Stabilize target_feature_11

[veluca93]

Stabilization report

This is an updated version of #116114, which is itself a redo of #99767. Most of this commit and report were copied from those PRs. Thanks @LeSeulArtichaut and @calebzulawski!

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 generally 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.

// 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();
}

Moreover, once #135504 is merged, they can be converted to safe function pointers in a context in which calling them is safe:

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

fn foo() -> fn() {
    // Converting `avx2` to fn() is a compilation error here.
    avx2
}

#[target_feature(enable = "avx2")]
fn bar() -> fn() {
    // `avx2` coerces to fn() here
    avx2
}

See the section "Closures" below for justification of this behaviour.

Test cases

Tests for this feature can be found in tests/ui/target_feature/.

Edge cases

Closures

• target-feature 1.1: should closures inherit target-feature annotations? #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.

#[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 guarantee that the target-feature is available while the function, any closures defined inside it, as well as any safe function pointers obtained from target-feature functions inside it, execute.

This is usually ensured because target features are assumed to never disappear, and:

• on any unsafe call to a #[target_feature] function, presence of the target feature is guaranteed by the programmer through the safety requirements of the unsafe call.
• on any safe call, this is guaranteed recursively by the caller.

If you work in an environment where target features can be disabled, it is your responsibility to ensure that no code inside a target feature function (including inside a closure) runs after this (until the feature is enabled again).

Note: this has an effect on existing code, as nowadays closures do not inherit features from the enclosing function, and thus this strengthens a safety requirement. It was originally proposed in #73631 to solve this by adding a new type of UB: “taking a target feature away from your process after having run code that uses that target feature is UB” .
This was motivated by userspace code already assuming in a few places that CPU features never disappear from a program during execution (see i.e. https://github.com/rust-lang/stdarch/blob/2e29bdf90832931ea499755bb4ad7a6b0809295a/crates/std_detect/src/detect/arch/x86.rs); however, concerns were raised in the context of the Linux kernel; thus, we propose to relax that requirement to "causing the set of usable features to be reduced is unsafe; when doing so, the programmer is required to ensure that no closures or safe fn pointers that use removed features are still in scope".

• Fix #[inline(always)] on closures with target feature 1.1 #111836

Closures accept #[inline(always)], even within functions marked with #[target_feature]. Since these attributes conflict, #[inline(always)] wins out to maintain compatibility.

ABI concerns

• The extern "C" ABI of SIMD vector types depends on target features #116558

The ABI of some types can change when compiling a function with different target features. This could have introduced unsoundness with target_feature_11, but recent fixes (#133102, #132173) either make those situations invalid or make the ABI no longer dependent on features. Thus, those issues should no longer occur.

Special functions

The #[target_feature] attribute is forbidden from a variety of special functions, such as main, current and future lang items (e.g. #[start], #[panic_handler]), safe default trait implementations and safe trait methods.

This was not disallowed at the time of the first stabilization PR for target_features_11, and resulted in the following issues/PRs:

• #[target_feature] is allowed on main #108645
• #[target_feature] is allowed on default implementations #108646
• #[target_feature] is allowed on #[panic_handler] with target_feature 1.1 #109411
• Prevent using #[target_feature] on lang item functions #115910

Documentation

• Reference: Document the target_feature_11 feature reference#1181

---

cc tracking issue #69098
cc @workingjubilee
cc @RalfJung
r? @rust-lang/lang

[RalfJung]

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.

Oddly, this means this compiles

#[target_feature(enable = "avx2")]
fn bar() -> fn() {
    || avx2()
}

but this does not

#[target_feature(enable = "avx2")]
fn bar() -> fn() {
    avx2
}

That's not a blocker but might be worth an issue. (But changing this may mean we have to refine the safety condition.)

This means that in order to call a function with #[target_feature], you must guarantee that the target-feature is available while the function executes and for as long as any returned closures (or function pointers) that inherit features from that function live.

The definition of when something "live"s is subtle. I would make it about execution: you must guarantee that the target feature is available while the function or any closure defined inside that function executes. This is generally ensured because target features, once available, cannot usually be taken back; if you work in an environment where they can be taken back, it is your responsibility to ensure that no code inside a target feature function (including inside a closure) runs after this (until the feature is enabled again).

The #[target_feature] attribute is forbidden from a variety of special functions

There is an enumeration saying that the attribute is allowed on main etc, only to then state that it is forbidden. This is confusing.

Have we covered all special functions?

[traviscross]

Actually, I'm going to...

r? @oli-obk

...as he had already looked at this.

[oli-obk]

@bors r+

[bors]

📌 Commit 44b2e6c has been approved by oli-obk

It is now in the queue for this repository.