Keyword async

Returns a Future instead of blocking the current thread.

Use async in front of fn, closure, or a block to turn the marked code into a Future. As such the code will not be run immediately, but will only be evaluated when the returned future is .awaited.

We have written an async book detailing async/await and trade-offs compared to using threads.

Control Flow

return statements and ? operators within async blocks do not cause a return from the parent function; rather, they cause the Future returned by the block to return with that value.

For example, the following Rust function will return 5, causing x to take the ! type:

#[expect(unused_variables)]
fn example() -> i32 {
    let x = {
        return 5;
    };
}

In contrast, the following asynchronous function assigns a Future<Output = i32> to x, and only returns 5 when x is .awaited:

async fn example() -> i32 {
    let x = async {
        return 5;
    };

    x.await
}

Code using ? behaves similarly - it causes the async block to return a Result without affecting the parent function.

Note that you cannot use break or continue from within an async block to affect the control flow of a loop in the parent function.

Control flow in async blocks is documented further in the async book.

Editions

async is a keyword from the 2018 edition onwards.

It is available for use in stable Rust from version 1.39 onwards.