Async Traits

Async methods in traits are not yet supported in the stable channel (An experimental feature exists in nightly and should be stabilized in the mid term.)

The crate async_trait provides a workaround through a macro:

use async_trait::async_trait;
use std::time::Instant;
use tokio::time::{sleep, Duration};

trait Sleeper {
    async fn sleep(&self);

struct FixedSleeper {
    sleep_ms: u64,

impl Sleeper for FixedSleeper {
    async fn sleep(&self) {

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();
            println!("slept for {}ms", start.elapsed().as_millis());

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 deep Rust and probably not worth describing in-depth. Niko Matsakis did a good job of explaining them in this post if you are interested in digging deeper.

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