The Problem
With the current lack of object-safety on async traits1, Dynamic Dispatch on Service instances can only be achieved by converting to classic Tower traits for boxing.
With a single layer, into_classic(), tower::ServiceExt::boxed(), and a few extra where constraints are sufficient. But, with multiple layers, it seems to require not only async_fn_in_trait but also return_type_notation to achieve, which then massively convolutes usage at the call site.
Even with all of this, I've only managed to come up with a non-generic wrapper for my individual use case, which continues to be difficult to utilize due to a large number of impenetrable trait solver failures.
Feature Request
An API for correctly boxing via classic-service wrappers would be very helpful, especially if it can handle the multi-layer boxing problem2 involved in boxing a tower_async::MakeService.
If it ends up being nightly-only due to a requirement on return_type_notation, that would still be very nice even behind a feature flag.
Footnotes
-
As mentioned in the RFC to stabilize it, with no active or draft RFC to add
dyn-async-traits, so far as I am aware. The tracking issue for eventual work on this is rust-lang/rust#107011. ↩ -
My attempt to do so with an intermediate
MapResponsecall to box the outgoing services was, as mentioned above, unsuccessful. ↩
How does funding with Polar work?
Pay now to fund the work behind this issue.
Get updates on progress being made.
Maintainer is rewarded once the issue is completed.
Backer
You're funding impactful open source efforts
Contributor
You want to contribute to this effort
Maintainer
You want to get funding like this too