Michael's Notes & Blog

Rust Async Notes

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.await ing on a Multithreaded Executor

Future may move between threads, so any variables used in async bodies must be able to travel between threads. So it is not safe to use Rc, RefCell or any other types that don't implement the Send trait, (it is possible to use these types so long as they aren't in scope during a call to .await.). Similarly, it isn't a good idea to hold a traditional non-futures-aware lock across an .await, as it can cause the threadpool to lock up: one task could take out a lock, .await and yield to the executor, allowing another task to attempt to take the lock and cause a deadlock. To avoid this, use the Mutex in futures::lock


  1. For pinned data, you have to maitain the invariant that its memory will not get invalidated or repurposed from the moment it gets pinned until when drop is called. so the question here is if I moved the pinned data, what can I expect? a compile error or a runtime error or undefined behavior? how can I do an experiment on this

  2. Pinning a !Unpin object to the stack requires unsafe, why?

  3. Getting a &mut T to a pinned T requires unsafe if T: !Unpin.

  4. If T: Unpin (which is the default), then Pin<'a, T> is entirely equivalent to &'a mut T. In other words: Unpin means it's OK for this type to be moved even when pinned, so Pin will have no effect on such a type .