Are we calling macOS "open" with a wrong argument type?

[RalfJung]

In this line, we are calling open64 (which on macOS is open renamed):

rust/src/libstd/sys/unix/fs.rs

Line 706 in de27cd7

let fd = cvt_r(|| unsafe { open64(path.as_ptr(), flags, opts.mode as c_int) })?;

As third argument, we are passing something of type c_int. However, according to this document, the type should be mode_t instead, and on macOS these types do not seem identical -- the libc crate has

src/unix/bsd/apple/mod.rs
10:pub type mode_t = u16;

Seems like we are using the wrong argument type here?
(open is variadic so the compiler cannot check this. But Miri complained when I started to check argument sizes.)

[tmiasko]

The types are correct. In C, the default argument promotions are performed on trailing arguments.

[RalfJung]

In C, the default argument promotions are performed on trailing arguments.

I don't know what that means, but the man page explicitly says mode_t -- so is the man page wrong then?

[hanna-kruppe]

The third argument is passed as variadic argument, so the man page is not saying the function has signature open(const char*, int, mode_t), just that C programmers should use mode_t for representing the mode if one is needed -- presumably not just w.r.t. open but more generally in all APIs. But at ABI level, the third argument is passed as int because of the C rule that @tmiasko alludes to.

[RalfJung]

So would a comment like this be appropriate? #71921