Rust's char is not guaranteed to be ABI-compatible with C++'s uint32_t

[anforowicz]

Today cbindgen translates Rust's char into C++'s uint32_t (see here). (It used to translate into char32_t, but this was changed in a PR a few years ago.) This seems to work, but AFAIU Rust makes no guarantees wrt ABI behavior of Rust's built-in char type. In particular, the ABI may ask to use different registers depending on whether the type is classified as an integer or not (and Rust doesn't guarantee any particular classification of Rust's char).

Other notes:

• improper_ctypes warning fires for char (see example Rust playground)
• System V ABI documentation can be found here, but a Stack Overflow answer here provides a shorter and possibly easier-to-understand summary.
• AFAIK cxx doesn't support the char type
• Other FFI tools that do support char may also have the same bug (e.g. Crubit and Diplomat)

/cc @Manishearth who has kindly brought this issue to my attention