proposal: consolidate libc::mode_t abstraction

[nyurik]

I was hacking on #7728, trying to get rid of numerous suspiscious u32 casts, and realized that many of them have to do with libc::mode_t alias. In most unix-like OS, it uses u32, but some use u16 and Redux uses i32, making this somewhat problematic.

Coreutils code constantly casts from mode_t to u32 and back. I think a better paradigm might be to introduce a dedicated type, probably based on bitflags? In general, a strongly typed flag type should be better than a generic u32 passed around. My only reservation is that maybe this should be part of libc itself?

See also rust-lang/libc#4404

[sylvestre]

yeah, makes sense!

[BenWiederhake]

Sounds like a good idea! Let's wait and see what libc does.

[tertsdiepraam]

I doubt libc will do that, because they have 0 dependencies and are just meant to be a raw wrapper. But the nix crate already has something like this: https://docs.rs/nix/latest/nix/sys/stat/struct.Mode.html. We should probably use that instead of raw libc in most cases.