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On some architectures, vector types may have a different ABI depending on whether the relevant target features are enabled. (The ABI when the feature is disabled is often not specified, but LLVM implements some de-facto ABI.)

As discussed in rust-lang/lang-team#235, this turns out to very easily lead to unsound code.

This commit makes it a post-monomorphization future-incompat warning to declare or call functions using those vector types in a context in which the corresponding target features are disabled, if using an ABI for which the difference is relevant. This ensures that these functions are always called with a consistent ABI.

See the nomination comment for more discussion.

Part of #116558

r? RalfJung

rustbot added S-waiting-on-review

Status: Awaiting review from the assignee but also interested parties.

T-compiler

Relevant to the compiler team, which will review and decide on the PR/issue.

labels

Oct 26, 2024

This comment has been minimized.

bors added a commit to rust-lang-ci/rust that referenced this pull request

Oct 26, 2024

Emit warning when calling/declaring functions with unavailable vectors.

As discussed in rust-lang/lang-team#235, this turns out to very easily lead to unsound code.

See the [nomination comment](rust-lang#127731 (comment)) for more discussion.

Part of rust-lang#116558

r? RalfJung

The collector always runs, so likely we'll have to make this new check a query to avoid the perf issues.

For the declaration-site check this should be fairly easy, we can pass in the monomorphized instance and that has everything we need. The call-site check is more tricky since the inputs currently are (callee_ty, *fn_span, self.body.source.instance). AFAIK we usually avoid passing a span into a query as those are quite unstable, but not sure what else to do here?
Cc @compiler-errors

☀️ Try build successful - checks-actions
Build commit: bbf9ed8 (bbf9ed8a41f053260da986cd0252b156f3866520)

This comment has been minimized.

Finished benchmarking commit (bbf9ed8): comparison URL.

Overall result: ❌ regressions - please read the text below

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR may lead to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

mean	range	count
Regressions ❌ (primary)	5.1%	[0.3%, 16.8%]	75
Regressions ❌ (secondary)	4.8%	[0.1%, 29.4%]	30
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	5.1%	[0.3%, 16.8%]	75

Max RSS (memory usage)

Results (primary 4.2%, secondary 3.1%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

mean	range	count
Regressions ❌ (primary)	4.2%	[1.0%, 10.1%]	68
Regressions ❌ (secondary)	3.4%	[0.9%, 6.1%]	25
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.0%	[-3.0%, -3.0%]	1
All ❌✅ (primary)	4.2%	[1.0%, 10.1%]	68

Cycles

Results (primary 11.1%, secondary 12.1%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

mean	range	count
Regressions ❌ (primary)	11.1%	[1.2%, 26.2%]	56
Regressions ❌ (secondary)	17.3%	[2.9%, 38.6%]	9
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.6%	[-4.9%, -2.9%]	3
All ❌✅ (primary)	11.1%	[1.2%, 26.2%]	56

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 785.03s -> 787.479s (0.31%)
Artifact size: 333.74 MiB -> 333.57 MiB (-0.05%)

This comment has been minimized.

You'll probably need to fix the compilation error to make it buildable, but yes

The call-site check is more tricky since the inputs currently are (callee_ty, *fn_span, self.body.source.instance).

@RalfJung: Why not just make the query something like (callee_ty, instance) which then returns some "status" or something that captures "should we emit a lint?" that you then use at the call-site to turn into a lint, rather than making the query responsible for emitting the lint? I agree that you almost never want to pass a span to a query.

The call-site check is more tricky since the inputs currently are (callee_ty, *fn_span, self.body.source.instance).

@RalfJung: Why not just make the query something like (callee_ty, instance) which then returns some "status" or something that captures "should we emit a lint?" that you then use at the call-site to turn into a lint, rather than making the query responsible for emitting the lint? I agree that you almost never want to pass a span to a query.

I thought of doing the same too - I also gave up on the previous attempt since that got in a somewhat annoying rabbit hole.
Should be ready for a perf run now!

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bors added a commit to rust-lang-ci/rust that referenced this pull request

Oct 26, 2024

Emit warning when calling/declaring functions with unavailable vectors.

As discussed in rust-lang/lang-team#235, this turns out to very easily lead to unsound code.

See the [nomination comment](rust-lang#127731 (comment)) for more discussion.

Part of rust-lang#116558

r? RalfJung

☀️ Try build successful - checks-actions
Build commit: 95e2c91 (95e2c91a1f2db67bbad2800a9838d921ab01cbbb)

This comment has been minimized.

Finished benchmarking commit (95e2c91): comparison URL.

Overall result: ❌ regressions - please read the text below

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

mean	range	count
Regressions ❌ (primary)	1.7%	[0.2%, 3.8%]	48
Regressions ❌ (secondary)	1.9%	[1.0%, 2.7%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.7%	[0.2%, 3.8%]	48

Max RSS (memory usage)

Results (primary 3.1%, secondary -0.8%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

mean	range	count
Regressions ❌ (primary)	3.4%	[1.4%, 6.5%]	21
Regressions ❌ (secondary)	4.4%	[4.4%, 4.4%]	1
Improvements ✅ (primary)	-1.8%	[-1.8%, -1.8%]	1
Improvements ✅ (secondary)	-1.6%	[-4.0%, -0.4%]	6
All ❌✅ (primary)	3.1%	[-1.8%, 6.5%]	22

Cycles

Results (primary 3.2%, secondary 3.0%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

mean	range	count
Regressions ❌ (primary)	3.2%	[1.6%, 6.3%]	29
Regressions ❌ (secondary)	3.0%	[3.0%, 3.1%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.2%	[1.6%, 6.3%]	29

