Can someone explain to a non-rustacean what Tokio introduces that's not part of Rust? It looks like Rust provides the async/await semantics, so I'm guessing this is an event loop and dispatching system?

Replying to myself, I found this bit in the docs explaining how Tokio decorates the main:

> An async fn is used as we want to enter an asynchronous context. However, asynchronous functions must be executed by a runtime. The runtime contains the asynchronous task scheduler, provides evented I/O, timers, etc.

steveklabnik

To provide some more color on why this isn't built in, different runtimes provide different kinds of guarantees and performance profiles. A webapp has very different requirements than an embedded system, and so we don't want to provide a single runtime. The language contains the basic things needed for the ecosystem to exist, and interoperation points, and then leaves the rest to said ecosystem.

(Some of those interoperation points are still being worked out, so it's not perfect yet.)

olalonde

When learning Rust a few months ago, I built a small client library for a REST API using reqwest (which uses Tokio). I then started writing a web app using Tide (https://github.com/http-rs/tide). I eventually realized that it would be difficult to use the library I had built earlier since Tide uses the async-std runtime rather than Tokio. That was very disappointing. Is there any plan to make it easier to write "runtime agnostic" libraries in the future?