Outputting integral type from fragment shader causes Vulkan validation error

[Wumpf]

Validation layer trips off since Naga emits flat qualifier for integer outputs of fragment shader.
Note that wgpu correctly forces output to be an integer type when writing to a "true" (non-snorm/unorm) target.

Happens with a shader like this:

@fragment
fn fs_main_outline_mask(in: VertexOut) -> @location(0) vec2<u32> {
    return batch.outline_mask;
}

More details in this comment rerun-io/rerun#1532 (comment)

Not familiar with Naga's code, but it looks like there's a check missing here for fragment outputs https://github.com/gfx-rs/naga/blob/master/src/back/spv/writer.rs#L1415

[teoxoy]

Thanks for the report!

Not familiar with Naga's code, but it looks like there's a check missing here for fragment outputs https://github.com/gfx-rs/naga/blob/master/src/back/spv/writer.rs#L1415

Indeed, adding the check there should fix it; as we seem to handle VUID-StandaloneSpirv-Flat-06202 but not VUID-StandaloneSpirv-Flat-06201.

A PR would be welcome!