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 783.187s -> 786.602s (0.44%)
Artifact size: 333.73 MiB -> 333.78 MiB (0.02%)

bors mentioned this pull request

Nov 10, 2024

bors mentioned this pull request

Nov 10, 2024

Finished benchmarking commit (7660aed): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

mean	range	count
Regressions ❌ (primary)	0.5%	[0.2%, 1.0%]	36
Regressions ❌ (secondary)	0.6%	[0.2%, 0.9%]	6
Improvements ✅ (primary)	-0.4%	[-0.4%, -0.4%]	1
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	1
All ❌✅ (primary)	0.5%	[-0.4%, 1.0%]	37

Max RSS (memory usage)

Results (primary 3.2%, secondary 0.3%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

mean	range	count
Regressions ❌ (primary)	3.8%	[0.8%, 11.6%]	8
Regressions ❌ (secondary)	4.1%	[2.6%, 6.1%]	3
Improvements ✅ (primary)	-1.8%	[-1.8%, -1.8%]	1
Improvements ✅ (secondary)	-3.4%	[-4.6%, -1.3%]	3
All ❌✅ (primary)	3.2%	[-1.8%, 11.6%]	9

Cycles

Results (primary 1.2%, secondary 2.3%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

mean	range	count
Regressions ❌ (primary)	1.2%	[0.6%, 1.9%]	8
Regressions ❌ (secondary)	2.3%	[1.7%, 3.0%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.2%	[0.6%, 1.9%]	8

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 780.152s -> 784.572s (0.57%)
Artifact size: 335.33 MiB -> 335.21 MiB (-0.03%)

That's about as expected -- it's the best we managed after a whole bunch of experimentation: one fully cached extra query per monomorphized function.

mati865 pushed a commit to mati865/rust that referenced this pull request

Nov 12, 2024

…iler-errors

Emit warning when calling/declaring functions with unavailable vectors.

As discussed in rust-lang/lang-team#235, this turns out to very easily lead to unsound code.

See the [nomination comment](rust-lang#127731 (comment)) for more discussion.

Part of rust-lang#116558

r? RalfJung

bors added a commit to rust-lang-ci/rust that referenced this pull request

Nov 16, 2024

jhpratt added a commit to jhpratt/rust that referenced this pull request

Nov 17, 2024

jieyouxu added a commit to jieyouxu/rust that referenced this pull request

Nov 17, 2024

rust-timer added a commit to rust-lang-ci/rust that referenced this pull request

Nov 17, 2024

matthiaskrgr added a commit to matthiaskrgr/rust that referenced this pull request

Nov 21, 2024

rust-timer added a commit to rust-lang-ci/rust that referenced this pull request

Nov 21, 2024

eddyb mentioned this pull request

Dec 3, 2024

This was referenced

Dec 23, 2024

fmease added a commit to fmease/rust that referenced this pull request

Feb 11, 2025

Stabilize target_feature_11

Stabilization report

This is an updated version of rust-lang#116114, which is itself a redo of rust-lang#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 rust-lang#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? rust-lang#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 rust-lang#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 rust-lang#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 rust-lang#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 (rust-lang#133102, rust-lang#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:

Documentation

Reference: Document the target_feature_11 feature reference#1181

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

jhpratt added a commit to jhpratt/rust that referenced this pull request

Feb 13, 2025

Stabilize target_feature_11

Stabilization report

This is an updated version of rust-lang#116114, which is itself a redo of rust-lang#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.

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 rust-lang#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? rust-lang#73631

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

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.

Fix #[inline(always)] on closures with target feature 1.1 rust-lang#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 rust-lang#116558

Special functions

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

Documentation

Reference: Document the target_feature_11 feature reference#1181

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

jhpratt added a commit to jhpratt/rust that referenced this pull request

Feb 13, 2025

Stabilize target_feature_11

Stabilization report

This is an updated version of rust-lang#116114, which is itself a redo of rust-lang#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.

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 rust-lang#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? rust-lang#73631

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

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.

Fix #[inline(always)] on closures with target feature 1.1 rust-lang#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 rust-lang#116558

Special functions

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

Documentation

Reference: Document the target_feature_11 feature reference#1181

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

rust-timer added a commit to rust-lang-ci/rust that referenced this pull request

Feb 13, 2025

Rollup merge of rust-lang#134090 - veluca93:stable-tf11, r=oli-obk

Stabilize target_feature_11

Stabilization report

This is an updated version of rust-lang#116114, which is itself a redo of rust-lang#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.

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 rust-lang#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? rust-lang#73631

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

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.

Fix #[inline(always)] on closures with target feature 1.1 rust-lang#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 rust-lang#116558

Special functions

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

Documentation

Reference: Document the target_feature_11 feature reference#1181

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

github-actions bot pushed a commit to tautschnig/verify-rust-std that referenced this pull request

Mar 11, 2025

Stabilize target_feature_11

Stabilization report

This is an updated version of rust-lang#116114, which is itself a redo of rust-lang#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.

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 rust-lang#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? rust-lang#73631

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

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.

Fix #[inline(always)] on closures with target feature 1.1 rust-lang#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 rust-lang#116558

Special functions

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

Documentation

Reference: Document the target_feature_11 feature reference#1181

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

github-actions bot pushed a commit to tautschnig/verify-rust-std that referenced this pull request

Mar 11, 2025

Stabilize target_feature_11

Stabilization report

This is an updated version of rust-lang#116114, which is itself a redo of rust-lang#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.

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 rust-lang#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? rust-lang#73631

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

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.

Fix #[inline(always)] on closures with target feature 1.1 rust-lang#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 rust-lang#116558

Special functions

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

Documentation

Reference: Document the target_feature_11 feature reference#1181

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