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

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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/`](b67ba9ba20/src/test/ui/rfcs/rfc-2396-target_feature-11/).

### Edge cases

- https://github.com/rust-lang/rust/issues/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: https://github.com/rust-lang/reference/pull/1181

---
cc tracking issue #69098
r? `@ghost`
This commit is contained in:
bors 2023-02-28 01:14:56 +00:00
commit b583ede652
28 changed files with 78 additions and 151 deletions
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47
compiler/rustc_codegen_ssa/src/codegen_attrs.rs
View file

@ -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