src/librustc_error_codes/error_codes/E0733.md

   1 Recursion in an `async fn` requires boxing. For example, this will not compile:
   2
   3 ```edition2018,compile_fail,E0733
   4 async fn foo(n: usize) {
   5     if n > 0 {
   6         foo(n - 1).await;
   7     }
   8 }
   9 ```
  10
  11 To achieve async recursion, the `async fn` needs to be desugared
  12 such that the `Future` is explicit in the return type:
  13
  14 ```edition2018,compile_fail,E0720
  15 use std::future::Future;
  16 fn foo_desugared(n: usize) -> impl Future<Output = ()> {
  17     async move {
  18         if n > 0 {
  19             foo_desugared(n - 1).await;
  20         }
  21     }
  22 }
  23 ```
  24
  25 Finally, the future is wrapped in a pinned box:
  26
  27 ```edition2018
  28 use std::future::Future;
  29 use std::pin::Pin;
  30 fn foo_recursive(n: usize) -> Pin<Box<dyn Future<Output = ()>>> {
  31     Box::pin(async move {
  32         if n > 0 {
  33             foo_recursive(n - 1).await;
  34         }
  35     })
  36 }
  37 ```
  38
  39 The `Box<...>` ensures that the result is of known size,
  40 and the pin is required to keep it in the same place in memory.