Currently, tower::buffer::Service is parameterized over the type of the inner Service inside the buffer worker. However, the Service struct does not actually need to know the type of the inner service, only the types of the inner service's Future and Error associated types.

In many cases, the Service type will likely be a significantly longer type than the Future and Error types --- when the service is composed of a number of chained middleware services, its type may be quite complex. Although futures will also often be composed of many nested future types, these will generally not be quite as large as Service types, for a few reasons:

• Some middleware Services may not wrap the inner service's future, and simply perform logic in call and/or poll_ready. These Services will be generic over the inner Service type, but will not wrap the Future type
• Some middleware may return boxed futures, erasing the inner Future type while still being generic over the inner Service type

Therefore, we may wish to consider changing the definition of the buffer::Service type to be generic over only the Future and Error types of the inner service. This could reduce type complexity that may impact compilation time (as well as binary size, if type names are ever displayed in the program). Of course, because this changes the type signature of buffer::Worker, this will have to wait until the next breaking change.

cc @olix0r, who initially suggested this.