diff --git a/actix-http/CHANGES.md b/actix-http/CHANGES.md
index e89208748..dd02b1760 100644
--- a/actix-http/CHANGES.md
+++ b/actix-http/CHANGES.md
@@ -2,18 +2,22 @@
 
 ## Unreleased - 2021-xx-xx
 ### Added
+* `BoxAnyBody`: a boxed message body with boxed errors. [#2183]
 * Re-export `http` crate's `Error` type as `error::HttpError`. [#2171]
 * Re-export `StatusCode`, `Method`, `Version` and `Uri` at the crate root. [#2171]
 * Re-export `ContentEncoding` and `ConnectionType` at the crate root. [#2171]
 
 ### Changed
+* The `MessageBody` trait now has an associated `Error` type. [#2183]
 * `header` mod is now public. [#2171]
 * `uri` mod is now public. [#2171]
 
 ### Removed
 * Stop re-exporting `http` crate's `HeaderMap` types in addition to ours. [#2171]
+* Down-casting for `MessageBody` types. [#2183]
 
 [#2171]: https://github.com/actix/actix-web/pull/2171
+[#2183]: https://github.com/actix/actix-web/pull/2183
 
 
 ## 3.0.0-beta.6 - 2021-04-17
diff --git a/actix-http/src/body/body.rs b/actix-http/src/body/body.rs
index e816f9cb4..dfb3da1a5 100644
--- a/actix-http/src/body/body.rs
+++ b/actix-http/src/body/body.rs
@@ -60,7 +60,7 @@ impl MessageBody for Body {
     fn poll_next(
         self: Pin<&mut Self>,
         cx: &mut Context<'_>,
-    ) -> Poll<Option<Result<Bytes, Error>>> {
+    ) -> Poll<Option<Result<Bytes, Self::Error>>> {
         match self.get_mut() {
             Body::None => Poll::Ready(None),
             Body::Empty => Poll::Ready(None),
@@ -74,7 +74,7 @@ impl MessageBody for Body {
             }
 
             // TODO: MSRV 1.51: poll_map_err
-            Body::Message(body) => match ready!(body.as_mut().poll_next(cx)) {
+            Body::Message(body) => match ready!(body.as_pin_mut().poll_next(cx)) {
                 Some(Err(err)) => Poll::Ready(Some(Err(err.into()))),
                 Some(Ok(val)) => Poll::Ready(Some(Ok(val))),
                 None => Poll::Ready(None),
@@ -194,7 +194,7 @@ impl BoxAnyBody {
     }
 
     /// Returns a mutable pinned reference to the inner message body type.
-    pub fn as_mut(
+    pub fn as_pin_mut(
         &mut self,
     ) -> Pin<&mut (dyn MessageBody<Error = Box<dyn StdError + 'static>>)> {
         self.0.as_mut()
@@ -218,6 +218,7 @@ impl MessageBody for BoxAnyBody {
         mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,
     ) -> Poll<Option<Result<Bytes, Self::Error>>> {
+        // TODO: MSRV 1.51: poll_map_err
         match ready!(self.0.as_mut().poll_next(cx)) {
             Some(Err(err)) => Poll::Ready(Some(Err(err.into()))),
             Some(Ok(val)) => Poll::Ready(Some(Ok(val))),
diff --git a/actix-http/src/body/body_stream.rs b/actix-http/src/body/body_stream.rs
index 0bb4b75c1..ebe872022 100644
--- a/actix-http/src/body/body_stream.rs
+++ b/actix-http/src/body/body_stream.rs
@@ -50,7 +50,7 @@ where
     fn poll_next(
         mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,
-    ) -> Poll<Option<Result<Bytes, Error>>> {
+    ) -> Poll<Option<Result<Bytes, Self::Error>>> {
         loop {
             let stream = self.as_mut().project().stream;
 
diff --git a/actix-http/src/body/response_body.rs b/actix-http/src/body/response_body.rs
index 699ea9384..855c742f2 100644
--- a/actix-http/src/body/response_body.rs
+++ b/actix-http/src/body/response_body.rs
@@ -5,7 +5,7 @@ use std::{
 };
 
 use bytes::Bytes;
-use futures_core::Stream;
+use futures_core::{ready, Stream};
 use pin_project::pin_project;
 
 use crate::error::Error;
@@ -77,7 +77,12 @@ where
         cx: &mut Context<'_>,
     ) -> Poll<Option<Self::Item>> {
         match self.project() {
-            ResponseBodyProj::Body(body) => body.poll_next(cx).map_err(Into::into),
+            // TODO: MSRV 1.51: poll_map_err
+            ResponseBodyProj::Body(body) => match ready!(body.poll_next(cx)) {
+                Some(Err(err)) => Poll::Ready(Some(Err(err.into()))),
+                Some(Ok(val)) => Poll::Ready(Some(Ok(val))),
+                None => Poll::Ready(None),
+            },
             ResponseBodyProj::Other(body) => Pin::new(body).poll_next(cx),
         }
     }
diff --git a/actix-http/src/body/sized_stream.rs b/actix-http/src/body/sized_stream.rs
index 79c09a427..4af132389 100644
--- a/actix-http/src/body/sized_stream.rs
+++ b/actix-http/src/body/sized_stream.rs
@@ -50,7 +50,7 @@ where
     fn poll_next(
         mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,
-    ) -> Poll<Option<Result<Bytes, Error>>> {
+    ) -> Poll<Option<Result<Bytes, Self::Error>>> {
         loop {
             let stream = self.as_mut().project().stream;
 
diff --git a/actix-http/src/encoding/encoder.rs b/actix-http/src/encoding/encoder.rs
index 705a61af1..bd8c9e991 100644
--- a/actix-http/src/encoding/encoder.rs
+++ b/actix-http/src/encoding/encoder.rs
@@ -131,7 +131,7 @@ where
                 None => Poll::Ready(None),
             },
             EncoderBodyProj::BoxedStream(ref mut b) => {
-                match ready!(b.as_mut().poll_next(cx)) {
+                match ready!(b.as_pin_mut().poll_next(cx)) {
                     Some(Err(err)) => {
                         Poll::Ready(Some(Err(EncoderError::Boxed(err.into()))))
                     }
diff --git a/src/middleware/logger.rs b/src/middleware/logger.rs
index f32732724..8a60d6c70 100644
--- a/src/middleware/logger.rs
+++ b/src/middleware/logger.rs
@@ -340,7 +340,7 @@ where
     fn poll_next(
         self: Pin<&mut Self>,
         cx: &mut Context<'_>,
-    ) -> Poll<Option<Result<Bytes, Error>>> {
+    ) -> Poll<Option<Result<Bytes, Self::Error>>> {
         let this = self.project();
         match this.body.poll_next(cx) {
             Poll::Ready(Some(Ok(chunk))) => {