> The new write-up from OP is that you can "forget" a future (or just hold onto it longer than you meant to), in which case the code in the async function stops running but the destructors are NOT executed.

If you're relying for global correctness on some future being continuously polled, you should just be spawning async tasks instead. Then the runtime takes care of the polling for you, you can't just neglect it - unless the whole thread is blocked, which really shouldn't happen. "Futures" are intentionally a lower-level abstraction than "async runtime tasks".