr_span, "target_feature applied to non-fn");
                         continue;
@@ -201,7 +201,7 @@ fn process_builtin_attrs(
                     let safe_target_features =
                         matches!(sig.header.safety, hir::HeaderSafety::SafeTargetFeatures);
                     codegen_fn_attrs.safe_target_features = safe_target_features;
-                    if safe_target_features {
+                    if safe_target_features && !was_forced {
                         if tcx.sess.target.is_like_wasm || tcx.sess.opts.actually_rustdoc {
                             // The `#[target_feature]` attribute is allowed on
                             // WebAssembly targets on all functions. Prior to stabilizing
@@ -232,6 +232,7 @@ fn process_builtin_attrs(
                         tcx,
                         did,
                         features,
+                        *was_forced,
                         rust_target_features,
                         &mut codegen_fn_attrs.target_features,
                     );
@@ -462,7 +463,7 @@ fn check_result(
             .collect(),
     ) {
         let span =
-            find_attr!(tcx.get_all_attrs(did), AttributeKind::TargetFeature(_, span) => *span)
+            find_attr!(tcx.get_all_attrs(did), AttributeKind::TargetFeature{attr_span: span, ..} => *span)
                 .unwrap_or_else(|| tcx.def_span(did));
 
         tcx.dcx()

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -3,7 +3,7 @@ use rustc_data_structures::unord::{UnordMap, UnordSet};
 use rustc_hir::attrs::InstructionSetAttr;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LOCAL_CRATE, LocalDefId};
-use rustc_middle::middle::codegen_fn_attrs::TargetFeature;
+use rustc_middle::middle::codegen_fn_attrs::{TargetFeature, TargetFeatureKind};
 use rustc_middle::query::Providers;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::Session;
@@ -22,6 +22,7 @@ pub(crate) fn from_target_feature_attr(
     tcx: TyCtxt<'_>,
     did: LocalDefId,
     features: &[(Symbol, Span)],
+    was_forced: bool,
     rust_target_features: &UnordMap<String, target_features::Stability>,
     target_features: &mut Vec<TargetFeature>,
 ) {
@@ -88,7 +89,14 @@ pub(crate) fn from_target_feature_attr(
                         }
                     }
                 }
-                target_features.push(TargetFeature { name, implied: name != feature })
+                let kind = if name != feature {
+                    TargetFeatureKind::Implied
+                } else if was_forced {
+                    TargetFeatureKind::Forced
+                } else {
+                    TargetFeatureKind::Enabled
+                };
+                target_features.push(TargetFeature { name, kind })
             }
         }
     }

compiler/rustc_feature/src/builtin_attrs.rs

@@ -744,6 +744,10 @@ pub static BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         template!(List: &["set"], "https://doc.rust-lang.org/reference/attributes/codegen.html#the-instruction_set-attribute"),
         ErrorPreceding, EncodeCrossCrate::No
     ),
+    gated!(
+        unsafe force_target_feature, Normal, template!(List: &[r#"enable = "name""#]),
+        DuplicatesOk, EncodeCrossCrate::No, effective_target_features, experimental!(force_target_feature)
+    ),
     gated!(
         sanitize, Normal, template!(List: &[r#"address = "on|off""#, r#"kernel_address = "on|off""#, r#"cfi = "on|off""#, r#"hwaddress = "on|off""#, r#"kcfi = "on|off""#, r#"memory = "on|off""#, r#"memtag = "on|off""#, r#"shadow_call_stack = "on|off""#, r#"thread = "on|off""#]), ErrorPreceding,
         EncodeCrossCrate::No, sanitize, experimental!(sanitize),

compiler/rustc_feature/src/unstable.rs

@@ -480,6 +480,8 @@ declare_features! (
     (unstable, doc_cfg_hide, "1.57.0", Some(43781)),
     /// Allows `#[doc(masked)]`.
     (unstable, doc_masked, "1.21.0", Some(44027)),
+    /// Allows features to allow target_feature to better interact with traits.
+    (incomplete, effective_target_features, "CURRENT_RUSTC_VERSION", Some(143352)),
     /// Allows the .use postfix syntax `x.use` and use closures `use |x| { ... }`
     (incomplete, ergonomic_clones, "1.87.0", Some(132290)),
     /// Allows exhaustive pattern matching on types that contain uninhabited types.

