https://www.reddit.com/r/rust/comments/6amj9g/generic_implementation_of_a_trait_with_associated/

Cargo.toml

[package]
name = "ruma-client-test"
version = "0.1.0"

[dependencies]
futures = "0.1.13"
tokio-service = "0.1.0"

[dependencies.hyper]
git = "https://github.com/hyperium/hyper"
rev = "fed04dfb58e19b408322d4e5ca7474871e848a35"

lib.rs

extern crate futures;
extern crate hyper;
extern crate tokio_service;

pub trait Endpoint {
    type Request: Into<hyper::Request>;
    type Response: From<hyper::Response>;
}

pub struct Client;
pub struct Error;

impl<E> tokio_service::Service for Client where E: Endpoint {
    type Request = E::Request;
    type Response = E::Response;
    type Error = Error;
    type Future = Box<futures::Future<Item = Self::Request, Error = Self::Error>>;

    fn call(&self, req: Self::Request) -> Self::Future {
        unimplemented!();
    }
}