[Feature Request] Allow the addition of custom Rust annotations to generated function definitions

[mstrydom]

We would like to wrap all our calls into the FFI boundary with a standard piece of code. In particular. we would like to use cee_scape to solve some long_jmp behaviour from the code that we are calling into. A nice way to be able to do this is to have an annotation on the generated FFI code from rust-bindgen. We can then use this proc macro to generate the wrappers that we want.

This seemed like something that other users of rust-bindgen could use, as it provides a lot of flexibility with, what I hope is, very little investment from rust-bindgen.

This issue is to see if the contributors of rust-bindgen would be interested in adding such a feature and what they would require to be added.

My view is that we allow add a setting that adds a custom annotation to the extern block, lets say extern_annotation("my_crate::my_annotation")

rust-bindgen would then generate all functions as:

[my_crate::my_annotation]
extern "C" {
    pub fn my_c_function(
        ctype: *const ::std::os::raw::c_char,
    ) -> ::std::os::raw::c_int;
}

After this point, a user of rust-bindgen can put what they want/need inside the annotation implementation. Something like:

pub fn my_annotation(_args: TokenStream, input: TokenStream) -> TokenStream {
    let input = parse_macro_input!(input as ItemForeignMod);

    quote! {
        fn <macro code to get wrapping function name> {
            #input

            unsafe { my_c_function(ctype) }
        }
    }
    .into()
}

I assume we would want to add include and exclude configuration in case there someone does not want to put the annotation on all functions. Or one could argue that they can implement exclusions in their macro, but its probably cleaner to add in rust-bindgen

If there is an existing way of doing this in rust-bindgen already that I just missed, please let me know.

I am more than happy to take a stab at producing a pull request for this, although I would appreciate some help in finding the right code.

Any thoughts/help would be appreciated.

[mstrydom]

I did just see this code: https://github.com/rust-lang/rust-bindgen/blob/main/bindgen/codegen/postprocessing/merge_extern_blocks.rs#L6 . Which looks like some merging of extern blocks happen. I am checking the version I have been using and I see I am on 0.63, so some of the local behaviour I assumed might have changed in later versions.

[emilio]

We have already a wasm_import_module_name option that adds such an attribute where you want, afaict. Extending / generalizing that seems better. Given we merge the blocks I think it'd be simpler if we didn't add extra exclusion logic or what not.

[pvdrz]

this could also be seen as a generalization of the custom derive callback. Maybe it would make sense to introduce a callback to annotate any rust items with custom attributes and implement the existing callbacks on top of it instead.

[pvdrz]

We have already a wasm_import_module_name option that adds such an attribute where you want, afaict. Extending / generalizing that seems better. Given we merge the blocks I think it'd be simpler if we didn't add extra exclusion logic or what not.

I think younger me implemented the extern blocks merge so it only happens if the extern blocks are identical, we would only need to extend this notion of identical to include attributes as well.