No deadlock reasoning:
- RwLock can block when reader and writer threads are contesting access
- When swap lock is acquired, we either have:
  - an exclusive write access from one of add, remove, or clear
  - a shared read access from call and one of add, remove, or clear
- Otherwise,
  - when a change lock is acquired, we have a single read access from one of add, remove, or clear
  - when no lock is acquired, there is no access
- Therefore delegates shouldn't deadlock

Assuming we have an exclusive access to the array

Assuming we have an exclusive access to the array

Assuming we have exclusive access to the array

use core::cell::UnsafeCell.

The `Option` that wraps `Arc<[Delegate<T>]>` will let us assign `None` instead of creating `Arc::new([])` with no space penalty.

Now we only need to allocate once for Arc

Should fail to compile if not

Co-authored-by: Kenny Kerr <[email protected]>