Structured caching in an actor

Using Swift
20

FWIW with SE-0420, we can now implement this using the latest toolchain:

func withCancellingContinuation<T>(
  isolation: isolated (any Actor)? = #isolation,
  operation: (CheckedContinuation<T, Error>) -> Void,
  onCancel handler: @Sendable () -> Void
) async throws -> T {
  try await withTaskCancellationHandler {
    try await withCheckedThrowingContinuation { continuation in
      _ = isolation
      operation(continuation)
    }
  } onCancel: {
    handler()
  }
}

Note: current version requires caller to explicitly pass #isolation:

try await withCancellingContinuation(isolation: #isolation) {
  self.assertIsolated()
  $0.resume()
} onCancel: {
  print("🐟")
}
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