Oops, forgot to run cargo fmt before. Sorry about that, new commit now.

Hi again! Was feeling unpleased with this solution and wanted to go back to it.
Attempting to simply use if let instead of getting a string from the spirv macro.

// rust-gpu/crates/spirv-std/macros/src/lib.rs/spirv()
    // prepend with #[rust_gpu::spirv(..)]
    let attr: proc_macro2::TokenStream = attr.into();
    tokens.extend(quote! { #[cfg_attr(target_arch="spirv", rust_gpu::spirv(#attr))] });

-   if attr.to_string().trim() == "fragment" {
-       let item_clone = item.clone();
-       let input = syn::parse_macro_input!(item_clone as ItemFn);
-       if !matches!(input.vis, Visibility::Public(_)) {
-           panic!("The `spirv` macro can only be applied to public functions.");
-       }
-   }

+   if let Ok(item_fn) = syn::parse::<ItemFn>(item.clone()) {
+       if !matches!(item_fn.vis, Visibility::Public(_)) {
+           panic!("The `spirv` macro can only be applied to public functions.");
+       }
+   }

    let item: proc_macro2::TokenStream = item.into();
    for tt in item {

This works great! However, there are some functions instead of the spirv_std source that are not public. I don't know much about these functions, so I'd love to know if that was intentional, or if we can go ahead and make them public. I've pushed a new commit with those changes to be viewed. However, if the two function changes are not wanted, we can do it another way!

Oops, apparently I missed a lot more private functions. I can fix this in the morning if that's the way that the code owners want to go with it

So! As far as I can tell, nothing close to this error message shows up anywhere in the rust-gpu repo. However, the exact same error message showed up in another repo, SPIRV-Tools#2538. They fixed is by going from SPV_ENV_VULKAN_1_0 to SPV_ENV_UNIVERSAL_1_3 whatever that means. This makes me think that the error is probably on the vulkan or spirv side of things, rather than the rust-gpu side.

In addition to this, when I change functions from public to private in the rust-gpu examples, such as compute-shader or mouse-shader, the error I get when using cargo build -p [crate] is simply

error: function `main_fs` is never used
   --> examples/shaders/mouse-shader/src/lib.rs:140:4
    |
140 | fn main_fs(
    |    ^^^^^^^

which makes it seem even more to me like it isn't being checked on the rust side.
Going to keep investigating, but figured I'd share some results. Happy Thanksgiving all!