Fix Location calculation for arrays in entrypoint interface

[Arc-blroth]

This PR fixes the calulation for Location decorations on array entrypoint parameters.

Both Vulkan (spec) and MLSL (page 44 of the spec) specify that array elements must take up consecutive locations, one for each element. Currently however, rust-gpu will always allocate 1 Location per entrypoint parameter, meaning that this code (playground link):

#[spirv(vertex)]
pub fn main_vs(
    data: Input<[f32; 4]>,
    more_data: Input<[f32; 4]>,
    mut out_data: Output<[f32; 4]>,
) {

will generate invalid SPIRV:

OpEntryPoint Vertex %1 "main_vs" %data %more_data %out_data
OpName %data "data"
OpName %more_data "more_data"
OpName %out_data "out_data"
OpDecorate %_arr_float_uint_4 ArrayStride 4
OpDecorate %data Location 0
OpDecorate %more_data Location 1
OpDecorate %out_data Location 0

since data and more_data have overlapping locations.

I'm not exactly that familiar with rustc and rustc_codegen_spirv, so there might be a better way to tell if an entrypoint parameter maps to a SPIRV array type than inspecting the generated type - please review :)

[XAMPPRocky]

Thank you for your PR! This looks good to me, can you also add two tests? One to ensure that the behaviour works, and one that generates the invalid entry param type error? You can see similar tests in tests/ui.

[Arc-blroth]

@eddyb pointed out that this PR is essentially blocked on #443 - I'll update this PR after that one is merged.

[Arc-blroth]

Sorry for taking so long, but I've updated to HEAD and added a test for fixing array locations. The CI seems to be failing because of commits that don't have any relationship to this PR, but I tested this PR and it works locally.

[XAMPPRocky]

LGTM!

[khyperia]

LGTM, just adding a note to our future selves that this is incomplete, the vulkan spec says:

64-bit three- and four-component vectors consume two consecutive locations.

so we'll need to fix that too eventually (not needed in this PR).

[khyperia]

Closing this for inactivity, if you'd like to revive this, please do!