Initial precompiled shaders implementation

[SupaMaggie70Incorporated]

Connections
Works towards #3103
Depends on #7831

Also, as this is the start of a big change, I'd like to ping @cwfitzgerald to at least make sure I haven't gone off in the wrong direction.

Description
This adds a CreateShaderModuleDescriptorPassthrough::Generic enum variant which contains code for multiple types of shader source,allows creating passthrough shaders without writing backend specific code (if on metal pass MSL, etc). This variant also includes an optional reflection thing. For now, I don't know exactly what reflection info is needed, but if possible, we should make sure this can live in wgpu-types or wgpu-core to avoid dependency on naga. It seems the best model for this is the wgpu_core::validation::Interface, we might just have to replace some of the naga types.

Using this requires enabling the EXPERIMENTAL_PRECOMPILED_SHADERS feature. This feature is only supported on DX12, Vulkan, and Metal. Logic on these backends is otherwise identical to the respective specific passthrough methods.

Nothing is currently done with the reflection info.

An overview of my approach can be found in this comment. If this process takes longer than expected we can make a tracking issue. For now I will be posting updates by editing that comment and referencing it in PRs.

Testing
No testing yet. However, the code seems somewhat small and robust.

Squash pls

Checklist

• Run cargo fmt.
• Run taplo format.
• 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.

[cwfitzgerald]

Hmm, I think in general I like the ideas put forward in your plan you wrote up, that makes sense to me. As for the code changes themselves, I don't think that we need another variant internally - this feels redundant when all the other variants exist. Maybe what would need to be added is an optional reflection info to each of the variants?

[SupaMaggie70Incorporated]

@cwfitzgerald The purpose of this variant is so that the user can pass a single shader without considering backend-specific code. This way we can also later on add a macro that creates this struct, filling fields for all the backends that are supported, and it doesn't need to be passed a Backend parameter. I can change the approach if you want, but it might make everything a little less clean from the user's point of view.

[cwfitzgerald]

Yeah, I agree that it makes sense in wgpu's api but wgpu-core and wgpu-hal should be able to use the original ones?

[SupaMaggie70Incorporated]

Yeah, I agree that it makes sense in wgpu's api but wgpu-core and wgpu-hal should be able to use the original ones?

True, didn't think about that to be honest. I'll rewrite that part of this PR

[jimblandy]

@cwfitzgerald When you're back - this has been waiting for a while.

[DJMcNab]

I'm really looking forward to this! Just some things which popped up!

[SupaMaggie70Incorporated]

@DJMcNab Thanks for the review! Upon further inspection it seems my PR was not super well thought through or checked, so I'll have another look over everything myself in addition to addressing your comments.

[SupaMaggie70Incorporated]

@DJMcNab I've just given this PR a final look and it looks good. Some of your comments I left open as I wasn't sure how to handle them. Let me know what you think, you get the final say of course!

Also pinging @cwfitzgerald because this hopefully can be merged soon

Another note: the only backend not currently supported is OpenGL. Once we get that done, maybe we should consider removing this as a feature? I'm not sure how that would interact with e.g. custom backends so I'll wait until later to inquire about that.

[DJMcNab]

I'm not planning to review this again - it needs to be reviewed by someone with write access, and I was providing a "first pass" review to help speed this along, but I can't give a fully "in-context" review.

Thanks for actioning the review feedback so well! I've found it easy to submit PRs which don't hold up when given a second reading myself as well - that's what review is for, after all!

[cwfitzgerald]

A few small comments, but this is exactly what I was looking for!

[cwfitzgerald]

This should be a major changes with a small things showing off the diff and the new API

[SupaMaggie70Incorporated]

I made a new changelog entry, let me know if you want more changes to it

[SupaMaggie70Incorporated]

Alright @cwfitzgerald, I think I've addressed most of the recent changes (and also updated the PR to latest trunk). If there's anything more let me know.