Implement SPIR-V passthrough

[Shfty]

Note: This is an adoption of #3996, originally authored by @molikto

Objective

Allow use of wgpu::Features::SPIRV_SHADER_PASSTHROUGH and the corresponding wgpu::Device::create_shader_module_spirv for SPIR-V shader assets.

This enables use-cases where naga is not sufficient to load a given (valid) SPIR-V module, i.e. cases where naga lacks support for a given SPIR-V feature employed by a third-party codegen backend like rust-gpu.

Solution

• Reimplemented the changes from Spirv shader bypass #3996, on account of the original branch having been deleted.
• Documented the new spirv_shader_passthrough feature flag with the appropriate platform support context from wgpu's documentation.

Changelog

• Adds a spirv_shader_passthrough feature flag to the following crates:

  • bevy
  • bevy_internal
  • bevy_render

• Extends RenderDevice::create_shader_module with a conditional call to wgpu::Device::create_shader_module_spirv if spirv_shader_passthrough is enabled and wgpu::Features::SPIRV_SHADER_PASSTHROUGH is present for the current platform.

• Documents the relevant wgpu platform support in docs/cargo_features.md

[Shfty]

Also, I've removed spirv_shader_passthrough from the default feature in the bevy crate:

Passthrough is inherently unsafe in the Rust sense, so making it an explicit choice seems like the wiser call.

Plus, enabling it by default implicitly creates cases whereby supported platforms use passthrough, and unsupported ones use naga, as per the wgpu::Features check added during the original pull.

This is desirable behaviour from a compatibility standpoint, but has the potential to catch users off-guard if their shader behaves differently across passthrough / non-passthrough (i.e. by crashing their app with a panic when naga can't translate a SPIR-V module).

So, it seems better as an opt-in - that positions the existing naga setup as the compatibility baseline, with the option to drop down to unsafe passthrough land made available for projects that require it.

[tychedelia]

I can't test because I'm on macOS, but this makes sense to me and seems very useful for certain users. It's gated behind a feature so should be safe either way.

[NthTensor]

Yep, looks reasonable.

[github-actions]

You added a new feature but didn't update the readme. Please run cargo run -p build-templated-pages -- update features to update it, and commit the file change.

[alice-i-cecile]

Once this is passing CI I'll merge it in. Sorry for the very long wait!

[bas-ie]

Couldn't resist adopting an adoption! #15352

[alice-i-cecile]

Closing as adopted.