Async trait methods
===================
[
](https://github.com/dtolnay/async-trait)
[
](https://crates.io/crates/async-trait)
[
](https://docs.rs/async-trait)
[
](https://github.com/dtolnay/async-trait/actions?query=branch%3Amaster)
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:
```rust
pub trait Trait {
async fn f(&self);
}
pub fn make() -> Box {
unimplemented!()
}
```
```console
error[E0038]: the trait `Trait` is not dyn compatible
--> src/main.rs:5:22
|
5 | pub fn make() -> Box {
| ^^^^^^^^^ `Trait` is not dyn compatible
|
note: for a trait to be dyn compatible it needs to allow building a vtable
for more information, visit
--> 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*][hard] for a deeper analysis
of how this implementation differs from what the compiler and language deliver
natively.
[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. We get to
have `Vec>` or `&[&dyn Advertisement]`, for
example.
```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.
## Explanation
Async fns get transformed into methods that return `Pin>` and delegate to an async block.
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,
{
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.
```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<'_>) {}
}
```
#### 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.