`is_x86_feature_detected!("avx512f")` fails to build on beta and nightly

[oconnor663]

The following program builds and runs on stable but fails to build on nightly:

fn main() {
    dbg!(is_x86_feature_detected!("avx512f"));
}

Here's the build error on rustc 1.43.0-nightly (58b834344 2020-02-05):

error[E0658]: use of unstable library feature 'stdsimd'
 --> src/main.rs:2:10
  |
2 |     dbg!(is_x86_feature_detected!("avx512f"));
  |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: for more information, see https://github.com/rust-lang/rust/issues/27731
  = help: add `#![feature(stdsimd)]` to the crate attributes to enable
  = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

But note that if I replace "avx512f" with "avx2", the error goes away.

[Mark-Simulacrum]

cc @gnzlbg

[oconnor663]

Here's the output of cargo-bisect-rustc:

searched nightlies: from nightly-2020-01-02 to nightly-2020-02-10
regressed nightly: nightly-2020-02-06
searched commits: from c9290dc to 58b8343
regressed commit: eda1a7a

That points to #68755, which is a large merge of stdarch. When I bisect stdarch, I find the error was introduced in rust-lang/stdarch@cb6fa68, possibly intentionally. Is it the case that AVX-512 feature detection was never intended to be stabilized? Or maybe it got caught up in a larger group of unstable features? Prior to this regression, it did in fact work :)

[oconnor663]

A quick search through https://github.com/search?q=is_x86_feature_detected%21%28%22avx512f%22%29&type=Code turns up at least one other crate besides BLAKE3 that fails to build on nightly for this reason:

git clone https://github.com/PoC-Consortium/scavenger
cd scavenger
cargo check --features=simd           # succeeds
cargo +nightly check --features=simd  # fails