Async Traits

Async methods in traits were stabilized in the 1.75 release. This required support for using return-position impl Trait in traits, as the desugaring for async fn includes -> impl Future<Output = ...>.

However, even with the native support, there are specific pitfalls around async fn:

• Return-position impl Trait captures all in-scope lifetimes (so certain patterns of borrowing cannot be expressed).

• Async traits cannot be used with trait objects (dyn Trait support).

The async_trait crate provides a workaround for dyn support through a macro, with specific caveats:

# // Copyright 2024 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
use async_trait::async_trait;
use std::time::Instant;
use tokio::time::{Duration, sleep};

#[async_trait]
trait Sleeper {
    async fn sleep(&self);
}

struct FixedSleeper {
    sleep_ms: u64,
}

#[async_trait]
impl Sleeper for FixedSleeper {
    async fn sleep(&self) {
        sleep(Duration::from_millis(self.sleep_ms)).await;
    }
}

async fn run_all_sleepers_multiple_times(
    sleepers: Vec<Box<dyn Sleeper>>,
    n_times: usize,
) {
    for _ in 0..n_times {
        println!("Running all sleepers...");
        for sleeper in &sleepers {
            let start = Instant::now();
            sleeper.sleep().await;
            println!("Slept for {} ms", start.elapsed().as_millis());
        }
    }
}

#[tokio::main]
async fn main() {
    let sleepers: Vec<Box<dyn Sleeper>> = vec![
        Box::new(FixedSleeper { sleep_ms: 50 }),
        Box::new(FixedSleeper { sleep_ms: 100 }),
    ];
    run_all_sleepers_multiple_times(sleepers, 5).await;
}

• async_trait is easy to use, but note that it's using heap allocations to achieve this. This heap allocation has performance overhead.

• The challenges in language support for async trait are too deep to describe in-depth in this class. See this blog post by Niko Matsakis if you are interested in digging deeper. See also these keywords:

  • RPIT: short for return-position impl Trait.
  • RPITIT: short for return-position impl Trait in trait (RPIT in trait).

• Try creating a new sleeper struct that will sleep for a random amount of time and adding it to the Vec.