That's a rare exception, and just a design choice of this particular library function. It had to intentionally implement a workaround integrated with the async runtime to survive normal cancellation. (BTW, the anti-cancellation workaround isn't compatible with Rust's temporary references, which can be painfully restrictive. When people say Rust's async sucks, they often actually mean `tokio:spawn()` made their life miserable).

Regular futures don't behave like this. They're passive, and can't force their owner to keep polling them, and can't prevent their owner from dropping them.

When a Future is dropped, it has only one chance to immediately do something before all of its memory is obliterated, and all of its inputs are invalidated. In practice, this requires immediately aborting all the work, as doing anything else would be either impossible (risking use-after-free bugs), or require special workarounds (e.g. io_uring can't work with the bare Future API, and requires an external drop-surviving buffer pool).