Async trait methods

The stabilization of async functions in traits in Rust 1.75 did not include support for using traits containing async functions as dyn Trait. Trying to use dyn with an async trait produces the following error:

pub trait Trait {
    async fn f(&self);
}

pub fn make() -> Box<dyn Trait> {
    unimplemented!()
}

error[E0038]: the trait `Trait` is not dyn compatible
 --> src/main.rs:5:22
  |
5 | pub fn make() -> Box<dyn Trait> {
  |                      ^^^^^^^^^ `Trait` is not dyn compatible
  |
note: for a trait to be dyn compatible it needs to allow building a vtable
      for more information, visit <https://doc.rust-lang.org/reference/items/traits.html#dyn-compatibility>
 --> src/main.rs:2:14
  |
1 | pub trait Trait {
  |           ----- this trait is not dyn compatible...
2 |     async fn f(&self);
  |              ^ ...because method `f` is `async`
  = help: consider moving `f` to another trait

This crate provides an attribute macro to make async fn in traits work with dyn traits.

Please refer to why async fn in traits are hard for a deeper analysis of how this implementation differs from what the compiler and language deliver natively.

Example

This example implements the core of a highly effective advertising platform using async fn in a trait.

The only thing to notice here is that we write an #[async_trait] macro on top of traits and trait impls that contain async fn, and then they work. We get to have Vec<Box<dyn Advertisement + Sync>> or &[&dyn Advertisement], for example.

use async_trait::async_trait;

#[async_trait]
trait Advertisement {
    async fn run(&self);
}

struct Modal;

#[async_trait]
impl Advertisement for Modal {
    async fn run(&self) {
        self.render_fullscreen().await;
        for _ in 0..4u16 {
            remind_user_to_join_mailing_list().await;
        }
        self.hide_for_now().await;
    }
}

struct AutoplayingVideo {
    media_url: String,
}

#[async_trait]
impl Advertisement for AutoplayingVideo {
    async fn run(&self) {
        let stream = connect(&self.media_url).await;
        stream.play().await;

        // Video probably persuaded user to join our mailing list!
        Modal.run().await;
    }
}

Supported features

It is the intention that all features of Rust traits should work nicely with #[async_trait], but the edge cases are numerous. Please file an issue if you see unexpected borrow checker errors, type errors, or warnings. There is no use of unsafe in the expanded code, so rest assured that if your code compiles it can't be that badly broken.

• 👍 Self by value, by reference, by mut reference, or no self;
• 👍 Any number of arguments, any return value;
• 👍 Generic type parameters and lifetime parameters;
• 👍 Associated types;
• 👍 Having async and non-async functions in the same trait;
• 👍 Default implementations provided by the trait;
• 👍 Elided lifetimes.

Explanation

Async fns get transformed into methods that return Pin<Box<dyn Future + Send + 'async_trait>> and delegate to an async block.

For example the impl Advertisement for AutoplayingVideo above would be expanded as:

impl Advertisement for AutoplayingVideo {
    fn run<'async_trait>(
        &'async_trait self,
    ) -> Pin<Box<dyn std::future::Future<Output = ()> + Send + 'async_trait>>
    where
        Self: Sync + 'async_trait,
    {
        Box::pin(async move {
            /* the original method body */
        })
    }
}

Non-threadsafe futures

Not all async traits need futures that are dyn Future + Send. To avoid having Send and Sync bounds placed on the async trait methods, invoke the async trait macro as #[async_trait(?Send)] on both the trait and the impl blocks.

Elided lifetimes

Be aware that async fn syntax does not allow lifetime elision outside of & and &mut references. (This is true even when not using #[async_trait].) Lifetimes must be named or marked by the placeholder '_.

Fortunately the compiler is able to diagnose missing lifetimes with a good error message.

type Elided<'a> = &'a usize;

#[async_trait]
trait Test {
    async fn test(not_okay: Elided, okay: &usize) {}
}

error[E0726]: implicit elided lifetime not allowed here
 --> src/main.rs:9:29
  |
9 |     async fn test(not_okay: Elided, okay: &usize) {}
  |                             ^^^^^^- help: indicate the anonymous lifetime: `<'_>`

The fix is to name the lifetime or use '_.

#[async_trait]
trait Test {
    // either
    async fn test<'e>(elided: Elided<'e>) {}
    // or
    async fn test(elided: Elided<'_>) {}
}

License

^{Licensed under either of Apache License, Version 2.0 or MIT license at your option.}
_{Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this crate by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.}