Add CxxChar to bind to C++ char32_t type

[edsrzf]

This makes it possible to implement functions in Rust that accept a char32_t. The CxxChar type is necessary because C++ does not guarantee that char32_t contains a valid Unicode scalar value.

There is not yet a way for C++ code to create a Rust char.

I'll update the docs after getting feedback on the design and implementation.

Partly addresses #592.

[edsrzf]

The problem with using straight u32 for the Rust type is that if we do this:

fn rust_fn(ch: u32) {}

#[cxx::bridge]
pub mod ffi {
    extern "Rust" {
        fn rust_fn(ch: u32);
    }
}

...then the generated C++ declaration will look something like this:

void rust_fn(::std::uint32_t ch) noexcept;

...and uint32_t and char32_t are distinct C++ types, mangled differently. (Reference link for the IA64 ABI followed by gcc and clang.)

It's not specifically important to me to provide a wrapper type, but it is important to me to be able to generate C++ function signatures compatible with char32_t. If there's a better way to accomplish this, then I'm all ears.

[edsrzf]

So just to be clear, would you expect to be able to write:

fn rust_fn(ch: u32) {}

#[cxx::bridge]
pub mod ffi {
    extern "Rust" {
        fn rust_fn(ch: char32_t);
    }
}

...and then generate a C++ declaration using char32_t?

[dtolnay]

Right. That is how the standard library and libc do C integer types.

https://doc.rust-lang.org/1.50.0/std/os/raw/index.html https://docs.rs/libc/0.2.91/libc/#types

Canonically the rust_fn implementation would be written using cxx::num::char32_t though.