Documentation unclear with Fn trait on extern "C" function pointers

[MarkMcCaskey]

The relevant docs are here and here in the repo:

In addition, function pointers of any signature, ABI, or safety are Copy, and all safe function pointers implement Fn, FnMut, and FnOnce. This works because these traits are specially known to the compiler.

The code:

extern "C" fn abc(x: u32) {
    println!("Hello, {}", x);
}

fn do_it<F: Fn(u32)>(f: F, n: u32) {
    f(n)
}

fn main() {
    do_it(abc, 5);
}

Does not compile even though abc can be called without unsafe, which the docs seem to suggest is sufficient for this to work.

Rust playground demonstrating the issue

I'd be happy to update the docs, but unfortunately I think I don't understand why this doesn't work well enough to do so.

[josephlr]

See #40710

[hkBst]

#40710 now works, but this case still doesn't, so there is some difference.