Implement RFC 2396: #[target_feature] 1.1

src/librustc_feature/active.rs

@@ -559,6 +559,9 @@ declare_features! (
     /// Allow negative trait implementations.
     (active, negative_impls, "1.44.0", Some(68318), None),
 
+    /// Allows the use of `#[target_feature]` on safe functions.
+    (active, target_feature_11, "1.45.0", Some(69098), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

src/librustc_middle/ty/error.rs

@@ -3,11 +3,13 @@ use rustc_ast::ast;
 use rustc_errors::{pluralize, Applicability, DiagnosticBuilder};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
+use rustc_span::symbol::sym;
 use rustc_span::Span;
 use rustc_target::spec::abi;
 
 use std::borrow::Cow;
 use std::fmt;
+use std::ops::Deref;
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq, TypeFoldable)]
 pub struct ExpectedFound<T> {
@@ -58,6 +60,8 @@ pub enum TypeError<'tcx> {
     ConstMismatch(ExpectedFound<&'tcx ty::Const<'tcx>>),
 
     IntrinsicCast,
+    /// Safe `#[target_feature]` functions are not assignable to safe function pointers.
+    TargetFeatureCast(DefId),
 }
 
 pub enum UnconstrainedNumeric {
@@ -183,6 +187,10 @@ impl<'tcx> fmt::Display for TypeError<'tcx> {
                 write!(f, "expected `{}`, found `{}`", values.expected, values.found)
             }
             IntrinsicCast => write!(f, "cannot coerce intrinsics to function pointers"),
+            TargetFeatureCast(_) => write!(
+                f,
+                "cannot coerce functions with `#[target_feature]` to safe function pointers"
+            ),
             ObjectUnsafeCoercion(_) => write!(f, "coercion to object-unsafe trait object"),
         }
     }
@@ -193,7 +201,8 @@ impl<'tcx> TypeError<'tcx> {
         use self::TypeError::*;
         match self {
             CyclicTy(_) | UnsafetyMismatch(_) | Mismatch | AbiMismatch(_) | FixedArraySize(_)
-            | Sorts(_) | IntMismatch(_) | FloatMismatch(_) | VariadicMismatch(_) => false,
+            | Sorts(_) | IntMismatch(_) | FloatMismatch(_) | VariadicMismatch(_)
+            | TargetFeatureCast(_) => false,
 
             Mutability
             | TupleSize(_)
@@ -489,6 +498,18 @@ impl Trait for X {
                     );
                 }
             }
+            TargetFeatureCast(def_id) => {
+                let attrs = self.get_attrs(*def_id);
+                let target_spans = attrs
+                    .deref()
+                    .iter()
+                    .filter(|attr| attr.has_name(sym::target_feature))
+                    .map(|attr| attr.span);
+                db.note(
+                    "functions with `#[target_feature]` can only be coerced to `unsafe` function pointers"
+                );
+                db.span_labels(target_spans, "`#[target_feature]` added here");
+            }
             _ => {}
         }
     }

src/librustc_middle/ty/structural_impls.rs

@@ -645,6 +645,7 @@ impl<'a, 'tcx> Lift<'tcx> for ty::error::TypeError<'a> {
             ExistentialMismatch(ref x) => return tcx.lift(x).map(ExistentialMismatch),
             ConstMismatch(ref x) => return tcx.lift(x).map(ConstMismatch),
             IntrinsicCast => IntrinsicCast,
+            TargetFeatureCast(ref x) => TargetFeatureCast(*x),
             ObjectUnsafeCoercion(ref x) => return tcx.lift(x).map(ObjectUnsafeCoercion),
         })
     }

src/librustc_mir/transform/check_unsafety.rs

@@ -19,6 +19,7 @@ use crate::util;
 
