Async trait methods
===================
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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/
<br>
## 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;
}
}
```
<br>
## 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.
<br>
## 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:
```rust
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))
}
}
```
<br>
## 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.
<br>
## 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<'_>) {}
}
```
<br>
## 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;
```
<br>
#### License
<sup>
Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
</sup>
<br>
<sub>
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.
</sub>