Async fn in dyn trait

Welcome! This document explores how to combine dyn and impl Trait in return position. This is crucial pre-requisite for async functions in traits. As a motivating example, consider the trait AsyncIterator:

#![allow(unused)]
fn main() {
trait AsyncIterator {
    type Item;
    
    async fn next(&mut self) -> Option<Self::Item>;
}
}

The async fn here is, of course, short for a function that returns impl Future:

#![allow(unused)]
fn main() {
trait AsyncIterator {
    type Item;
    
    fn next(&mut self) -> impl Future<Output = Option<Self::Item>>;
}
}

The focus of this document is on how we can support dyn AsyncIterator. For an examination of why this is difficult, see this blog post.

Key details

Here is a high-level summary of the key details of our approach:

• Natural usage:
  • To use dynamic dispatch, just write &mut dyn AsyncIterator, same as any other trait.
  • Similarly, on the impl side, just write impl AsyncIterator for MyType, same as any other trait.
• Allocation by default, but not required:
  • By default, trait functions that return -> impl Trait will allocate a Box to store the trait, but only when invoked through a dyn Trait (static dispatch is unchanged).
  • To support no-std or high performance scenarios, types can customize how an -> impl Trait function is dispatch through dyn. We show how to implement an InlineAsyncIterator type, for example, that wraps another AsyncIterator and stores the resulting futures on pre-allocated stack space.
    • rust-lang will publish a crate, dyner, that provides several common strategies.
• Separation of concerns:
  • Users of a dyn AsyncIterator do not need to know (or care) whether the impl allocates a box or uses some other allocation strategy.
  • Similarly, authors of a type that implements AsyncIterator can just write an impl. That code can be used with any number of allocation adapters.