You also have to take into account that Rust has a borrow checker that makes opaque C calls 2nd class citizens anyway. Zig doesn't have that as far as I can tell.

So it means that even if Rust had a standard C header include directive it still wouldn't be completely transparent because most of the time you want to wrap around C APIs in Rust to provide safe, borrow checked and pointer-free alternatives. There's a different philosophy I think.