Build fails on nightly-2024-02-05

[leighmcculloch]

Builds using curve25519-dalek started to fail on nightly-2024-02-05. These builds passed on nightly-2024-02-03.

Here's an extract of the build error, from a GitHub Actions CI run where the full build is captured here.

error[E0658]: use of unstable library feature 'stdarch_x86_avx512'
  --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/curve25519-dalek-4.1.1/src/backend/vector/ifma/field.rs:26:5
   |
26 |     _mm256_madd52lo_epu64(z.into(), x.into(), y.into()).into()
   |     ^^^^^^^^^^^^^^^^^^^^^
   |
   = 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   = note: this compiler was built on 2024-02-05; consider upgrading it if it is out of date

error[E0658]: use of unstable library feature 'stdarch_x86_avx512'
  --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/curve25519-dalek-4.1.1/src/backend/vector/ifma/field.rs:25:9
   |
25 |     use core::arch::x86_64::_mm256_madd52lo_epu64;
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = 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   = note: this compiler was built on 2024-02-05; consider upgrading it if it is out of date

error[E0658]: use of unstable library feature 'stdarch_x86_avx512'
  --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/curve25519-dalek-4.1.1/src/backend/vector/ifma/field.rs:34:5
   |
34 |     _mm256_madd52hi_epu64(z.into(), x.into(), y.into()).into()
   |     ^^^^^^^^^^^^^^^^^^^^^
   |
   = 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   = note: this compiler was built on 2024-02-05; consider upgrading it if it is out of date

error[E0658]: use of unstable library feature 'stdarch_x86_avx512'
  --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/curve25519-dalek-4.1.1/src/backend/vector/ifma/field.rs:33:9
   |
33 |     use core::arch::x86_64::_mm256_madd52hi_epu64;
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = 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 2024-02-05; consider upgrading it if it is out of date

error[E0635]: unknown feature `stdsimd`
  --> /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/curve25519-dalek-4.1.1/src/lib.rs:13:70
   |
13 | #![cfg_attr(all(curve25519_dalek_backend = "simd", nightly), feature(stdsimd))]
   |                                                                      ^^^^^^^

Some errors have detailed explanations: E0635, E0658.
For more information about an error, try `rustc --explain E0635`.
error: could not compile `curve25519-dalek` (lib) due to 5 previous errors

[i18nsite]

same error on mac x86

[jimmygchen]

I've submitted a PR to fix this: #619