If an API leaks memory, then I’d like it to be deemed unsafe. That way, leaking a future would be unsafe, so the borrow checker could infer (transitively) that freeing the future means that any references it had are now dead (as it can already infer when a synchronous function call pops returns).

Am I missing something subtle?

Edit: Rc with cycles would be a problem. I rarely intentionally use Rc though (certainly less often than I create a future).

Edit 2: maybe an auto trait could statically disallow Rc cycles somehow?