chore: use #[non_exhaustive] instead of private unit field (#3320)

tokio-util/src/codec/framed.rs

@@ -219,7 +219,6 @@ impl<T, U> Framed<T, U> {
             codec: self.inner.codec,
             read_buf: self.inner.state.read.buffer,
             write_buf: self.inner.state.write.buffer,
-            _priv: (),
         }
     }
 }
@@ -282,7 +281,7 @@ where
 ///
 /// [`Framed`]: crate::codec::Framed
 #[derive(Debug)]
-#[allow(clippy::manual_non_exhaustive)]
+#[non_exhaustive]
 pub struct FramedParts<T, U> {
     /// The inner transport used to read bytes to and write bytes to
     pub io: T,
@@ -295,10 +294,6 @@ pub struct FramedParts<T, U> {
 
     /// A buffer with unprocessed data which are not written yet.
     pub write_buf: BytesMut,
-
-    /// This private field allows us to add additional fields in the future in a
-    /// backwards compatible way.
-    _priv: (),
 }
 
 impl<T, U> FramedParts<T, U> {
@@ -312,7 +307,6 @@ impl<T, U> FramedParts<T, U> {
             codec,
             read_buf: BytesMut::new(),
             write_buf: BytesMut::new(),
-            _priv: (),
         }
     }
 }