> does this work well with async or is it purely sync tail calls for now?

The current RFC generally does not allow `become` to be used with `async` for now [0]:

> Tail calling from async functions or async blocks is not allowed. This is due to the high implementation effort as it requires special handling for the async state machine. This restriction can be relaxed by a future RFC.

> Using `become` on a `.await` expression, such as `become f().await`, is also not allowed. This is because `become` requires a function call and `.await` is not a function call, but is a special construct.

> Note that tail calling async functions from sync code is possible but the return type for async functions is `impl Future`, which is unlikely to be interesting.

[0]: https://github.com/phi-go/rfcs/blob/guaranteed-tco/text/0000...