Fix nightly_portable_simd after LaneCount removal.

[anforowicz]

rust-lang/rust@0eaef59 (which included rust-lang/portable-simd#485) was merged on 2026-01-27 and removed core::simd::LaneCount. This bytemuck commit ensures that nightly_portable_simd feature of bytemuck can build with nightly Rust before and after the LaneCount change.

Fixes #343

[anforowicz]

Hmmm... it seems that https://doc.rust-lang.org/cargo/reference/features.html says that the dep:rustversion is only available starting with Rust 1.60. I am not sure what this PR should do to address this. Maybe making rustversion dependency unconditional is okay? Let me try that.

[anforowicz]

Hmmm... it seems that https://doc.rust-lang.org/cargo/reference/features.html says that the dep:rustversion is only available starting with Rust 1.60. I am not sure what this PR should do to address this. Maybe making rustversion dependency unconditional is okay? Let me try that.

Actually, in local testing it seems that just replacing dep:rustversion with rustversion works. (And this seems okay, because bytemuck's Cargo.toml doesn't declare a feature called rustversion, so there is no conflict.)

[Lokathor]

I can't get to this for a few hours but I will try to get to it tonight.

[anforowicz]

I can't get to this for a few hours but I will try to get to it tonight.

Thanks! FWIW Chromium's Rust toolchain roll should be unblocked at this point by https://crrev.com/c/7535942/2..3, so (at least from Chromium perspective) this PR isn't very urgent - totally fine to get to it in the next few days or so. (i.e. Chromium can temporarily carry a Chromium-specific patch for bytemuck and then absorb the long-term fix [e.g. from this PR] when a new version of bytemuck is released to crates.io.)

[Lokathor]

released in 1.25