 pub struct UnsafetyChecker<'a, 'tcx> {
     body: &'a Body<'tcx>,
+    body_did: LocalDefId,
     const_context: bool,
     min_const_fn: bool,
     violations: Vec<UnsafetyViolation>,
@@ -35,6 +36,7 @@ impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
         const_context: bool,
         min_const_fn: bool,
         body: &'a Body<'tcx>,
+        body_did: LocalDefId,
         tcx: TyCtxt<'tcx>,
         param_env: ty::ParamEnv<'tcx>,
     ) -> Self {
@@ -44,6 +46,7 @@ impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
         }
         Self {
             body,
+            body_did,
             const_context,
             min_const_fn,
             violations: vec![],
@@ -87,6 +90,10 @@ impl<'a, 'tcx> Visitor<'tcx> for UnsafetyChecker<'a, 'tcx> {
                         UnsafetyViolationKind::GeneralAndConstFn,
                     )
                 }
+
+                if let ty::FnDef(func_id, _) = func_ty.kind {
+                    self.check_target_features(func_id);
+                }
             }
         }
         self.super_terminator(terminator, location);
@@ -436,6 +443,22 @@ impl<'a, 'tcx> UnsafetyChecker<'a, 'tcx> {
             }
         }
     }
+
+    /// Checks whether calling `func_did` needs an `unsafe` context or not, i.e. whether
+    /// the called function has target features the calling function hasn't.
+    fn check_target_features(&mut self, func_did: DefId) {
+        let callee_features = &self.tcx.codegen_fn_attrs(func_did).target_features;
+        let self_features = &self.tcx.codegen_fn_attrs(self.body_did).target_features;
+
+        // Is `callee_features` a subset of `calling_features`?
+        if !callee_features.iter().all(|feature| self_features.contains(feature)) {
+            self.require_unsafe(
+                "call to function with `#[target_feature]`",
+                "can only be called if the required target features are available",
+                UnsafetyViolationKind::GeneralAndConstFn,
+            )
+        }
+    }
 }
 
 pub(crate) fn provide(providers: &mut Providers<'_>) {
@@ -502,7 +525,8 @@ fn unsafety_check_result(tcx: TyCtxt<'_>, def_id: LocalDefId) -> UnsafetyCheckRe
         }
         hir::BodyOwnerKind::Const | hir::BodyOwnerKind::Static(_) => (true, false),
     };
-    let mut checker = UnsafetyChecker::new(const_context, min_const_fn, body, tcx, param_env);
+    let mut checker =
+        UnsafetyChecker::new(const_context, min_const_fn, body, def_id, tcx, param_env);
     checker.visit_body(&body);
 
     check_unused_unsafe(tcx, def_id, &checker.used_unsafe, &mut checker.inherited_blocks);

src/librustc_span/symbol.rs

@@ -722,6 +722,7 @@ symbols! {
         suggestion,
         sync_trait,
         target_feature,
+        target_feature_11,
         target_has_atomic,
         target_has_atomic_load_store,
         target_thread_local,

src/librustc_typeck/check/coercion.rs

@@ -688,12 +688,22 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b);
 
         match b.kind {
-            ty::FnPtr(_) => {
+            ty::FnPtr(b_sig) => {
                 let a_sig = a.fn_sig(self.tcx);
                 // Intrinsics are not coercible to function pointers
                 if a_sig.abi() == Abi::RustIntrinsic || a_sig.abi() == Abi::PlatformIntrinsic {
                     return Err(TypeError::IntrinsicCast);
                 }
+
+                // Safe `#[target_feature]` functions are not assignable to safe fn pointers (RFC 2396).
+                if let ty::FnDef(def_id, _) = a.kind {
+                    if b_sig.unsafety() == hir::Unsafety::Normal
+                        && !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
+                    {
+                        return Err(TypeError::TargetFeatureCast(def_id));
+                    }
+                }
+
                 let InferOk { value: a_sig, mut obligations } =
                     self.normalize_associated_types_in_as_infer_ok(self.cause.span, &a_sig);
 

