Rust's stability story should apply to bool in FFI

[hsivonen]

@matklad

I am not really an expert in FFI, but could we, instead of bool is _Bool, guarantee that bool is a a single byte with two valid values 1, 0, which happens to be the representation of _Bool for most platforms?

This quite probably is exactly the thing being proposed, but the phrasing is a bit different:

Assuming that my current understanding that MSVC on Windows and gcc/clang on Linux/BSD on today-relevant CPUs represent _Bool that way is correct (I don't know my way around GCC and clang sources well enough to check), that would work for me.

@matklad

instead of tying Rust and C, we directly specify bool ABI, and it becomes your responsibility, as a programmer, to ensure that C's _Bool ABI matches Rust ABI for your platforms.

I think it would be terribly inefficient to actually leave it up to every programmer to research this, so even if the normative definition was as formulated above, I think it would be good for Rust documentation to say for which targets the definition matches C _Bool. Hopefully at present the answer is "on all supported targets".