[msl-out] atomic CompareExchange is not implemented

[guusw]

Doesn't seem like there's any tracking issue for this currently, I just encountered this while trying to use atomicCompareExchangeWeak

[0216 14:17:07.454] [wgpu] [error] Device::create_render_pipeline error: Internal error in ShaderStages(VERTEX) shader: MSL: FeatureNotImplemented("atomic CompareExchange")
[0216 14:17:07.454] [wgpu] [error] Shader translation error for stage ShaderStages(VERTEX): MSL: FeatureNotImplemented("atomic CompareExchange")
[0216 14:17:07.454] [wgpu] [error] Please report it to https://github.com/gfx-rs/wgpu
[0216 14:17:07.454] [wgpu] [error] Validation Error

Caused by:
    In wgpuDeviceCreateRenderPipeline
    Internal error in ShaderStages(VERTEX) shader: MSL: FeatureNotImplemented("atomic CompareExchange")
 (1)

[teoxoy]

#4364 tracks this but is more like a meta issue.

[guusw]

For now I implemented a working version for metal for my project using wgpu here:
shards-lang@5f43672

Just leaving that here for anyone looking for a temporary workaround.

[teoxoy]

Would you be open to start a PR to add it upstream?

[guusw]

Yeah I'm considering it once I have time to clean up that commit.

[9291Sam]

Any progress on a PR?

[9291Sam]

I'm going to start work on this.