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Cancellation commonly drops a future when its owner stops polling it. Local values are dropped, but external effects already performed are not automatically reversed. That makes cleanup automatic but means a future must be cancel-safe — losing a half-consumed buffer or a partially applied write between awaits is the classic bug.
Cancellation in async Rust commonly occurs when the owner drops a future, often after an awaited operation yields. The future's local values are dropped, which can release in-process resources such as a mutex guard. External effects are different: a write already accepted by a database or a message already sent is not rolled back by dropping the future. Even an operation still running in another task or service may continue. A timeout tells the caller it stopped waiting; it does not prove the work never happened.
Draw a two-step operation: save an order, then send a confirmation.
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