Async trait methods =================== [github](https://github.com/dtolnay/async-trait) [crates.io](https://crates.io/crates/async-trait) [docs.rs](https://docs.rs/async-trait) [build status](https://github.com/dtolnay/async-trait/actions?query=branch%3Amaster) 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: ```rust trait MyTrait { async fn f() {} } ``` ```console 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*][hard] for a deeper analysis of how this implementation differs from what the compiler and language hope to deliver in the future. [hard]: https://smallcultfollowing.com/babysteps/blog/2019/10/26/async-fn-in-traits-are-hard/
## 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. ```rust 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>` and delegate to a private async freestanding function. For example the `impl Advertisement for AutoplayingVideo` above would be expanded as: ```rust impl Advertisement for AutoplayingVideo { fn run<'async_trait>( &'async_trait self, ) -> Pin + 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. ```rust type Elided<'a> = &'a usize; #[async_trait] trait Test { async fn test(not_okay: Elided, okay: &usize) {} } ``` ```console 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 `'_`. ```rust #[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. ```rust #[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: ```console 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: ```rust #[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`: ```rust #[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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