Async trait methods

The initial round of stabilizations for the async/await language feature in Rust 1.39 did not include support for async fn in traits. Trying to include an async fn in a trait produces the following error:

trait MyTrait {
    async fn f() {}
}

error[E0706]: trait fns cannot be declared `async`
 --> src/main.rs:4:5
  |
4 |     async fn f() {}
  |     ^^^^^^^^^^^^^^^

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

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 hope to deliver in the future.

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.

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;
• 👍 Dyn-capable traits.

Explanation

Async fns get transformed into methods that return Pin<Box<dyn Future + Send + 'async_trait>> and delegate to a private async freestanding function.

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,
    {
        async fn run(_self: &AutoplayingVideo) {
            /* the original method body */
        }

        Box::pin(run(self))
    }
}

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<'_>) {}
}

Dyn traits

Traits with async methods can be used as trait objects as long as they meet the usual requirements for dyn -- no methods with type parameters, no self by value, no associated types, etc.

#[async_trait]
pub trait ObjectSafe {
    async fn f(&self);
    async fn g(&mut self);
}

impl ObjectSafe for MyType {...}

let value: MyType = ...;
let object = &value as &dyn ObjectSafe;  // make trait object

The one wrinkle is in traits that provide default implementations of async methods. In order for the default implementation to produce a future that is Send, the async_trait macro must emit a bound of Self: Sync on trait methods that take &self and a bound Self: Send on trait methods that take &mut self. An example of the former is visible in the expanded code in the explanation section above.

If you make a trait with async methods that have default implementations, everything will work except that the trait cannot be used as a trait object. Creating a value of type &dyn Trait will produce an error that looks like this:

error: the trait `Test` cannot be made into an object
 --> src/main.rs:8:5
  |
8 |     async fn cannot_dyn(&self) {}
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For traits that need to be object safe and need to have default implementations for some async methods, there are two resolutions. Either you can add Send and/or Sync as supertraits (Send if there are &mut self methods with default implementations, Sync if there are &self methods with default implementions) to constrain all implementors of the trait such that the default implementations are applicable to them:

#[async_trait]
pub trait ObjectSafe: Sync {  // added supertrait
    async fn can_dyn(&self) {}
}

let object = &value as &dyn ObjectSafe;

or you can strike the problematic methods from your trait object by bounding them with Self: Sized:

#[async_trait]
pub trait ObjectSafe {
    async fn cannot_dyn(&self) where Self: Sized {}

    // presumably other methods
}

let object = &value as &dyn ObjectSafe;

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