Type alias impl trait [0]. Async fns return an existential type `impl Future`. You cannot return that type in traits yet (on stable), so getting a concrete return type requires dynamic dispatch and a heap allocation. On top of that, async fns capture from any references they take as arguments. To do that in traits would require GATs (generic associated types) [1]:

    impl Foo for Bar {
        //       GAT ↓     TiiT ↓
        type Output<'a>      = impl Future<Output = Foo> + 'a;

        fn foo<'a>(&'a self) -> Self::Output<'a> { ... }
    }

The above is possible on nightly, whereas on stable you would have to box the future, and because you can't capture from self, you would have to use a ref counted pointer or clone the parts of self you need. Eventually, this entire transformation would allow for async fns directly in traits, but that is probably a long way off [2].

[0]: https://github.com/rust-lang/rust/issues/63063

[1]: https://github.com/rust-lang/rust/issues/44265

[2]: http://smallcultfollowing.com/babysteps/blog/2019/10/26/asyn...