WGSL parsing for mesh shaders

[inner-daemons]

Connections
#7197

Description
Adds WGSL parsing for mesh shaders to naga.

This PR is quite a bit smaller than it looks, just a few hundred lines. The rest of the "changes" come from snapshots.

Testing
There are mesh shader tests (that don't actually write to any backend)

Also, #8456 proves that this is enough information to write proper SPIR-V shaders

Squash or Rebase?
Squash

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.

[inner-daemons]

@cwfitzgerald This PR still needs some cleanup, in terms of making the code easier to read and updating the spec, but I think this represents what I imagine the approach to mesh shaders & WGSL would look like (see naga/tests/in/wgsl/mesh-shader.wgsl). So if I could get a preliminary overview kind of review soon-ish that would be appreciated, but of course not 100% necessary :)

[cwfitzgerald]

No promises before the 12th ofc but I should be able to get to it before then

[jimblandy]

I'm encouraged by the way this concentrates almost all the validation work in valid/interface.rs, the other valid submodules have lots of delicious deletions.

[Copilot]

Pull Request Overview

This PR adds WGSL parsing support for mesh shaders to Naga, implementing a new approach where mesh shader outputs are written to workgroup variables rather than through function calls. The changes enable parsing of @task and @mesh entry points, new builtins (@builtin(vertices), @builtin(primitives), etc.), the task_payload address space, and the @per_primitive attribute.

Key changes:

• New mesh shader builtins (VertexCount, Vertices, PrimitiveCount, Primitives) for workgroup variable fields
• New TaskPayload address space for inter-stage communication
• Removal of MeshFunction statement type in favor of direct workgroup variable access
• Updated validation to analyze mesh output variable structure
• Frontend support for @task, @mesh, and @payload attributes

Reviewed Changes

Copilot reviewed 31 out of 39 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
naga/src/ir/mod.rs	Adds new BuiltIn variants and modifies MeshStageInfo to include output_variable field
naga/src/valid/interface.rs	Adds validation for mesh output variables and capability checks for new builtins
naga/src/proc/mod.rs	Adds analyze_mesh_shader_info function to extract mesh info from workgroup variables
naga/src/front/wgsl/parse/mod.rs	Adds parsing for @task, @mesh, @payload attributes and @per_primitive decorator
naga/src/front/wgsl/parse/directive/enable_extension.rs	Adds MeshShader enable extension
naga/src/valid/analyzer.rs	Removes FunctionMeshShaderInfo and related mesh function analysis
naga/tests/in/wgsl/mesh-shader.wgsl	Adds test input for mesh shader parsing
docs/api-specs/mesh_shading.md	Updates specification to reflect workgroup variable-based approach
Multiple backend files	Updates to handle new builtins with unreachable!() for unsupported cases

Comments suppressed due to low confidence (1)

naga/src/ir/mod.rs:2587

• Corrected spelling: 'renderedas' should be 'rendered as' in the doc comment.

    Lines,

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.