WGSL: Nested structures are not allowed for entry point IO

[aleino-nv]

This issue was split from #5173.

Affected tests under tests/compute (currently excluded):

1. compile-time-loop

Output from slang-test, for compile-time-loop:

...
WGPU error: Error while parsing WGSL: :38:6 error: nested structures cannot be used for entry point IO
     assembledVertex_0 : AssembledVertex_0,
     ^^^^^^^^^^^^^^^^^

:42:1 note: while analyzing entry point 'vertexMain'
fn vertexMain( input_0 : VertexStageInput_0) -> VertexStageOutput_0
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
{
^
    var output_0 : VertexStageOutput_0;
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    output_0.coarseVertex_0.color_0 = input_0.assembledVertex_0.color_1;
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
var _S2 : vec4<f32> = (((vec4<f32>(input_0.assembledVertex_0.position_0, 1.0f)) * (unpackStorage_0(Uniforms_0.modelViewProjection_0))));
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    output_0.sv_position_0 = _S2;
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    output_0.coarseVertex_0.uv_0 = input_0.assembledVertex_0.uv_1;
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    return output_0;
^^^^^^^^^^^^^^^^^^^^
}
^


 - While validating [ShaderModuleDescriptor]
 - While calling [Device].CreateShaderModule([ShaderModuleDescriptor]).
...

Generated WGSL for slangc -o test.wgsl -target wgsl -stage vertex -entry vertexMain %SLANG_SRC%\tests\compute\compile-time-loop.slang:

...
struct AssembledVertex_0
{
     position_0 : vec3<f32>,
     color_1 : vec3<f32>,
     uv_1 : vec2<f32>,
};

struct VertexStageInput_0
{
     assembledVertex_0 : AssembledVertex_0,
};

@vertex
fn vertexMain( input_0 : VertexStageInput_0) -> VertexStageOutput_0
{
    var output_0 : VertexStageOutput_0;
    output_0.coarseVertex_0.color_0 = input_0.assembledVertex_0.color_1;
    output_0.sv_position_0 = (((vec4<f32>(input_0.assembledVertex_0.position_0, 1.0f)) * (unpackStorage_0(Uniforms_0.modelViewProjection_0))));
    output_0.coarseVertex_0.uv_0 = input_0.assembledVertex_0.uv_1;
    return output_0;
}
...

[aleino-nv]

@csyonghe The GLSL output seems to get rid of this kind of nesting. Does a similar restriction apply to GLSL?

[aleino-nv]

I think I found he passage that forbids nesting of input structures
in the section on entry point declarations (https://www.w3.org/TR/WGSL/#entry-point-decl):

"The type of each formal parameter, and the entry point’s return type, must be one of: ... a structure whose member types are any of bool, numeric scalar, or numeric vector. ..."

[aleino-nv]

Maybe we can re-use the same legalization pass that's used for GLSL to fix this WGSL issue.