How do you encode the locking issue in the type system, it seems magical? Can you just never hold any locks when calling await, is it smart enough to know that this scheduler might move work between threads?
More the latter. The lock guard is not `Send`, so holding it across the await point makes the `impl Future` returned by the async function also not `Send`. Therefore it can't be passed to a scheduler which does work stealing, since that scheduler is typed to require futures to be `Send`.