Support x86 AVX-12 SIMD types without `simd-nightly`

[joshlf]

As of Rust 1.89.0, the stdarch_x86_avx512 feature has been stabilized, and so we no longer need the simd-nightly feature in order to support the types gated by that feature.

A first attempt at this is in #2574, but it runs into an issue:

• We add version detection for 1.89.0; specifically, we add the following entry to Cargo.toml's [package.metadata.build-rs] table: zerocopy-simd-x86-avx12-1-89-0 = "1.89.0"
• We add zerocopy-simd-x86-avx12-1-89-0 to the list of toolchains in our GitHub CI build matrix
• Since (as of this writing, 2025-06-10) 1.89.0 has not been released stably, CI fails when trying to install 1.89.0 (e.g. see this failure)

Thus, our plan is the following:

• Temporarily add in Expect stable_features lint for stable SIMD types #2584
  • #![cfg_attr(all(feature = "simd-nightly", target_arch = "x86"), expect(stable_features))]
    • This will have the effect of alerting us if stabilization is rolled back before 1.89.0 is stabilized
• Once 1.89.0 is released as stable, land Support x86 AVX-12 SIMD types on 1.89.0 stable #2574

[jswrenn]

You can improve on this slightly by using expect(stable_features) instead, since that will ensure we'll get a CI failure if stabilization is rolled back.

[joshlf]

You can improve on this slightly by using expect(stable_features) instead, since that will ensure we'll get a CI failure if stabilization is rolled back.

I believe we'd want to only enable that in CI, right? (Could be dangerous/brittle if we use it when customers are building.) But yeah, agreed.

[jswrenn]

Cargo caps lints. This is why it's safe for us to deny clippy warnings without breaking our dependents when they run clippy.

[joshlf]

Cargo caps lints. This is why it's safe for us to deny clippy warnings without breaking our dependents when they run clippy.

And that's true of expect as well?