Rust libraries can have implementations for each async runtime and then you can pick between them using features. For example, when using sqlx with tokio I would have this in my Cargo.toml:

    [dependencies]
    sqlx = { version = "0.5", features = [ "runtime-tokio-rustls", "sqlite", "migrate" ] }

But I also could use async-std with:

    [dependencies]
    sqlx = { version = "0.5", features = [ "runtime-async-std-rustls", "sqlite", "migrate" ] }

So you should be able to get all your deps on a single runtime.

In practice both major runtimes have long-term stability guarantees (e.g. tokio has committed to maintained 1.0 for at least 5 years), so if you use libraries compatible with Tokio 1.0 then you're unlikely to have issues with this for some time.

Tokio 1 is the only async runtime used in production at scale, there's very little reason to use anything else. So you can seek out libraries that use tokio 1 and ignore anything else.

I haven't used much Python recently, but iirc you can just import your own runtimes, too. Twisted, gevent, that sort of thing. Having some sort of sane defaults bundled gives you a really nice baseline for interop, but doesn't preclude you from picking things that fit your use case better.

Definitely feels like one place where Rust kinda dropped the ball, at least from a user perspective in $CURRENTYEAR.

True, what I meant is that asyncio is part of the interpreter so you are very unlikely to have trouble with incompatible versions of asyncio. Twisted and gevent don't use async/await, but there's Trio which does and is saner than asyncio, but thankfully library authors aren't forcing it's usage. It's also possible to write libraries that use async but you bring your own runtime (trio or asyncio) with AnyIO.

It's not possible to have multiple 1.x.y versions of the same crate in your project, so you would need a really old library that depends on Tokio 0.2.x for that to happen. This isn't something that normally comes up in practice.