I was trying to look for what channel I used back in the days I needed to write so called half-async code, but I can't find it anymore. If I remember correctly, at least the 0.1 version of futures had a channel with the other end being sync and the other async.

Today I'd maybe take a look into crossbeam and their queue implementations for this kind of synchronization.

https://docs.rs/crossbeam/0.8.0/crossbeam/