Bonus points if it has the ability for users to define static analysis a la borrow checking.
Async makes things possible that are hard or impossible to do with synch programming.
It’s a real game changer for python especially. Cant comment on rust, hopefully its implementation is smooth.
Python is so slow you gain nothing with async. I have a plenty of cool stories how I've fixed crumbled products because they were async and all connection load instead of nginx/backend goes directly into db.
You rarely need long connections and if you choose async for python because it's fast then it's a wrong choice because it's not.
BTW asyncio is awful bloated implementation with huge overhead around task management.