Procedural macro for recursive async functions.
Consider the following recursive implementation of the fibonacci numbers:
rust,ignore
async fn fib(n : u32) -> u64 {
match n {
0 => panic!("zero is not a valid argument to fib()!"),
1 | 2 => 1,
3 => 2,
_ => fib(n-1).await + fib(n-2).await
}
}
The compiler helpfully tells us that:
console
error[E0733]: recursion in an `async fn` requires boxing
--> src/main.rs:1:26
|
1 | async fn fib(n : u32) -> u64 {
| ^^^ 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.
```rust use asyncrecursion::asyncrecursion;
async fn fib(n : u32) -> u64 { match n { 0 => panic!("zero is not a valid argument to fib()!"), 1 | 2 => 1, 3 => 2, _ => 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:
```rust
async fn example() { // ... } ```
In detail:
#[async_recursion] modifies your function to return a [BoxFuture], and#[async_recursion(?Send)] modifies your function to return a [LocalBoxFuture].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.