Tokio

Tokio provides:

A multi-threaded runtime for executing asynchronous code.
An asynchronous version of the standard library.
A large ecosystem of libraries.

use tokio::time;

async fn count_to(count: i32) {
    for i in 1..=count {
        println!("Count in task: {i}!");
        time::sleep(time::Duration::from_millis(5)).await;
    }
}

#[tokio::main]
async fn main() {
    tokio::spawn(count_to(10));

    for i in 1..5 {
        println!("Main task: {i}");
        time::sleep(time::Duration::from_millis(5)).await;
    }
}

With the tokio::main macro we can now make main async.
The spawn function creates a new, concurrent "task".
Note: spawn takes a Future, you don't call .await on count_to.

Further exploration:

Why does count_to not (usually) get to 10? This is an example of async cancellation. tokio::spawn returns a handle which can be awaited to wait until it finishes.
Try count_to(10).await instead of spawning.
Try awaiting the task returned from tokio::spawn.