Inline async fn


  • Able to create async functions whose storage is stored in the receiver, rather than being returned to the caller
  • Resulting future is Send if receiver is Send
  • Resulting trait is dyn safe without any limitations or compromises

Design notes

Short version: make it possible to have async fn where the state is stored in the Self type (detailed writeup). This is equivalent to writing a poll function. Like a poll function, it makes the trait dyn safe; it also has the advantage that Self: Send implies that the returned future is also Send.

Frequently asked questions

What aspects of the design are unresolved?

Primarily bikeshed. How should we designate that an async function is 'inline', and can we come up with a less overloaded name?

There is also the concern that the overall complexity of having varieties of "async functions" is too much.

Do we really need "inline" async functions?

They are needed to manage AsyncDrop at minimum, but they make sense for any trait which is the "primary purpose" of the types that implement it. Basically, anywhere that we used a "poll" function today. After all, poll functions today push all the "intermediate state" into the self type in exactly the same fashion as an inline async function.

Why not just have "inline" async functions?

Async functions aren't a good match for traits that have a lot of methods, since that would put a lot of state into the self type. Also, inline async functions cannot express method-level generics (beyond lifetime parameters).

Why do need both inline async functions and GATs?

Inline async functions are only suitable for non-generic async functions and for async fn with &mut self. If an async function is generic, for example:

fn main() {
trait Foo {
    async fn foo<T>(&mut self);

then we cannot store its state in self because we would need distinct copies for each value of T.