Naga transpiling to wgsl

[Firestar99]

Requires #71 and Rust-GPU/rust-gpu#264 (and base retarget)

pub fn main() -> Result<(), Box<dyn std::error::Error>> {
    let shader_crate = PathBuf::from("./shaders");

    // install the toolchain and build the `rustc_codegen_spirv` codegen backend with it
    let backend = cargo_gpu::Install::from_shader_crate(shader_crate.clone()).run()?;

    // build the shader crate
    let builder = SpirvBuilder {
        print_metadata: MetadataPrintout::DependencyOnly,
        spirv_metadata: SpirvMetadata::Full,
        ..backend.to_spirv_builder(shader_crate, "spirv-unknown-vulkan1.2")
    };
    let spv_result = builder.build()?;

    // transpile the spv binaries to wgsl
    let wgsl_result = spv_result
        .naga_transpile(Capabilities::empty())?
        .to_wgsl(WriterFlags::empty())?;
    let path_to_wgsl = wgsl_result.module.unwrap_single();

    // emit path to wgsl into env var, used in `quad.rs` like this:
    // > include_str!(env!("COLORBUBBLE_WGSL_SHADER_PATH"))
    println!(
        "cargo::rustc-env=COLORBUBBLE_WGSL_SHADER_PATH={}",
        path_to_wgsl.display()
    );

    // you could also generate some rust source code into the `std::env::var("OUT_DIR")` dir
    // and use `include!(concat!(env!("OUT_DIR"), "/shader_symbols.rs"));` to include it
    Ok(())
}

https://github.com/Firestar99/colorbubble/blob/eedd20ecc8155f9be15bc95b0fcd68b5002b94f3/build.rs

I've separated the transpiling into two steps: naga_transpile and to_wgsl. This would allow us to add more naga transpiling targets in the future, without having to reparse the spv again or cause major code breakage.

[tombh]

Great idea!

Just wondering if you knew about being able to base PRs off branches other than main? It's just a little change in the Github PR:

It's not at all necessary, just FYI. And it doesn't mean you have to actually merge into the new base branch (unless you want to of course), because you can always change the base branch back to main before the actual merge. That way you get a nice diff in the Github UI whilst it's being developed.

[Firestar99]

@tombh feel free to comment on the build script example in the description, or try out the linked project on branch cargo-gpu :D

[LegNeato]

I'm not sure we want to do this. Is there any way this can be pushed "down" into rust-gpu? It feels like surfacing unnecessary impl details. To users, the transpiling is basically part of the compilation step...except targeting something like wgsl-unknown-unknown