fix(msl-out): use namer for <fun>{Input,Output} structs

[ErichDonGubler]

Connections

• This appears to be a long-standing issue in the Metal backend.
• Discovered with bug 1870208, comment 3, which had the audacity to use uses the names vs for an entrypoint with a local variable called vsOutput.
• Resolves struct <fun>Output conflicts with fn <fun> on Metal #6439.

Description

Names generated for <fun>{Input,Output} structures in the Metal backend are not escaped using the namer API. This means the naming does not avoid conflicts with shader-provided identifiers that may use the same names.

Fix this by using the namer API.

Testing

• One can reproduce this issue by using the following shader in a render pipeline:

     struct OurVertexShaderOutput {
       @builtin(position) position: vec4f,
       @location(0) texcoord: vec2f,
     };
   
     @vertex fn vs(
       @location(0) xy: vec2f
     ) -> OurVertexShaderOutput {
       var vsOutput: OurVertexShaderOutput;
       vsOutput.position = vec4f(xy, 0.0, 1.0);
       return vsOutput;
     }
   
     @fragment fn fs() -> @location(0) vec4f {
       return vec4f(1.0, 0.0, 0.0, 1.0);
     }

  When the pipeline is created, this (rather confusing) Metal compiler error is emitted:

   Uncaptured WebGPU error: Internal error in ShaderStages(VERTEX) shader: Metal: program_source:44:12: error: no viable conversion from returned value of type 'OurVertexShaderOutput' to function return type 'vsOutput'
       return vsOutput { _tmp.position, _tmp.texcoord };
              ^~~~~~~~
   program_source:25:8: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'OurVertexShaderOutput' to 'const vsOutput &' for 1st argument
   struct vsOutput {
          ^
   program_source:25:8: note: candidate c
  

• A regression test has been added using the above case, whose snapshot output is adjusted with this fix.

• I've consumed this change in Firefox, and validated that the issue noted at https://webgpufundamentals.org/webgpu/lessons/webgpu-fundamentals.html#drawing-triangles-to-textures is fixed.

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[teoxoy]

Nice!

[ErichDonGubler]

I didn't delve into naga-repo-era history, but it seems the most recent author to touch the code in question is @jimblandy, and his changes were mostly moving the code around. Tagging @gfx-rs/naga for awareness: let's be more suspicious of identifiers that don't go through the namer.