Stabilise is_aarch64_feature_detected! under simd_aarch64 feature

[adamgemmell]

Initial implementation, looking for feedback on the approach here. #86941

One point I noticed was that I haven't seen different "since" versions for the same feature - does this mean that other features can't be added to to the simd_aarch64 feature once this is in stable? If so it might need a more specific name.

r? @Amanieu

[adamgemmell]

CI should fail until the stdarch PR is committed and updated here

[Amanieu]

We can have different since versions for the same feature. So adding new target features in the future is not a problem.

[bors]

☔ The latest upstream changes (presumably #92587) made this pull request unmergeable. Please resolve the merge conflicts.

[Amanieu]

Should this be removed for aarch64 as well? You might need to put this behind cfg(bootstrap) so the stage0 compiler still accepts it.

[adamgemmell]

True - I mistakenly removed it when making pauth temporarily unstable again.

[adamgemmell]

Actually, it does need to remain there while pauth is unstable. I'll fix it up when rust-lang/stdarch#1259 is merged, and update the submodule at the same time.

[Amanieu]

@bors r+ rollup=never

[bors]

📌 Commit 93b5bfb has been approved by Amanieu

[bors]

⌛ Testing commit 93b5bfb with merge e789f3a...

[bors]

☀️ Test successful - checks-actions
Approved by: Amanieu
Pushing e789f3a to master...

[rust-timer]

Finished benchmarking commit (e789f3a): comparison url.

Summary: This benchmark run did not return any relevant results.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression

[ehuss]

Just a heads up, the stabilization machinery may cause confusion here. The docs say this is still unstable: https://doc.rust-lang.org/nightly/std/arch/macro.is_aarch64_feature_detected.html

This is because the https://doc.rust-lang.org/ docs are built for x86_64. For some reason, using the macro from an incompatible target raises an unstable error. For example, using is_aarch64_feature_detected on x86_64 will display:

error[E0658]: use of unstable library feature 'stdsimd'
 --> src/main.rs:2:8
  |
2 |     if std::arch::is_aarch64_feature_detected!("paca") {
  |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: see issue #27731 <https://github.com/rust-lang/rust/issues/27731> for more information
  = help: add `#![feature(stdsimd)]` to the crate attributes to enable

error:
               is_aarch64_feature_detected can only be used on AArch64 targets.
               You can prevent it from being used in other architectures by
               guarding it behind a cfg(target_arch) as follows:

                   #[cfg(target_arch = "aarch64")] {
                       if is_aarch64_feature_detected(...) { ... }
                   }

 --> src/main.rs:2:8
  |
2 |     if std::arch::is_aarch64_feature_detected!("paca") {
  |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: this error originates in the macro `std::arch::is_aarch64_feature_detected` (in Nightly builds, run with -Z macro-backtrace for more info)

Is that intentional?