Shared Type Question

[scott-wilson]

Hey, this may not be the right direction to go, but feel free to correct me.

I am wrapping a C++ library, and they've defined a struct like this:

struct Thing {
    unsigned char someenum;
}

I want to be able to directly access the struct with a Rust struct like this:

enum SomeEnum {
    A,
    B,
    C
}
struct Thing {
    someenum: SomeEnum,
}

As a test, I've tried doing something like this:

#[cxx::bridge]
mod ffi {
    enum SomeEnum {
        A,
        B,
        C,
    }

    struct Thing {
        someenum: SomeEnum,
    }
}

unsafe impl cxx::ExternType for ffi::Thing {
    type Id = cxx::type_id!("Thing");
}

But I'm probably misunderstanding how ExternType works.

Either way, is this (or something like this) possible to do in cxx currently (or will be possible in the near future)?

Thanks!

[dtolnay]

Shared structs produce both a Rust definition and a C++ definition. If a C++ definition already exists, you can't use a shared struct for the same thing, but you can use an extern C++ type.

#[repr(u8)]
pub enum SomeEnum {
    A,
    B,
    C,
}

#[repr(C)]
pub struct Thing {
    someenum: SomeEnum,
}

unsafe impl cxx::ExternType for Thing {
    type Id = cxx::type_id!("Thing");
}

#[cxx::bridge]
mod ffi {
    extern "C" {
        type Thing = crate::Thing;
    }
}

[scott-wilson]

That did the trick! Also, this is probably out of scope, but it looks like when rustfmt runs, the type Thing = crate::Thing; gets converted to type Thing;. Do you know the rustfmt rule causing that to happen, or have you run into this and know of a solution to prevent that from happening? I can create a minimal example project to demo this.

[dtolnay]

That is a rustfmt bug rust-lang/rustfmt#4159 which was fixed in rust-lang/rustfmt#4164 but has not been released yet (:slightly_frowning_face: for a long time). I've opened rust-lang/rustfmt#4447 to backport the fix to the current released version of rustfmt.

For now you can either switch to rustfmt v2.0.0-rc.2 which contains the fix, or use #[rustfmt::skip].

[scott-wilson]

I'll use the skip. Thanks!