Auto merge of #99767 - LeSeulArtichaut:stable-target-feature-11, r=estebank

Stabilize `#![feature(target_feature_11)]`

## Stabilization report

### Summary

Allows for safe functions to be marked with `#[target_feature]` attributes.

Functions marked with `#[target_feature]` are generally considered as unsafe functions: they are unsafe to call, cannot be assigned to safe function pointers, and don't implement the `Fn*` traits.

However, calling them from other `#[target_feature]` functions with a superset of features is safe.

```rust
// Demonstration function
#[target_feature(enable = "avx2")]
fn avx2() {}

fn foo() {
    // Calling `avx2` here is unsafe, as we must ensure
    // that AVX is available first.
    unsafe {
        avx2();
    }
}

#[target_feature(enable = "avx2")]
fn bar() {
    // Calling `avx2` here is safe.
    avx2();
}
```

### Test cases

Tests for this feature can be found in [`src/test/ui/rfcs/rfc-2396-target_feature-11/`](https://github.com/rust-lang/rust/tree/b67ba9ba208ac918228a18321fc3a11a99b1c62b/src/test/ui/rfcs/rfc-2396-target_feature-11/).

### Edge cases

- #73631

Closures defined inside functions marked with `#[target_feature]` inherit the target features of their parent function. They can still be assigned to safe function pointers and implement the appropriate `Fn*` traits.

```rust
#[target_feature(enable = "avx2")]
fn qux() {
    let my_closure = || avx2(); // this call to `avx2` is safe
    let f: fn() = my_closure;
}
```

This means that in order to call a function with `#[target_feature]`, you must show that the target-feature is available while the function executes *and* for as long as whatever may escape from that function lives.

### Documentation

- Reference: rust-lang/reference#1181

---
cc tracking issue #69098
r? `@ghost`

Author: bors

compiler/rustc_codegen_ssa/src/codegen_attrs.rs

@@ -224,34 +224,23 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: DefId) -> CodegenFnAttrs {
             if !tcx.is_closure(did.to_def_id())
                 && tcx.fn_sig(did).skip_binder().unsafety() == hir::Unsafety::Normal
             {
-                if tcx.sess.target.is_like_wasm || tcx.sess.opts.actually_rustdoc {
-                    // The `#[target_feature]` attribute is allowed on
-                    // WebAssembly targets on all functions, including safe
-                    // ones. Other targets require that `#[target_feature]` is
-                    // only applied to unsafe functions (pending the
-                    // `target_feature_11` feature) because on most targets
-                    // execution of instructions that are not supported is
-                    // considered undefined behavior. For WebAssembly which is a
-                    // 100% safe target at execution time it's not possible to
-                    // execute undefined instructions, and even if a future
-                    // feature was added in some form for this it would be a
-                    // deterministic trap. There is no undefined behavior when
-                    // executing WebAssembly so `#[target_feature]` is allowed
-                    // on safe functions (but again, only for WebAssembly)
-                    //
-                    // Note that this is also allowed if `actually_rustdoc` so
-                    // if a target is documenting some wasm-specific code then
-                    // it's not spuriously denied.
-                } else if !tcx.features().target_feature_11 {
-                    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(did), "not an `unsafe` function");
-                    err.emit();
-                } else {
+                // The `#[target_feature]` attribute is allowed on
+                // WebAssembly targets on all functions, including safe
+                // ones. Other targets have conditions on the usage of
+                // `#[target_feature]` because on most targets
+                // execution of instructions that are not supported is
+                // considered undefined behavior. For WebAssembly which is a
+                // 100% safe target at execution time it's not possible to
+                // execute undefined instructions, and even if a future
+                // feature was added in some form for this it would be a
+                // deterministic trap. There is no undefined behavior when
+                // executing WebAssembly so `#[target_feature]` is allowed
+                // on safe functions (but again, only for WebAssembly)
+                //
+                // Note that this is also allowed if `actually_rustdoc` so
+                // if a target is documenting some wasm-specific code then
+                // it's not spuriously denied.
+                if !(tcx.sess.target.is_like_wasm || tcx.sess.opts.actually_rustdoc) {
                     check_target_feature_trait_unsafe(tcx, did, attr.span);
                 }
             }
@@ -478,7 +467,7 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: DefId) -> CodegenFnAttrs {
     });
 
     // #73631: closures inherit `#[target_feature]` annotations
-    if tcx.features().target_feature_11 && tcx.is_closure(did.to_def_id()) {
+    if tcx.is_closure(did.to_def_id()) {
         let owner_id = tcx.parent(did.to_def_id());
         if tcx.def_kind(owner_id).has_codegen_attrs() {
             codegen_fn_attrs

compiler/rustc_feature/src/accepted.rs

@@ -312,6 +312,8 @@ declare_features! (
     (accepted, struct_variant, "1.0.0", None, None),
     /// Allows `#[target_feature(...)]`.
     (accepted, target_feature, "1.27.0", None, None),
+    /// Allows the use of `#[target_feature]` on safe functions.
+    (accepted, target_feature_11, "CURRENT_RUSTC_VERSION", Some(69098), None),
     /// Allows `fn main()` with return types which implements `Termination` (RFC 1937).
     (accepted, termination_trait, "1.26.0", Some(43301), None),
     /// Allows `#[test]` functions where the return type implements `Termination` (RFC 1937).

compiler/rustc_feature/src/active.rs

@@ -513,8 +513,6 @@ declare_features! (
     (active, strict_provenance, "1.61.0", Some(95228), None),
     /// Allows string patterns to dereference values to match them.
     (active, string_deref_patterns, "1.67.0", Some(87121), None),
-    /// Allows the use of `#[target_feature]` on safe functions.
-    (active, target_feature_11, "1.45.0", Some(69098), None),
     /// Allows using `#[thread_local]` on `static` items.
     (active, thread_local, "1.0.0", Some(29594), None),
     /// Allows defining `trait X = A + B;` alias items.

