Expand description
Type erasure for async trait methods
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:
trait MyTrait {
async fn f() {}
}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 for a deeper analysis of how this implementation differs from what the compiler and language hope to deliver in the future.
§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.
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<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:
impl Advertisement for AutoplayingVideo {
fn run<'async_trait>(
&'async_trait self,
) -> Pin<Box<dyn core::future::Future<Output = ()> + 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.
type Elided<'a> = &'a usize;
#[async_trait]
trait Test {
async fn test(not_okay: Elided, okay: &usize) {}
}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 '_.
#[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.
#[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 objectThe 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:
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:
#[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:
#[async_trait]
pub trait ObjectSafe {
async fn cannot_dyn(&self) where Self: Sized {}
// presumably other methods
}
let object = &value as &dyn ObjectSafe;