Comment by dminik

Sure. Let's do an imaginary scenario. Let's say that you are the author of a http request library.

Async hasn't been added yet, so you're using `std::net::TcpStream`.

All is well until async comes along. Now, you have a problem. If you use async, your previous sync users won't be able to (easily) call your functions. You're looking at an API redesign.

So, you swallow your pride and add an async variant of your functionality. Since Tokio is most popular, you use `tokio::net::TcpStream`.

All is well, until a user comes in and says "Hey, I would like to use your library with smol (a different async runtime)". Now what do you do? Add a third variant of your code using `smol::net::TcpStream`? It's getting a bit ridiculous, and smol isn't the only alternative runtime.

One solution is to do what Zig does, but there isn't really an agreed upon solution. The stdlib does not even provide AsyncRead/AsyncWrite so you could invert your code and just work with streams provided from above and keep your libary executor agnostic.