# Error code E0733

An `async` function used recursion without boxing.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    async fn foo(n: usize) {
        if n > 0 {
            foo(n - 1).await;
        }
    }
    }

To perform async recursion, the `async fn` needs to be desugared such that the `Future` is explicit in the return type:

    #![allow(unused)]
    fn main() {
    use std::future::Future;
    fn foo_desugared(n: usize) -> impl Future<Output = ()> {
        async move {
            if n > 0 {
                foo_desugared(n - 1).await;
            }
        }
    }
    }

Finally, the future is wrapped in a pinned box:

    #![allow(unused)]
    fn main() {
    use std::future::Future;
    use std::pin::Pin;
    fn foo_recursive(n: usize) -> Pin<Box<dyn Future<Output = ()>>> {
        Box::pin(async move {
            if n > 0 {
                foo_recursive(n - 1).await;
            }
        })
    }
    }

The `Box<...>` ensures that the result is of known size, and the pin is required to keep it in the same place in memory.