Naga mesh shader SPIR-V writer

[inner-daemons]

Connections
Works towards #7197
Builds on #8370

Description

Add a SPIR-V writer for mesh shaders

All major changes here are in the SPIR-V backend for naga and naga snapshots. Other "changes" are inherited from #8370

Testing

Mesh shader test WGSL is now written to SPIR-V as a snapshot. Mesh shader tests & example use naga-written spirv on vulkan backend.

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.

[Copilot]

Pull Request Overview

This PR adds WGSL mesh shader support to wgpu, enabling the use of mesh shaders on the Vulkan backend through naga's WGSL frontend and SPIR-V backend. The implementation introduces new shader stages (Task and Mesh), new built-ins, a task_payload address space, and updates the API to use a workgroup output variable approach instead of imperative setVertex/setPrimitive calls.

Key changes:

• Adds mesh shader capability validation in wgpu-core
• Implements WGSL parsing for mesh shader syntax including @mesh, @task, @payload, and @per_primitive attributes
• Adds SPIR-V backend support for mesh shader emission
• Removes GLSL compilation in favor of WGSL for Vulkan mesh shaders
• Updates documentation and adds comprehensive test coverage

Reviewed Changes

Copilot reviewed 39 out of 51 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
wgpu-core/src/device/mod.rs	Adds MESH_SHADER capability to validator based on feature flag
naga/src/ir/mod.rs	Adds new built-ins (VertexCount, Vertices, etc.) and restructures MeshStageInfo to include output_variable
naga/src/front/wgsl/parse/mod.rs	Implements parsing for mesh shader attributes and directives
naga/src/valid/interface.rs	Adds validation for mesh shader constraints and output types
naga/src/back/spv/writer.rs	Implements SPIR-V emission for mesh shaders including output variable setup
tests/tests/wgpu-gpu/mesh_shader/mod.rs	Replaces GLSL compilation with WGSL, removes MESH_DISABLED test
tests/tests/wgpu-gpu/mesh_shader/shader.wgsl	New WGSL test shader for mesh shaders
docs/api-specs/mesh_shading.md	Updates documentation to reflect new workgroup variable approach
CHANGELOG.md	Documents the new feature additions

Comments suppressed due to low confidence (1)

naga/src/back/spv/writer.rs:1

• Using NonMaxU32::new(0).unwrap() to create a null handle is fragile and unclear. Consider defining a constant like NULL_TYPE_HANDLE or using a more explicit pattern to represent invalid/placeholder handles.

use alloc::{string::String, vec, vec::Vec}; 

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