[ Feature Request ] Resolve trait associated types

[sphaerophoria]

I've written some derive code that generates the equivalent C api structs for me. After running cargo expand the structs look like

    pub struct Notification {
        pub color: Color,
        pub message: String,
        pub message_format: MessageFormat,
    }
    #[repr(C)]
    pub struct CNotification {
        pub color: <Color as CRepr>::Output,
        pub message: <String as CRepr>::Output,
        pub message_format: <MessageFormat as CRepr>::Output,
    }
    impl CRepr for Notification {
        type Output = CNotification;
        fn to_c_repr(&self) -> Self::Output {
            CNotification{color: self.color.to_c_repr(),
                          message: self.message.to_c_repr(),
                          message_format: self.message_format.to_c_repr(),}
        }
    }
}

Unfortunately there is currently no way for me to automatically generate the equivalent C header bindings as no bindings generator can currently handle resolving what ::Output means. Currently I've worked around this by generating type names for each associated type but it feels ugly to do it that way, and my generated C headers get a lot of extra typedefs.

It would be nice if the bindings generator could resolve associated types like it does regular types.

If you have an idea of where to start I don't mind taking a stab at it

[sjeohp-zz]

I am also interested in this. Does anyone have an idea of how feasible it is?

e.g. I have various functions like

pub extern "C" fn some_func() -> <Option<bool> as Return<'static>>::Ext

which currently generate headers like

Ext some_func();

[emilio]

cbindgen doesn't hook into the rust compiler so it doesn't seem particularly easy to do it in the general case.

[mversic]

I require this feature badly. After having written most of this library for generating FFI out of existing rust structures I come here to find that generating C bindings would prove more of a challenge. My problem is twofold:

1. cbindgen doesn't expand macros (my extern fns are macro generated):
I seem to be able to sidestep this issue with cargo expand. As far as I was able to see cbindgen didn't have any problem with parsing the generated file
2. cbindgen doesn't resolve associated types (the library I mentioned relies heavily on associated types):
seems to me that I should be able to use RUSTFLAGS="--emit mir" cargo build(which resolves associated types) and integrate that output with cbindgen.

Do you find this approach feasible and do you think that 2. be integrated with cbindgen?

[orangeng]

Please assign me to this issue. I have already almost finished the implementation of the solution. Thank you