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.
How is this not more natural than creating various state machines and passing around all sorts of weird highly abstract Future-objects?
However, many real-world programs are inherently complicated state machines, where each individual state waits on one or more nested state machines to progress and/or complete -- async is often the most reasonable way to express that.
I wrote a post detailing a non-trivial production example use of async that might be interesting, regarding the cargo-nextest test runner that I wrote and maintain: https://sunshowers.io/posts/nextest-and-tokio/.
Nextest is not "web-scale" (c10k), since the number of tests one is concurrently running is usually bounded by the number of processor hyperthreads. So the fact that async tasks are more lightweight than OS threads doesn't have much bearing in this case. But even beyond that, being able to express state machines via async makes life so much simpler.
Over a year after switching nextest to using async, I have no regrets. The state machine has gotten around twice as complicated since then--for example, nextest now handles SIGTSTP and SIGCONT carefully--and async has coped admirably with it.
(There are currently somewhat serious issues with Rust async, such as a really messy cancellation story. But that's generally not what gets talked about on places like HN.)