Code shared from the Rust Playground

playground.rs

extern fn foo() {}

extern "C" {
    // This doesn't cast `Some(foo)` to the correct type
    fn printf(fmt: *const u8, ...);
    
    // This works correctly
    //fn printf(fmt: *const u8, _: Option<extern fn()>);
}

fn main() {
    unsafe {
        // ERROR: printf(b"foo:%p\n\x00".as_ptr(), foo);
        printf(b"foo:%p\n\x00".as_ptr(), foo as extern fn());
        printf(b"foo:%p\n\x00".as_ptr(), Some(foo));
        printf(b"foo:%p\n\x00".as_ptr(), Some(foo as extern fn()));
    }
}