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; } } }

The recursive invocation can be boxed:

#![allow(unused)] fn main() { async fn foo(n: usize) { if n > 0 { Box::pin(foo(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.

Alternatively, the body can be boxed:

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