1 files changed, 44 insertions, 0 deletions
diff --git a/compiler/rustc_error_codes/src/error_codes/E0733.md b/compiler/rustc_error_codes/src/error_codes/E0733.md
new file mode 100644
index 000000000..051b75148
--- /dev/null
+++ b/compiler/rustc_error_codes/src/error_codes/E0733.md
@@ -0,0 +1,44 @@
+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