I previously introduced the “send bound” problem, which refers to the need to add a Send bound to the future returned by an async function. This post continues my tour over the various solutions that are available. This post covers “Trait Transformers”. This proposal arose from a joint conversation with myself, Eric Holk, Yoshua Wuyts, Oli Scherer, and Tyler Mandry. It’s a variant of Eric Holk’s inferred async send bounds proposal as well as the work that Yosh/Oli have been doing in the keyword generics group. Those posts are worth reading as well, lots of good ideas there.¹

In the previous post, I introduced the “send bound” problem, which refers to the need to add a Send bound to the future returned by an async function. I want to start talking about some of the ideas that have been floating around for how to solve this problem. I consider this a bit of an open problem, in that I think we know a lot of the ingredients, but there is a bit of a “delicate balance” to finding the right syntax and so forth. To start with, though, I want to introduce Return Type Notation, which is an idea that Tyler Mandry and I came up with for referring to the type returned by a trait method.

Nightly Rust now has support for async functions in traits, so long as you limit yourself to static dispatch. That’s super exciting! And yet, for many users, this support won’t yet meet their needs. One of the problems we need to resolve is how users can conveniently specify when they need an async function to return a Send future. This post covers some of the background on send futures, why we don’t want to adopt the solution from the async_trait crate for the language, and the general direction we would like to go. Follow-up posts will dive into specific solutions.