diff --git a/src/client/conn.rs b/src/client/conn.rs
index 6072a866..c969d839 100644
--- a/src/client/conn.rs
+++ b/src/client/conn.rs
@@ -397,7 +397,7 @@ where
                 };
 
                 let (io, buf, _) = h1.into_inner();
-                pending.fulfill(Upgraded::new(Box::new(io), buf));
+                pending.fulfill(Upgraded::new(io, buf));
                 Poll::Ready(Ok(()))
             }
         }
diff --git a/src/common/io/rewind.rs b/src/common/io/rewind.rs
index 9acbe325..322ee3f0 100644
--- a/src/common/io/rewind.rs
+++ b/src/common/io/rewind.rs
@@ -36,6 +36,10 @@ impl<T> Rewind<T> {
     pub(crate) fn into_inner(self) -> (T, Bytes) {
         (self.inner, self.pre.unwrap_or_else(Bytes::new))
     }
+
+    pub(crate) fn get_mut(&mut self) -> &mut T {
+        &mut self.inner
+    }
 }
 
 impl<T> AsyncRead for Rewind<T>
diff --git a/src/server/conn.rs b/src/server/conn.rs
index ad01fd22..a2c5c094 100644
--- a/src/server/conn.rs
+++ b/src/server/conn.rs
@@ -955,7 +955,7 @@ mod upgrades {
                         };
 
                         let (io, buf, _) = h1.into_inner();
-                        pending.fulfill(Upgraded::new(Box::new(io), buf));
+                        pending.fulfill(Upgraded::new(io, buf));
                         return Poll::Ready(Ok(()));
                     }
                     Err(e) => match *e.kind() {
diff --git a/src/upgrade.rs b/src/upgrade.rs
index e8377322..3b47cf68 100644
--- a/src/upgrade.rs
+++ b/src/upgrade.rs
@@ -11,7 +11,7 @@ use std::fmt;
 use std::io;
 use std::marker::Unpin;
 
-use bytes::Bytes;
+use bytes::{Buf, Bytes};
 use tokio::io::{AsyncRead, AsyncWrite};
 use tokio::sync::oneshot;
 
@@ -73,39 +73,15 @@ pub(crate) fn pending() -> (Pending, OnUpgrade) {
     (Pending { tx }, OnUpgrade { rx: Some(rx) })
 }
 
-pub(crate) trait Io: AsyncRead + AsyncWrite + Unpin + 'static {
-    fn __hyper_type_id(&self) -> TypeId {
-        TypeId::of::<Self>()
-    }
-}
-
-impl dyn Io + Send {
-    fn __hyper_is<T: Io>(&self) -> bool {
-        let t = TypeId::of::<T>();
-        self.__hyper_type_id() == t
-    }
-
-    fn __hyper_downcast<T: Io>(self: Box<Self>) -> Result<Box<T>, Box<Self>> {
-        if self.__hyper_is::<T>() {
-            // Taken from `std::error::Error::downcast()`.
-            unsafe {
-                let raw: *mut dyn Io = Box::into_raw(self);
-                Ok(Box::from_raw(raw as *mut T))
-            }
-        } else {
-            Err(self)
-        }
-    }
-}
-
-impl<T: AsyncRead + AsyncWrite + Unpin + 'static> Io for T {}
-
 // ===== impl Upgraded =====
 
 impl Upgraded {
-    pub(crate) fn new(io: Box<dyn Io + Send>, read_buf: Bytes) -> Self {
+    pub(crate) fn new<T>(io: T, read_buf: Bytes) -> Self
+    where
+        T: AsyncRead + AsyncWrite + Unpin + Send + 'static,
+    {
         Upgraded {
-            io: Rewind::new_buffered(io, read_buf),
+            io: Rewind::new_buffered(Box::new(ForwardsWriteBuf(io)), read_buf),
         }
     }
 
@@ -115,9 +91,9 @@ impl Upgraded {
     /// `Upgraded` back.
     pub fn downcast<T: AsyncRead + AsyncWrite + Unpin + 'static>(self) -> Result<Parts<T>, Self> {
         let (io, buf) = self.io.into_inner();
-        match io.__hyper_downcast() {
+        match io.__hyper_downcast::<ForwardsWriteBuf<T>>() {
             Ok(t) => Ok(Parts {
-                io: *t,
+                io: t.0,
                 read_buf: buf,
                 _inner: (),
             }),
@@ -151,6 +127,14 @@ impl AsyncWrite for Upgraded {
         Pin::new(&mut self.io).poll_write(cx, buf)
     }
 
