Merge pull request #226 from VVsxmja/master

Update async fn recursion pattern

Author: nrc

examples/07_05_recursion/src/lib.rs

@@ -11,3 +11,10 @@ fn recursive() -> BoxFuture<'static, ()> {
     }.boxed()
 }
 // ANCHOR_END: example
+
+// ANCHOR: example_pinned
+async fn recursive_pinned() {
+    Box::pin(recursive_pinned()).await;
+    Box::pin(recursive_pinned()).await;
+}
+// ANCHOR_END: example_pinned

src/07_workarounds/04_recursion.md

@@ -37,21 +37,38 @@ This won't work—we've created an infinitely-sized type!
 The compiler will complain:
 
 ```
-error[E0733]: recursion in an `async fn` requires boxing
- --> src/lib.rs:1:22
+error[E0733]: recursion in an async fn requires boxing
+ --> src/lib.rs:1:1
   |
 1 | async fn recursive() {
-  |                      ^ an `async fn` cannot invoke itself directly
+  | ^^^^^^^^^^^^^^^^^^^^
   |
-  = note: a recursive `async fn` must be rewritten to return a boxed future.
+  = note: a recursive `async fn` call must introduce indirection such as `Box::pin` to avoid an infinitely sized future
 ```
 
 In order to allow this, we have to introduce an indirection using `Box`.
-Unfortunately, compiler limitations mean that just wrapping the calls to
+
+Prior to Rust 1.77, due to compiler limitations, just wrapping the calls to
 `recursive()` in `Box::pin` isn't enough. To make this work, we have
 to make `recursive` into a non-`async` function which returns a `.boxed()`
 `async` block:
 
 ```rust,edition2018
 {{#include ../../examples/07_05_recursion/src/lib.rs:example}}
 ```
+
+In newer version of Rust, [that compiler limitation has been lifted].
+
+Since Rust 1.77, support for recursion in `async fn` with allocation
+indirection [becomes stable], so recursive calls are permitted so long as they
+use some form of indirection to avoid an infinite size for the state of the
+function.
+
+This means that code like this now works:
+
+```rust,edition2021
+{{#include ../../examples/07_05_recursion/src/lib.rs:example_pinned}}
+```
+
+[becomes stable]: https://blog.rust-lang.org/2024/03/21/Rust-1.77.0.html#support-for-recursion-in-async-fn
+[that compiler limitation has been lifted]: https://github.com/rust-lang/rust/pull/117703/