WGSL/rasterization: How to properly decorate structs used in parameters to entry points with semantics

[brussig-tud]

I'm having trouble wrapping my head around how to add semantics to the fields in a struct which I want to output from a vertex shader and input into the fragment shader. My problem is that the generated WGSL code ends up not having proper @location attributes for each non-builtin field, so I am probably not doing it correctly. The documentation does seem to imply I don't have to do anything special (→WGSL specific functionalities):

Parameters without semantics are given automatic location indices. (See the https://github.com/location attribute.)

However, no location attributes are generated for any of my fields with non-builtin semantics. I figured that page might just contain outdated info so I tried adding semantics. I am really unsure how to do it correctly though, as documentation for this is very fragmented. From various examples, I pieced together that you can probably just invent any custom semantics yourself, giving it any name you want, and Slang just uses it as a marker of sorts to (in the case of WGSL) assign matching bind points. (If this is explained somewhere, I'd appreciate it if you can point me to the relevant part of the docs)
In any case, this is what I have:

struct ViewportVertex {
	float4 pos_clip : SV_Position;
	float2 uv : F_TexCoord;
}

/// Viewport-covering clip-space vertices
static const var vpVertices: ViewportVertex[] = {
	{float4(-1,-1, 0, 1), float2(0, 1)}, {float4(1,-1, 0, 1), float2(1, 1)},
	{float4(-1, 1, 0, 1), float2(0, 0)}, {float4(1, 1, 0, 1), float2(1, 0)}
};

/// Vertex shader entry point
[shader("vertex")]
func vertexMain (uint vertexID: SV_VertexID) -> ViewportVertex {
	return vpVertices[vertexID];
}

This however still results in WGSL code that does not specify any @location(..) for the uv field (same as if I didn't use custom semantics at all):

struct ViewportVertex_0
{
    @builtin(position) pos_clip_0 : vec4<f32>,
     uv_0 : vec2<f32>,
};

Consequently, both Chrome and Firefox complain about my entry point producing output without a binding, e.g. Chrome:

Error while parsing WGSL: :8:6 error: missing entry point IO attribute
     uv_0 : vec2<f32>,
     ^^^^

:20:1 note: while analyzing entry point 'vertexMain'
fn vertexMain(@builtin(vertex_index) vertexID_0 : u32) -> ViewportVertex_0
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

What am I doing wrong? How can I tell Slang to generate a binding for my uv field? For what it's worth, the same Slang code generates perfectly functional SPIR-V that I can use no problem with WGPU on desktop (Vulkan backend).

/Edit: Tested with Slang 2025.17.1

[brussig-tud]

I just noticed that the very sensible @location(0) is actually being created for my fragment shader:

[shader("fragment")]
func fragmentMain (vpIn: ViewportVertex) -> float4 {
	/* snip */
}

does result in the following WGSL:

struct pixelOutput_0
{
    @location(0) output_0 : vec4<f32>,
};

struct pixelInput_0
{
    @location(0) uv_2 : vec2<f32>,
};

@fragment
fn fragmentMain( _S2 : pixelInput_0, @builtin(position) pos_clip_2 : vec4<f32>) -> pixelOutput_0
/* snip */

I'm beginning to think I might have stumbled upon a bug?

[Devon7925]

I believe this is #8388. You can use an older version of slang until it's fixed.

[brussig-tud]

Yes, that must be it (sorry for having overlooked that there already is an existing issue for this). For the record, the correct link is #8388. Closing this as a duplicate of that issue.