+    fn poll_write_buf<B: Buf>(
+        mut self: Pin<&mut Self>,
+        cx: &mut task::Context<'_>,
+        buf: &mut B,
+    ) -> Poll<io::Result<usize>> {
+        Pin::new(self.io.get_mut()).poll_write_dyn_buf(cx, buf)
+    }
+
     fn poll_flush(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<io::Result<()>> {
         Pin::new(&mut self.io).poll_flush(cx)
     }
@@ -230,3 +214,156 @@ impl StdError for UpgradeExpected {
         "upgrade expected but not completed"
     }
 }
+
+// ===== impl Io =====
+
+struct ForwardsWriteBuf<T>(T);
+
+pub(crate) trait Io: AsyncRead + AsyncWrite + Unpin + 'static {
+    fn poll_write_dyn_buf(
+        &mut self,
+        cx: &mut task::Context<'_>,
+        buf: &mut dyn Buf,
+    ) -> Poll<io::Result<usize>>;
+
+    fn __hyper_type_id(&self) -> TypeId {
+        TypeId::of::<Self>()
+    }
+}
+
+impl dyn Io + Send {
+    fn __hyper_is<T: Io>(&self) -> bool {
+        let t = TypeId::of::<T>();
+        self.__hyper_type_id() == t
+    }
+
+    fn __hyper_downcast<T: Io>(self: Box<Self>) -> Result<Box<T>, Box<Self>> {
+        if self.__hyper_is::<T>() {
+            // Taken from `std::error::Error::downcast()`.
+            unsafe {
+                let raw: *mut dyn Io = Box::into_raw(self);
+                Ok(Box::from_raw(raw as *mut T))
+            }
+        } else {
+            Err(self)
+        }
+    }
+}
+
+impl<T: AsyncRead + Unpin> AsyncRead for ForwardsWriteBuf<T> {
+    unsafe fn prepare_uninitialized_buffer(&self, buf: &mut [std::mem::MaybeUninit<u8>]) -> bool {
+        self.0.prepare_uninitialized_buffer(buf)
+    }
+
+    fn poll_read(
+        mut self: Pin<&mut Self>,
+        cx: &mut task::Context<'_>,
+        buf: &mut [u8],
+    ) -> Poll<io::Result<usize>> {
+        Pin::new(&mut self.0).poll_read(cx, buf)
+    }
+}
+
+impl<T: AsyncWrite + Unpin> AsyncWrite for ForwardsWriteBuf<T> {
+    fn poll_write(
+        mut self: Pin<&mut Self>,
+        cx: &mut task::Context<'_>,
+        buf: &[u8],
+    ) -> Poll<io::Result<usize>> {
+        Pin::new(&mut self.0).poll_write(cx, buf)
+    }
+
+    fn poll_write_buf<B: Buf>(
+        mut self: Pin<&mut Self>,
+        cx: &mut task::Context<'_>,
+        buf: &mut B,
+    ) -> Poll<io::Result<usize>> {
+        Pin::new(&mut self.0).poll_write_buf(cx, buf)
+    }
+
+    fn poll_flush(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<io::Result<()>> {
+        Pin::new(&mut self.0).poll_flush(cx)
+    }
+
+    fn poll_shutdown(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<io::Result<()>> {
+        Pin::new(&mut self.0).poll_shutdown(cx)
+    }
+}
+
+impl<T: AsyncRead + AsyncWrite + Unpin + 'static> Io for ForwardsWriteBuf<T> {
+    fn poll_write_dyn_buf(
+        &mut self,
+        cx: &mut task::Context<'_>,
+        mut buf: &mut dyn Buf,
+    ) -> Poll<io::Result<usize>> {
+        Pin::new(&mut self.0).poll_write_buf(cx, &mut buf)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use tokio::io::AsyncWriteExt;
+
+    #[test]
+    fn upgraded_downcast() {
+        let upgraded = Upgraded::new(Mock, Bytes::new());
+
+        let upgraded = upgraded.downcast::<std::io::Cursor<Vec<u8>>>().unwrap_err();
+
+        upgraded.downcast::<Mock>().unwrap();
+    }
+
+    #[tokio::test]
+    async fn upgraded_forwards_write_buf() {
+        // sanity check that the underlying IO implements write_buf
+        Mock.write_buf(&mut "hello".as_bytes()).await.unwrap();
+
+        let mut upgraded = Upgraded::new(Mock, Bytes::new());
+        upgraded.write_buf(&mut "hello".as_bytes()).await.unwrap();
+    }
+
+    // TODO: replace with tokio_test::io when it can test write_buf
+    struct Mock;
+
+    impl AsyncRead for Mock {
+        fn poll_read(
+            self: Pin<&mut Self>,
+            _cx: &mut task::Context<'_>,
+            _buf: &mut [u8],
+        ) -> Poll<io::Result<usize>> {
+            unreachable!("Mock::poll_read")
+        }
+    }
+
+    impl AsyncWrite for Mock {
+        fn poll_write(
+            self: Pin<&mut Self>,
+            _cx: &mut task::Context<'_>,
+            _buf: &[u8],
+        ) -> Poll<io::Result<usize>> {
+            panic!("poll_write shouldn't be called");
+        }
+
+        fn poll_write_buf<B: Buf>(
+            self: Pin<&mut Self>,
+            _cx: &mut task::Context<'_>,
+            buf: &mut B,
+        ) -> Poll<io::Result<usize>> {
+            let n = buf.remaining();
+            buf.advance(n);
+            Poll::Ready(Ok(n))
+        }
+
+        fn poll_flush(self: Pin<&mut Self>, _cx: &mut task::Context<'_>) -> Poll<io::Result<()>> {
+            unreachable!("Mock::poll_flush")
+        }
+
+        fn poll_shutdown(
+            self: Pin<&mut Self>,
+            _cx: &mut task::Context<'_>,
+        ) -> Poll<io::Result<()>> {
+            unreachable!("Mock::poll_shutdown")
+        }
+    }
+}