compiler/rustc_hir/src/attrs/data_structures.rs

@@ -524,8 +524,9 @@ pub enum AttributeKind {
     /// Represents `#[rustc_std_internal_symbol]`.
     StdInternalSymbol(Span),
 
-    /// Represents `#[target_feature(enable = "...")]`
-    TargetFeature(ThinVec<(Symbol, Span)>, Span),
+    /// Represents `#[target_feature(enable = "...")]` and
+    /// `#[unsafe(force_target_feature(enable = "...")]`.
+    TargetFeature { features: ThinVec<(Symbol, Span)>, attr_span: Span, was_forced: bool },
 
     /// Represents `#[track_caller]`
     TrackCaller(Span),

compiler/rustc_hir/src/attrs/encode_cross_crate.rs

@@ -78,7 +78,7 @@ impl AttributeKind {
             SpecializationTrait(..) => No,
             Stability { .. } => Yes,
             StdInternalSymbol(..) => No,
-            TargetFeature(..) => No,
+            TargetFeature { .. } => No,
             TrackCaller(..) => Yes,
             TypeConst(..) => Yes,
             UnsafeSpecializationMarker(..) => No,

compiler/rustc_middle/src/middle/codegen_fn_attrs.rs

@@ -72,13 +72,23 @@ pub struct CodegenFnAttrs {
     pub patchable_function_entry: Option<PatchableFunctionEntry>,
 }
 
+#[derive(Copy, Clone, Debug, TyEncodable, TyDecodable, HashStable, PartialEq, Eq)]
+pub enum TargetFeatureKind {
+    /// The feature is implied by another feature, rather than explicitly added by the
+    /// `#[target_feature]` attribute
+    Implied,
+    /// The feature is added by the regular `target_feature` attribute.
+    Enabled,
+    /// The feature is added by the unsafe `force_target_feature` attribute.
+    Forced,
+}
+
 #[derive(Copy, Clone, Debug, TyEncodable, TyDecodable, HashStable)]
 pub struct TargetFeature {
     /// The name of the target feature (e.g. "avx")
     pub name: Symbol,
-    /// The feature is implied by another feature, rather than explicitly added by the
-    /// `#[target_feature]` attribute
-    pub implied: bool,
+    /// The way this feature was enabled.
+    pub kind: TargetFeatureKind,
 }
 
 #[derive(Copy, Clone, Debug, TyEncodable, TyDecodable, HashStable)]

compiler/rustc_mir_build/src/check_unsafety.rs

@@ -8,7 +8,7 @@ use rustc_errors::DiagArgValue;
 use rustc_hir::attrs::AttributeKind;
 use rustc_hir::def::DefKind;
 use rustc_hir::{self as hir, BindingMode, ByRef, HirId, Mutability, find_attr};
-use rustc_middle::middle::codegen_fn_attrs::TargetFeature;
+use rustc_middle::middle::codegen_fn_attrs::{TargetFeature, TargetFeatureKind};
 use rustc_middle::mir::BorrowKind;
 use rustc_middle::span_bug;
 use rustc_middle::thir::visit::Visitor;
@@ -522,7 +522,7 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
                             .iter()
                             .copied()
                             .filter(|feature| {
-                                !feature.implied
+                                feature.kind == TargetFeatureKind::Enabled
                                     && !self
                                         .body_target_features
                                         .iter()

compiler/rustc_passes/src/check_attr.rs

@@ -167,7 +167,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                 Attribute::Parsed(AttributeKind::Deprecation { .. }) => {
                     self.check_deprecated(hir_id, attr, span, target)
                 }
-                Attribute::Parsed(AttributeKind::TargetFeature(_, attr_span)) => {
+                Attribute::Parsed(AttributeKind::TargetFeature{ attr_span, ..}) => {
                     self.check_target_feature(hir_id, *attr_span, target, attrs)
                 }
                 Attribute::Parsed(AttributeKind::RustcObjectLifetimeDefault) => {

