Rust error E0733: recursion in an async fn requires boxing

What it meansAn [async] function used recursion without boxing.

What rustc prints

error[E0733]: recursion in an async fn requires boxing
 --> main.rs:1:1
  |
1 | async fn foo(n: usize) {
  | ^^^^^^^^^^^^^^^^^^^^^^
2 |     if n > 0 {
3 |         foo(n - 1).await;
  |         ---------------- recursive call here
  |
  = note: a recursive `async fn` call must introduce indirection such as `Box::pin` to avoid an infinitely sized future

Real output of rustc 1.99.0 compiling the official example below (2021 edition). Your names and line numbers will differ; the first line is the part to match.

E0733Everything else

Activein rustc 1.99.0

7related errors linked below

The compiler team's explanation

An async function used recursion without boxing.

Erroneous code example:

async fn foo(n: usize) {
    if n > 0 {
        foo(n - 1).await;
    }
}

The recursive invocation can be boxed:

async fn foo(n: usize) {
    if n > 0 {
        Box::pin(foo(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.

Alternatively, the body can be boxed:

use std::future::Future;
use std::pin::Pin;
fn foo(n: usize) -> Pin<Box<dyn Future<Output = ()>>> {
    Box::pin(async move {
        if n > 0 {
            foo(n - 1).await;
        }
    })
}

From rustc --explain E0733 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.

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Rust error E0733: recursion in an async fn requires boxing

What rustc prints

The compiler team's explanation

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