async runtime
The library that runs async tasks: it polls futures and wakes them when their I/O is ready.
Rust ships no runtime; Tokio provides an executor (a scheduler of tasks on one or many worker threads), timers, and non-blocking I/O, started by #[tokio::main].
Examples
async fn mainon its own fails with E0752 (mainfunction is not allowed to beasync);#[tokio::main]wraps it in a runtime that polls it.futures::executor::block_on(fut)is the smallest executor: it runs one future on the current thread until it finishes.