feat: implement Framed::map_codec (#4427)

tokio-util/src/codec/framed.rs

@@ -204,6 +204,26 @@ impl<T, U> Framed<T, U> {
         &mut self.inner.codec
     }
 
+    /// Maps the codec `U` to `C`, preserving the read and write buffers
+    /// wrapped by `Framed`.
+    ///
+    /// Note that care should be taken to not tamper with the underlying codec
+    /// as it may corrupt the stream of frames otherwise being worked with.
+    pub fn map_codec<C, F>(self, map: F) -> Framed<T, C>
+    where
+        F: FnOnce(U) -> C,
+    {
+        // This could be potentially simplified once rust-lang/rust#86555 hits stable
+        let parts = self.into_parts();
+        Framed::from_parts(FramedParts {
+            io: parts.io,
+            codec: map(parts.codec),
+            read_buf: parts.read_buf,
+            write_buf: parts.write_buf,
+            _priv: (),
+        })
+    }
+
     /// Returns a mutable reference to the underlying codec wrapped by
     /// `Framed`.
     ///

tokio-util/src/codec/framed_read.rs

@@ -108,6 +108,27 @@ impl<T, D> FramedRead<T, D> {
         &mut self.inner.codec
     }
 
+    /// Maps the decoder `D` to `C`, preserving the read buffer
+    /// wrapped by `Framed`.
+    pub fn map_decoder<C, F>(self, map: F) -> FramedRead<T, C>
+    where
+        F: FnOnce(D) -> C,
+    {
+        // This could be potentially simplified once rust-lang/rust#86555 hits stable
+        let FramedImpl {
+            inner,
+            state,
+            codec,
+        } = self.inner;
+        FramedRead {
+            inner: FramedImpl {
+                inner,
+                state,
+                codec: map(codec),
+            },
+        }
+    }
+
     /// Returns a mutable reference to the underlying decoder.
     pub fn decoder_pin_mut(self: Pin<&mut Self>) -> &mut D {
         self.project().inner.project().codec

tokio-util/src/codec/framed_write.rs

@@ -88,6 +88,27 @@ impl<T, E> FramedWrite<T, E> {
         &mut self.inner.codec
     }
 
+    /// Maps the encoder `E` to `C`, preserving the write buffer
+    /// wrapped by `Framed`.
+    pub fn map_encoder<C, F>(self, map: F) -> FramedWrite<T, C>
+    where
+        F: FnOnce(E) -> C,
+    {
+        // This could be potentially simplified once rust-lang/rust#86555 hits stable
+        let FramedImpl {
+            inner,
+            state,
+            codec,
+        } = self.inner;
+        FramedWrite {
+            inner: FramedImpl {
+                inner,
+                state,
+                codec: map(codec),
+            },
+        }
+    }
+
     /// Returns a mutable reference to the underlying encoder.
     pub fn encoder_pin_mut(self: Pin<&mut Self>) -> &mut E {
         self.project().inner.project().codec