Don't check for recursion in generator witness fields

compiler/rustc_error_codes/src/error_codes/E0733.md

@@ -13,7 +13,7 @@ async fn foo(n: usize) {
 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
+```edition2018,compile_fail,E0733
 use std::future::Future;
 fn foo_desugared(n: usize) -> impl Future<Output = ()> {
     async move {
@@ -41,4 +41,18 @@ fn foo_recursive(n: usize) -> Pin<Box<dyn Future<Output = ()>>> {
 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 recursive call-site can be boxed:
+
+```edition2018
+use std::future::Future;
+use std::pin::Pin;
+fn foo_recursive(n: usize) -> impl Future<Output = ()> {
+    async move {
+        if n > 0 {
+            Box::pin(foo_recursive(n - 1)).await;
+        }
+    }
+}
+```
+
 [`async`]: https://doc.rust-lang.org/std/keyword.async.html