Compile error with `stdarch_x86_avx512` feature not enable

[serendipity-crypto]

When I compile my codes with rand v0.9.0-beta.1 and a computer has avx512, the compiler return following error. It seems like that the attribute #![feature(stdarch_x86_avx512)] is not enabled correctly in rand.

   Compiling rand_chacha v0.9.0-beta.1
   Compiling rand v0.9.0-beta.1
error[E0658]: use of unstable library feature `stdarch_x86_avx512`
   --> E:\packages\rust\cargo\registry\src\rsproxy.cn-e3de039b2554c837\rand-0.9.0-beta.1\src\distr\utils.rs:183:39
    |
183 |                     let hi = unsafe { $mulhi(self.into(), x.into()) }.into();
    |                                       ^
...
196 |     wmul_impl_16! { u16x32, _mm512_mulhi_epu16, _mm512_mullo_epi16 }
    |     ---------------------------------------------------------------- in this macro invocation
    |
    = note: see issue #111137 <https://github.com/rust-lang/rust/issues/111137> for more information
    = help: add `#![feature(stdarch_x86_avx512)]` to the crate attributes to enable
    = note: this compiler was built on 2025-01-12; consider upgrading it if it is out of date
    = note: this error originates in the macro `wmul_impl_16` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0658]: use of unstable library feature `stdarch_x86_avx512`
   --> E:\packages\rust\cargo\registry\src\rsproxy.cn-e3de039b2554c837\rand-0.9.0-beta.1\src\distr\utils.rs:184:39
    |
184 |                     let lo = unsafe { $mullo(self.into(), x.into()) }.into();
    |                                       ^
...
196 |     wmul_impl_16! { u16x32, _mm512_mulhi_epu16, _mm512_mullo_epi16 }
    |     ---------------------------------------------------------------- in this macro invocation
    |
    = note: see issue #111137 <https://github.com/rust-lang/rust/issues/111137> for more information
    = help: add `#![feature(stdarch_x86_avx512)]` to the crate attributes to enable
    = note: this compiler was built on 2025-01-12; consider upgrading it if it is out of date
    = note: this error originates in the macro `wmul_impl_16` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0658`.
error: could not compile `rand` (lib) due to 2 previous errors

[dhardy]

Thanks for the report.

I think all we need to "fix" this for your CPU is to enable the feature flag:

diff --git a/src/lib.rs b/src/lib.rs
index 54ae884025..623637f1ef 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -49,6 +49,7 @@
 #![doc(test(attr(allow(unused_variables), deny(warnings))))]
 #![no_std]
 #![cfg_attr(feature = "simd_support", feature(portable_simd))]
+#![cfg_attr(all(feature = "simd_support", target_feature = "avx512bw"), feature(stdarch_x86_avx512))]
 #![cfg_attr(docsrs, feature(doc_auto_cfg))]
 #![allow(
     clippy::float_cmp,

Can you test this?

Aside: simd_support as a whole is experimental and unstable. We could add an stdarch_x86_avx512 feature flag to rand, but I don't think there's much point.

[serendipity-crypto]

Thank you, this fix has allowed my code to compile successfully without encountering the previous errors.

[dhardy]

Cool. Send a PR and I'll merge it. We don't need a CHANGELOG entry.

I'm investigating whether or not we should use run-time detection for CPU features, but that's another matter.

[dhardy]

(Fixed in #1551.)