I would like to have a Bool returning API to support isolated deinit. Implementation of the isolated deinit uses swift_task_isCurrentExecutor() to choose between fast path (no hopping) and slow path (hopping).
I wonder what are other examples of SerialExecutor implementations, aside from the Dispatch-based. Would they benefit from having var isIsolated: Bool? in the SerialExecutor protocol, even if Dispatch-based implementation just returns nil?
But even for Dispatch I see an opportunity. If code passes expected.checkIsolated() check, then we could set Task.current.executor = expected for the duration of the block passed to assumeIsolated(). Then inside the block calls to swift_task_isCurrentExecutor(expected) would return true, enabling fast path of the isolated deinit.