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An [`async`] function used recursion without boxing.
Erroneous code example:
```edition2018,compile_fail,E0733
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:
```edition2018,compile_fail,E0720
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:
```edition2018
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.
[`async`]: https://doc.rust-lang.org/std/keyword.async.html
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