src/librustc_typeck/collect.rs

@@ -29,7 +29,7 @@ use rustc_hir::def::{CtorKind, DefKind, Res};
 use rustc_hir::def_id::{DefId, LocalDefId, LOCAL_CRATE};
 use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
 use rustc_hir::weak_lang_items;
-use rustc_hir::{GenericParamKind, Node, Unsafety};
+use rustc_hir::{GenericParamKind, Node};
 use rustc_middle::hir::map::blocks::FnLikeNode;
 use rustc_middle::hir::map::Map;
 use rustc_middle::middle::codegen_fn_attrs::{CodegenFnAttrFlags, CodegenFnAttrs};
@@ -2413,13 +2413,12 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, id: DefId) -> CodegenFnAttrs {
                 codegen_fn_attrs.export_name = Some(s);
             }
         } else if attr.check_name(sym::target_feature) {
-            if tcx.is_closure(id) || tcx.fn_sig(id).unsafety() == Unsafety::Normal {
-                let msg = "`#[target_feature(..)]` can only be applied to `unsafe` functions";
-                tcx.sess
-                    .struct_span_err(attr.span, msg)
-                    .span_label(attr.span, "can only be applied to `unsafe` functions")
-                    .span_label(tcx.def_span(id), "not an `unsafe` function")
-                    .emit();
+            if !tcx.features().target_feature_11 {
+                check_target_feature_safe_fn(tcx, id, attr.span);
+            } else if let Some(local_id) = id.as_local() {
+                if tcx.fn_sig(id).unsafety() == hir::Unsafety::Normal {
+                    check_target_feature_trait_unsafe(tcx, local_id, attr.span);
+                }
             }
             from_target_feature(tcx, id, attr, &whitelist, &mut codegen_fn_attrs.target_features);
         } else if attr.check_name(sym::linkage) {
@@ -2666,3 +2665,39 @@ fn check_link_name_xor_ordinal(
         tcx.sess.err(msg);
     }
 }
+
+/// Checks the function annotated with `#[target_feature]` is unsafe,
+/// reporting an error if it isn't.
+fn check_target_feature_safe_fn(tcx: TyCtxt<'_>, id: DefId, attr_span: Span) {
+    if tcx.is_closure(id) || tcx.fn_sig(id).unsafety() == hir::Unsafety::Normal {
+        let mut err = feature_err(
+            &tcx.sess.parse_sess,
+            sym::target_feature_11,
+            attr_span,
+            "`#[target_feature(..)]` can only be applied to `unsafe` functions",
+        );
+        err.span_label(tcx.def_span(id), "not an `unsafe` function");
+        err.emit();
+    }
+}
+
+/// Checks the function annotated with `#[target_feature]` is not a safe
+/// trait method implementation, reporting an error if it is.
+fn check_target_feature_trait_unsafe(tcx: TyCtxt<'_>, id: LocalDefId, attr_span: Span) {
+    let hir_id = tcx.hir().as_local_hir_id(id);
+    let node = tcx.hir().get(hir_id);
+    if let Node::ImplItem(hir::ImplItem { kind: hir::ImplItemKind::Fn(..), .. }) = node {
+        let parent_id = tcx.hir().get_parent_item(hir_id);
+        let parent_item = tcx.hir().expect_item(parent_id);
+        if let hir::ItemKind::Impl { of_trait: Some(_), .. } = parent_item.kind {
+            tcx.sess
+                .struct_span_err(
+                    attr_span,
+                    "`#[target_feature(..)]` cannot be applied to safe trait method",
+                )
+                .span_label(attr_span, "cannot be applied to safe trait method")
+                .span_label(tcx.def_span(id), "not an `unsafe` function")
+                .emit();
+        }
+    }
+}

src/test/ui/macros/issue-68060.stderr

@@ -1,11 +1,14 @@
-error: `#[target_feature(..)]` can only be applied to `unsafe` functions
+error[E0658]: `#[target_feature(..)]` can only be applied to `unsafe` functions
   --> $DIR/issue-68060.rs:6:13
    |
 LL |             #[target_feature(enable = "")]
-   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can only be applied to `unsafe` functions
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 ...
 LL |             |_| (),
    |             ------ not an `unsafe` function
+   |
+   = note: see issue #69098 <https://github.com/rust-lang/rust/issues/69098> for more information
+   = help: add `#![feature(target_feature_11)]` to the crate attributes to enable
 
 error: the feature named `` is not valid for this target
   --> $DIR/issue-68060.rs:6:30
@@ -21,4 +24,5 @@ LL |             #[track_caller]
 
 error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0737`.
+Some errors have detailed explanations: E0658, E0737.
+For more information about an error, try `rustc --explain E0658`.

