async/await

At a high level, async Rust code looks very much like “normal” sequential code:

use futures::executor::block_on;

async fn count_to(count: i32) {
    for i in 1..=count {
        println!("Count is: {i}!");
    }
}

async fn async_main(count: i32) {
    count_to(count).await;
}

fn main() {
    block_on(async_main(10));
}

Key points:

  • Note that this is a simplified example to show the syntax. There is no long running operation or any real concurrency in it!

  • What is the return type of an async call?

    • Use let future: () = async_main(10); in main to see the type.

  • The “async” keyword is syntactic sugar. The compiler replaces the return type with a future.

  • You cannot make main async, without additional instructions to the compiler on how to use the returned future.

  • You need an executor to run async code. block_on blocks the current thread until the provided future has run to completion.

  • .await asynchronously waits for the completion of another operation. Unlike block_on, .await doesn’t block the current thread.

  • .await can only be used inside an async function (or block; these are introduced later).