Support associated methods

[dtolnay]

Currently only freestanding functions are supported by this crate, not methods.

I like the approach of using self in the foreign function parameter list to signify a method receiver:

#[cxx::bridge]
mod ffi {
    extern "C" {
        type C;
        fn f(self: &C);
    }

    extern "Rust" {
        type R;
        fn g(self: &R);
    }
}

struct R {...}

impl R {
    fn g(&self) {...}
}

This would expose the methods r.g() to C++ and c.f() to Rust.

The above syntax would require rustc 1.43 or newer for rust-lang/rust#68728.

[myronahn]

Would it make sense to use the &self shorthand instead of self: &C?

[dtolnay]

That would be fine if there is only one extern type in the block.

[dtolnay]

Implemented in #121.

I filed #124 to follow up on &self and &mut self shorthand inside of extern blocks that contain exactly one extern type.