A rust snippet to convert an anonymous function (or closure?) into an FFI-compatible callback

ffi-fn-registration.rs

use std::os::raw::c_void;
extern {
    fn register_callback(callback: extern fn(*const c_void, u32), user: *const c_void);
}

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 {
        register_callback(handler::<F>, Box::into_raw(Box::new(f)) as *const c_void);
    }
}