library/core/src/lib.rs

@@ -232,7 +232,6 @@
 #![feature(simd_ffi)]
 #![feature(staged_api)]
 #![feature(stmt_expr_attributes)]
-#![feature(target_feature_11)]
 #![feature(trait_alias)]
 #![feature(transparent_unions)]
 #![feature(try_blocks)]

tests/mir-opt/inline/inline_compatibility.inlined_no_sanitize.Inline.diff

@@ -4,14 +4,14 @@
   fn inlined_no_sanitize() -> () {
       let mut _0: ();                      // return place in scope 0 at $DIR/inline_compatibility.rs:+0:37: +0:37
       let _1: ();                          // in scope 0 at $DIR/inline_compatibility.rs:+1:5: +1:18
-+     scope 1 (inlined no_sanitize) {      // at $DIR/inline_compatibility.rs:24:5: 24:18
++     scope 1 (inlined no_sanitize) {      // at $DIR/inline_compatibility.rs:23:5: 23:18
 +     }
 
       bb0: {
           StorageLive(_1);                 // scope 0 at $DIR/inline_compatibility.rs:+1:5: +1:18
 -         _1 = no_sanitize() -> bb1;       // scope 0 at $DIR/inline_compatibility.rs:+1:5: +1:18
 -                                          // mir::Constant
--                                          // + span: $DIR/inline_compatibility.rs:24:5: 24:16
+-                                          // + span: $DIR/inline_compatibility.rs:23:5: 23:16
 -                                          // + literal: Const { ty: unsafe fn() {no_sanitize}, val: Value(<ZST>) }
 -     }
 - 

tests/mir-opt/inline/inline_compatibility.inlined_target_feature.Inline.diff

@@ -4,14 +4,14 @@
   fn inlined_target_feature() -> () {
       let mut _0: ();                      // return place in scope 0 at $DIR/inline_compatibility.rs:+0:40: +0:40
       let _1: ();                          // in scope 0 at $DIR/inline_compatibility.rs:+1:5: +1:21
-+     scope 1 (inlined target_feature) {   // at $DIR/inline_compatibility.rs:13:5: 13:21
++     scope 1 (inlined target_feature) {   // at $DIR/inline_compatibility.rs:12:5: 12:21
 +     }
 
       bb0: {
           StorageLive(_1);                 // scope 0 at $DIR/inline_compatibility.rs:+1:5: +1:21
 -         _1 = target_feature() -> bb1;    // scope 0 at $DIR/inline_compatibility.rs:+1:5: +1:21
 -                                          // mir::Constant
--                                          // + span: $DIR/inline_compatibility.rs:13:5: 13:19
+-                                          // + span: $DIR/inline_compatibility.rs:12:5: 12:19
 -                                          // + literal: Const { ty: unsafe fn() {target_feature}, val: Value(<ZST>) }
 -     }
 - 

tests/mir-opt/inline/inline_compatibility.not_inlined_c_variadic.Inline.diff

@@ -12,7 +12,7 @@
           StorageLive(_1);                 // scope 0 at $DIR/inline_compatibility.rs:+1:9: +1:10
           _1 = sum(const 4_u32, const 4_u32, const 30_u32, const 200_u32, const 1000_u32) -> bb1; // scope 0 at $DIR/inline_compatibility.rs:+1:13: +1:52
                                            // mir::Constant
-                                           // + span: $DIR/inline_compatibility.rs:42:13: 42:16
+                                           // + span: $DIR/inline_compatibility.rs:41:13: 41:16
                                            // + literal: Const { ty: unsafe extern "C" fn(u32, ...) -> u32 {sum}, val: Value(<ZST>) }
       }
 

tests/mir-opt/inline/inline_compatibility.not_inlined_no_sanitize.Inline.diff

@@ -9,7 +9,7 @@
           StorageLive(_1);                 // scope 0 at $DIR/inline_compatibility.rs:+1:5: +1:18
           _1 = no_sanitize() -> bb1;       // scope 0 at $DIR/inline_compatibility.rs:+1:5: +1:18
                                            // mir::Constant
-                                           // + span: $DIR/inline_compatibility.rs:29:5: 29:16
+                                           // + span: $DIR/inline_compatibility.rs:28:5: 28:16
                                            // + literal: Const { ty: unsafe fn() {no_sanitize}, val: Value(<ZST>) }
       }
 

tests/mir-opt/inline/inline_compatibility.not_inlined_target_feature.Inline.diff

@@ -9,7 +9,7 @@
           StorageLive(_1);                 // scope 0 at $DIR/inline_compatibility.rs:+1:5: +1:21
           _1 = target_feature() -> bb1;    // scope 0 at $DIR/inline_compatibility.rs:+1:5: +1:21
                                            // mir::Constant
-                                           // + span: $DIR/inline_compatibility.rs:18:5: 18:19
+                                           // + span: $DIR/inline_compatibility.rs:17:5: 17:19
                                            // + literal: Const { ty: unsafe fn() {target_feature}, val: Value(<ZST>) }
       }
 

tests/mir-opt/inline/inline_compatibility.rs

@@ -4,7 +4,6 @@
 
 #![crate_type = "lib"]
 #![feature(no_sanitize)]
-#![feature(target_feature_11)]
 #![feature(c_variadic)]
 
 // EMIT_MIR inline_compatibility.inlined_target_feature.Inline.diff