src/test/ui/rfcs/rfc-2396-target_feature-11/check-pass.rs

@@ -0,0 +1,50 @@
+// Tests the new rules added by RFC 2396, including:
+// - applying `#[target_feature]` to safe functions is allowed
+// - calling functions with `#[target_feature]` is allowed in
+//   functions which have (at least) the same features
+// - calling functions with `#[target_feature]` is allowed in
+//   unsafe contexts
+// - functions with `#[target_feature]` can coerce to unsafe fn pointers
+
+// check-pass
+// only-x86_64
+
+#![feature(target_feature_11)]
+
+#[target_feature(enable = "sse2")]
+const fn sse2() {}
+
+#[cfg(target_feature = "sse2")]
+const SSE2_ONLY: () = unsafe {
+    sse2();
+};
+
+#[target_feature(enable = "sse2")]
+fn also_sse2() {
+    sse2();
+}
+
+#[target_feature(enable = "sse2")]
+#[target_feature(enable = "avx")]
+fn sse2_and_avx() {
+    sse2();
+}
+
+struct Foo;
+
+impl Foo {
+    #[target_feature(enable = "sse2")]
+    fn sse2(&self) {
+        sse2();
+    }
+}
+
+fn main() {
+    if cfg!(target_feature = "sse2") {
+        unsafe {
+            sse2();
+            Foo.sse2();
+        }
+    }
+    let sse2_ptr: unsafe fn() = sse2;
+}

src/test/ui/rfcs/rfc-2396-target_feature-11/feature-gate-target_feature_11.rs

@@ -0,0 +1,6 @@
+// only-x86_64
+
+#[target_feature(enable = "sse2")] //~ ERROR can only be applied to `unsafe` functions
+fn foo() {}
+
+fn main() {}

src/test/ui/rfcs/rfc-2396-target_feature-11/feature-gate-target_feature_11.stderr

@@ -0,0 +1,14 @@
+error[E0658]: `#[target_feature(..)]` can only be applied to `unsafe` functions
+  --> $DIR/feature-gate-target_feature_11.rs:3:1
+   |
+LL | #[target_feature(enable = "sse2")]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | fn foo() {}
+   | ----------- not an `unsafe` function
+   |
+   = note: see issue #69098 <https://github.com/rust-lang/rust/issues/69098> for more information
+   = help: add `#![feature(target_feature_11)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/rfcs/rfc-2396-target_feature-11/fn-ptr.rs

@@ -0,0 +1,10 @@
+// only-x86_64
+
+#![feature(target_feature_11)]
+
+#[target_feature(enable = "sse2")]
+fn foo() {}
+
+fn main() {
+    let foo: fn() = foo; //~ ERROR mismatched types
+}

src/test/ui/rfcs/rfc-2396-target_feature-11/fn-ptr.stderr

@@ -0,0 +1,18 @@
+error[E0308]: mismatched types
+  --> $DIR/fn-ptr.rs:9:21
+   |
+LL | #[target_feature(enable = "sse2")]
+   | ---------------------------------- `#[target_feature]` added here
+...
+LL |     let foo: fn() = foo;
+   |              ----   ^^^ cannot coerce functions with `#[target_feature]` to safe function pointers
+   |              |
+   |              expected due to this
+   |
+   = note: expected fn pointer `fn()`
+                 found fn item `fn() {foo}`
+   = note: functions with `#[target_feature]` can only be coerced to `unsafe` function pointers
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.