Revamp error handling model (#402)

* Revamp error handling model

* changelog improvements and typo fixes

* Fix a few more Infallible bounds

* minor docs fixes

src/service/future.rs

@@ -2,7 +2,6 @@
 
 use crate::{
     body::{box_body, BoxBody},
-    response::IntoResponse,
     util::{Either, EitherProj},
     BoxError,
 };
@@ -19,42 +18,6 @@ use std::{
 use tower::util::Oneshot;
 use tower_service::Service;
 
-pin_project! {
-    /// Response future for [`HandleError`](super::HandleError).
-    #[derive(Debug)]
-    pub struct HandleErrorFuture<Fut, F> {
-        #[pin]
-        pub(super) inner: Fut,
-        pub(super) f: Option<F>,
-    }
-}
-
-impl<Fut, F, E, E2, B, Res> Future for HandleErrorFuture<Fut, F>
-where
-    Fut: Future<Output = Result<Response<B>, E>>,
-    F: FnOnce(E) -> Result<Res, E2>,
-    Res: IntoResponse,
-    B: http_body::Body<Data = Bytes> + Send + Sync + 'static,
-    B::Error: Into<BoxError> + Send + Sync + 'static,
-{
-    type Output = Result<Response<BoxBody>, E2>;
-
-    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        let this = self.project();
-
-        match ready!(this.inner.poll(cx)) {
-            Ok(res) => Ok(res.map(box_body)).into(),
-            Err(err) => {
-                let f = this.f.take().unwrap();
-                match f(err) {
-                    Ok(res) => Ok(res.into_response().map(box_body)).into(),
-                    Err(err) => Err(err).into(),
-                }
-            }
-        }
-    }
-}
-
 pin_project! {
     /// The response future for [`OnMethod`](super::OnMethod).
     pub struct OnMethodFuture<S, F, B>