WakerPtr {
 ptr: AtomicPtr<Waker>,
}

impl WakerPtr {
 const fn new() -> Self { Self { ptr: AtomicPtr::new(null_ptr()) } }

 fn swap(&self, b: Box<Waker>) -> Option<Box<Waker>> {
  let new_waker = b.into_raw();
  let old = self.ptr.swap(new_waker);
  if old.is_null() {
   None
  } else {
   // SAFETY:
   // the pointer could be either `NULL` or valid,
   // and it is not `NULL` here.
   unsafe {
    Some(Box::from_raw(old))
   }
  }
 }
}

impl WakerPtr {
 // Sets the waker if it is unset.
 // Does nothing otherwise.
 fn set_once(&self, w: &Waker) {
  if self.ptr.load() == null_mut() {
   let new_waker = Box::into_raw(Box::new(w.clone()));
   if self.ptr.compare_exchange(null_mut(), new_waker).is_err() {
    // Some flaw in atomic fences, and another thread already set the waker.
    // SAFETY:
    // `new_waker` was *not* set, thus there's no owner anywhere and it is safe.
    unsafe { drop(Box::from_raw(new_waker)) }
   }
  }
 }
}