Procedural macro for recursive async functions.

• Documentation
• Cargo package: async-recursion

Consider the following recursive implementation of the fibonacci numbers:

async fn fib(n : u32) -> u32 {
   match n {
       0 | 1 => 1,
       _ => fib(n-1).await + fib(n-2).await
   }
}

The compiler helpfully tells us that:

error[E0733]: recursion in an `async fn` requires boxing
 --> src/main.rs:1:26
  |
1 | async fn fib(n : u32) -> u32 {
  |                          ^^^ recursive `async fn`
  |
  = note: a recursive `async fn` must be rewritten to return a boxed `dyn Future`
  = note: consider using the `async_recursion` crate: https://crates.io/crates/async_recursion

This crate provides an attribute macro to automatically convert an async function to one returning a boxed Future.

use async_recursion::async_recursion;

#[async_recursion]
async fn fib(n : u32) -> u32 {
   match n {
      0 | 1 => 1,
      _ => fib(n-1).await + fib(n-2).await
   }
}

The returned Future has a Send bound to make sure it can be sent between threads. If this is undesirable you can mark that the bound should be left out like so:

#[async_recursion(?Send)]
async fn returned_future_is_not_send() {
   // ...
}

The returned Future doesn't have a Sync bound as it is usually not required. You can include a Sync bound as follows:

#[async_recursion(Sync)]
async fn returned_future_is_sync() {
   // ...
}

In detail:

• #[async_recursion] modifies your function to return a boxed Future with a Send bound.
• #[async_recursion(?Send)] modifies your function to return a boxed Future without a Send bound.
• #[async_recursion(Sync)] modifies your function to return a boxed Future with a Send and Sync bound.

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.