Data kept alongside the actor internal state

Due to the borrowing approach, the actor state is split into two parts. The first part is outside the actor cell, and is accessible to any runtime call that has an actor reference:

• Weak reference count
• Strong reference count
• Actor state: Prep, Ready, Zombie
• Termination notifier Ret<StopCause>
• A Deferrer instance

The Deferrer is required in order to support dropping the last ActorOwn. It is also used for Fwd and Ret instances calling the actor, and to support call! when only the actor is mentioned. It also means that any drop handler that has access to an actor reference also has a Deferrer available.

Note that Rc is not used to handle the weak and strong references, because we need to keep some data outside the cell that is accessible to a weak reference even if the strong reference count has gone to zero. Also we need to be able to terminate the actor even when there are still strong references.

The second part is inside the actor cell, and so is only accessible when there is a &mut Stakker reference available. So this is not accessible within calls to other actors, where the &mut Stakker reference is occupied by the borrow that enables access to the actor cell for that call. The actor cell contains:

• For Prep: FnOnce queue to store calls attempted before the actor is Ready
• For Ready: Self value for the actor