[naga]: Switch off of LazyLock to once_cell::racy::OnceBox

[bushrat011899]

Connections

• Contributes to Support no_std where possible (specifically for wasm32v1-none) #6826

Description

• Added a custom alternative to LazyLock, RacyLock, which uses AtomicPtr internally. This type is a drop-in replacement for LazyLock, but it trades in blocking for potentially leaking the initialized value in cases where multiple threads attempt to initialize. The type is public but in a private module to prevent external use.
• Removed instances of LazyLock and replaced them with RacyLock.
• Removed hlsl_out, wgsl_out, and msl_out from the extern crate std; statement, decoupling those features from explicit std usage.

Testing

• CI

Squash or Rebase?

Squash

Checklist

• Run cargo fmt.
• Run taplo format. N/A
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry. N/A

Notes

This addition includes two unsafe blocks in order to dereference the pointer stored within an internal AtomicPtr. I considered this preferable to bringing in a 3rd party dependency, such as spin.

I also chose to not offer a feature to switch back to std::sync::LazyLock, since such a feature would increase Naga's complexity, and risk stagnating unsafe code.

[cwfitzgerald]

I think the idea of the change is fine, but I would rather not have the unsafe be our responsibility - once_cell::racy::OnceBox exists and is already in-tree - can we build our RacyLock on top of that, we can do it in entirely safe code?

[bushrat011899]

I think the idea of the change is fine, but I would rather not have the unsafe be our responsibility - once_cell::racy::OnceBox exists and is already in-tree - can we build our RacyLock on top of that, we can do it in entirely safe code?

Oh I didn't realise that was the case, and I wasn't familiar with OnceBox either! That's a much better fit, I'll update this PR later tonight to remove the unsafe code and use that instead.

[bushrat011899]

Switched to OnceBox as discussed, less code and no unsafe. Didn't even need to add the race feature haha

[cwfitzgerald]

Nits