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 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` cannot be made into an object
 --> src/main.rs:5:22
  |
5 | pub fn make() -> Box<dyn Trait> {
  |                      ^^^^^^^^^ `Trait` cannot be made into an object
  |
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
 --> src/main.rs:2:14
  |
1 | pub trait Trait {
  |           ----- this trait cannot be made into an object...
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<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;
    }
}
```


## 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<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 */
        })
    }
}
```


## 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 implementations)
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;
```


#### 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.

