In C++ you don't have the scenario like in Rust, where one is forced to use a specific async runtime for library xyz, because it depends on having tokio as runtime.
In Rust you only are forced to use a specific runtime if you want to use its API. For example to spawn new tasks, or to block on a future. I believe that would be the same in C++.
In Rust, you don't need to use a specific runtime if you just want to use async function in your library.
Or has that situation been sorted out by now?