Recursion in an async fn
requires boxing. For example, this will not compile:
async fn foo(n: usize) { if n > 0 { foo(n - 1).await; } }
To achieve async recursion, the async fn
needs to be desugared such that the Future
is explicit in the return type:
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:
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.