compiler/rustc_span/src/symbol.rs

@@ -901,6 +901,7 @@ symbols! {
         dynamic_no_pic: "dynamic-no-pic",
         e,
         edition_panic,
+        effective_target_features,
         effects,
         eh_catch_typeinfo,
         eh_personality,
@@ -1061,6 +1062,7 @@ symbols! {
         fn_ptr_addr,
         fn_ptr_trait,
         forbid,
+        force_target_feature,
         forget,
         format,
         format_args,

compiler/rustc_trait_selection/src/error_reporting/infer/note_and_explain.rs

@@ -537,7 +537,7 @@ impl<T> Trait<T> for X {
                 }
             }
             TypeError::TargetFeatureCast(def_id) => {
-                let target_spans = find_attr!(tcx.get_all_attrs(def_id), AttributeKind::TargetFeature(.., span) => *span);
+                let target_spans = find_attr!(tcx.get_all_attrs(def_id), AttributeKind::TargetFeature{attr_span: span, was_forced: false, ..} => *span);
                 diag.note(
                     "functions with `#[target_feature]` can only be coerced to `unsafe` function pointers"
                 );

src/librustdoc/clean/types.rs

@@ -1087,7 +1087,8 @@ pub(crate) fn extract_cfg_from_attrs<'a, I: Iterator<Item = &'a hir::Attribute>
 
     // treat #[target_feature(enable = "feat")] attributes as if they were
     // #[doc(cfg(target_feature = "feat"))] attributes as well
-    if let Some(features) = find_attr!(attrs, AttributeKind::TargetFeature(features, _) => features)
+    if let Some(features) =
+        find_attr!(attrs, AttributeKind::TargetFeature { features, .. } => features)
     {
         for (feature, _) in features {
             cfg &= Cfg::Cfg(sym::target_feature, Some(*feature));

src/librustdoc/json/conversions.rs

@@ -930,7 +930,7 @@ fn maybe_from_hir_attr(
         ),
         AK::ExportName { name, span: _ } => Attribute::ExportName(name.to_string()),
         AK::LinkSection { name, span: _ } => Attribute::LinkSection(name.to_string()),
-        AK::TargetFeature(features, _span) => Attribute::TargetFeature {
+        AK::TargetFeature { features, .. } => Attribute::TargetFeature {
             enable: features.iter().map(|(feat, _span)| feat.to_string()).collect(),
         },
 

src/tools/miri/src/machine.rs

@@ -16,6 +16,7 @@ use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 #[allow(unused)]
 use rustc_data_structures::static_assert_size;
 use rustc_hir::attrs::InlineAttr;
+use rustc_middle::middle::codegen_fn_attrs::TargetFeatureKind;
 use rustc_middle::mir;
 use rustc_middle::query::TyCtxtAt;
 use rustc_middle::ty::layout::{
@@ -1076,7 +1077,7 @@ impl<'tcx> Machine<'tcx> for MiriMachine<'tcx> {
                 .target_features
                 .iter()
                 .filter(|&feature| {
-                    !feature.implied && !ecx.tcx.sess.target_features.contains(&feature.name)
+                    feature.kind != TargetFeatureKind::Implied && !ecx.tcx.sess.target_features.contains(&feature.name)
                 })
                 .fold(String::new(), |mut s, feature| {
                     if !s.is_empty() {

tests/assembly-llvm/force-target-feature.rs

@@ -0,0 +1,33 @@
+//@ only-x86_64
+//@ assembly-output: emit-asm
+//@ compile-flags: -C opt-level=3 -C target-feature=-avx2
+//@ ignore-sgx Tests incompatible with LVI mitigations
+
+#![feature(effective_target_features)]
+
+use std::arch::x86_64::{__m256i, _mm256_add_epi32, _mm256_setzero_si256};
+use std::ops::Add;
+
+#[derive(Clone, Copy)]
+struct AvxU32(__m256i);
+
+impl Add<AvxU32> for AvxU32 {
+    type Output = Self;
+
+    #[no_mangle]
+    #[inline(never)]
+    #[unsafe(force_target_feature(enable = "avx2"))]
+    fn add(self, oth: AvxU32) -> AvxU32 {
+        // CHECK-LABEL: add:
+        // CHECK-NOT: callq
+        // CHECK: vpaddd
+        // CHECK: retq
+        Self(_mm256_add_epi32(self.0, oth.0))
+    }
+}
+
+fn main() {
+    assert!(is_x86_feature_detected!("avx2"));
+    let v = AvxU32(unsafe { _mm256_setzero_si256() });
+    v + v;
+}

