Combines withTaskCancellationHandler with a CheckedContinuation.

TaskCancellationHandler.swift

import Synchronization

func untilCancelled() async {
	print("1")
	let mutex: Mutex<CheckedContinuation<Void, Never>?> = .init(nil)
	
	await withTaskCancellationHandler {
		print("2")
		await withCheckedContinuation { continuation in
			print("3")
			mutex.withLock {
				if Task.isCancelled {
					print("abort")
					continuation.resume()
					return
				}
				
				print("4")
				$0 = continuation
			}
		}
	} onCancel: {
		let continuation = mutex.withLock {
			print("5")
			return $0.take()
		}
		continuation?.resume()
	}
	
	print("6")
}

func main() async throws {
	let hangsForever = Task {
		await untilCancelled()
	}
	
	// Comment out this line to test early cancellation.
	try await Task.sleep(for: .milliseconds(10))
	
	print("will cancel")
	hangsForever.cancel()
	print("did cancel")
	
	try await Task.sleep(for: .milliseconds(10))
}

try await main()

But why this isn't safe, I think there is something I am missing but what is it 😅 ?

This type, as implemented, makes it possible to send non-Sendable data. That doesn't mean it actually will happen, just that the compiler will not be able to prevent it.

Got you, thanks for the explanation