use std::os::raw::c_void;
extern {
    // context, data
    fn on_thing_happens(callback: extern fn(*const c_void, u32));
}

fn register<F: Fn(u32)>(f: F) {
    extern fn handler<F: Fn(u32)>(f_ptr: *const c_void, val: u32) {
        let f = unsafe { &*(f_ptr as *const F) };
        f(val)
    }
    unsafe {
        on_thing_happens(handler::<F>);
    }
}

fn main() {
    fn foo(a: u32) {
        println!("{}", a);
    }
    register(foo);
}