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Async trait methods
===================
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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<dyn Trait> {
unimplemented!()
}
```
```console
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*][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/
<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. We get to
have `Vec<Box<dyn Advertisement + Sync>>` 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;
}
}
```
<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.
- &#128077;&ensp;Self by value, by reference, by mut reference, or no self;
- &#128077;&ensp;Any number of arguments, any return value;
- &#128077;&ensp;Generic type parameters and lifetime parameters;
- &#128077;&ensp;Associated types;
- &#128077;&ensp;Having async and non-async functions in the same trait;
- &#128077;&ensp;Default implementations provided by the trait;
- &#128077;&ensp;Elided lifetimes.
<br>
## 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:
```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,
{
Box::pin(async move {
/* the original method body */
})
}
}
```
<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>
#### 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>