Comment by kurtbuilds

Can you elaborate on why the `mixing` approach doesn't work? Specifically, if you want send bounds, doing this:

    trait HttpService: Send {
        fn fetch(&self, url: Url)
        -> impl Future<Output = HtmlBody> + Send;
    }

    impl HttpService for MyService {
        async fn fetch(&self, url: Url) -> HtmlBody {
            // This works, as long as `do_fetch(): Send`!
            self.client.do_fetch(url).await.into_body()
        }
    }