tests/ui/feature-gates/feature-gate-effective-target-features.default.stderr

@@ -0,0 +1,37 @@
+error[E0658]: the `#[force_target_feature]` attribute is an experimental feature
+  --> $DIR/feature-gate-effective-target-features.rs:13:5
+   |
+LL |     #[unsafe(force_target_feature(enable = "avx2"))]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #143352 <https://github.com/rust-lang/rust/issues/143352> for more information
+   = help: add `#![feature(effective_target_features)]` 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: `#[target_feature(..)]` cannot be applied to safe trait method
+  --> $DIR/feature-gate-effective-target-features.rs:21:5
+   |
+LL |     #[target_feature(enable = "avx2")]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot be applied to safe trait method
+LL |
+LL |     fn foo(&self) {}
+   |     ------------- not an `unsafe` function
+
+error[E0053]: method `foo` has an incompatible type for trait
+  --> $DIR/feature-gate-effective-target-features.rs:23:5
+   |
+LL |     fn foo(&self) {}
+   |     ^^^^^^^^^^^^^ expected safe fn, found unsafe fn
+   |
+note: type in trait
+  --> $DIR/feature-gate-effective-target-features.rs:7:5
+   |
+LL |     fn foo(&self);
+   |     ^^^^^^^^^^^^^^
+   = note: expected signature `fn(&Bar2)`
+              found signature `#[target_features] fn(&Bar2)`
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0053, E0658.
+For more information about an error, try `rustc --explain E0053`.

tests/ui/feature-gates/feature-gate-effective-target-features.feature.stderr

@@ -0,0 +1,35 @@
+warning: the feature `effective_target_features` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/feature-gate-effective-target-features.rs:3:30
+   |
+LL | #![cfg_attr(feature, feature(effective_target_features))]
+   |                              ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #143352 <https://github.com/rust-lang/rust/issues/143352> for more information
+   = note: `#[warn(incomplete_features)]` on by default
+
+error: `#[target_feature(..)]` cannot be applied to safe trait method
+  --> $DIR/feature-gate-effective-target-features.rs:21:5
+   |
+LL |     #[target_feature(enable = "avx2")]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot be applied to safe trait method
+LL |
+LL |     fn foo(&self) {}
+   |     ------------- not an `unsafe` function
+
+error[E0053]: method `foo` has an incompatible type for trait
+  --> $DIR/feature-gate-effective-target-features.rs:23:5
+   |
+LL |     fn foo(&self) {}
+   |     ^^^^^^^^^^^^^ expected safe fn, found unsafe fn
+   |
+note: type in trait
+  --> $DIR/feature-gate-effective-target-features.rs:7:5
+   |
+LL |     fn foo(&self);
+   |     ^^^^^^^^^^^^^^
+   = note: expected signature `fn(&Bar2)`
+              found signature `#[target_features] fn(&Bar2)`
+
+error: aborting due to 2 previous errors; 1 warning emitted
+
+For more information about this error, try `rustc --explain E0053`.

tests/ui/feature-gates/feature-gate-effective-target-features.rs

@@ -0,0 +1,27 @@
+//@ revisions: default feature
+//@ only-x86_64
+#![cfg_attr(feature, feature(effective_target_features))]
+//[feature]~^ WARN the feature `effective_target_features` is incomplete and may not be safe to use and/or cause compiler crashes
+
+trait Foo {
+    fn foo(&self);
+}
+
+struct Bar;
+
+impl Foo for Bar {
+    #[unsafe(force_target_feature(enable = "avx2"))]
+    //[default]~^ ERROR the `#[force_target_feature]` attribute is an experimental feature
+    fn foo(&self) {}
+}
+
+struct Bar2;
+
+impl Foo for Bar2 {
+    #[target_feature(enable = "avx2")]
+    //~^ ERROR `#[target_feature(..)]` cannot be applied to safe trait method
+    fn foo(&self) {}
+    //~^ ERROR method `foo` has an incompatible type for trait
+}
+
+fn main() {}