chore: Revert "use #[non_exhaustive] instead of private unit field" (#3323)

This reverts commit 575938d457.

tokio-util/src/codec/bytes_codec.rs

@@ -42,13 +42,12 @@ use std::io;
 /// ```
 ///
 #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash, Default)]
-#[non_exhaustive]
-pub struct BytesCodec;
+pub struct BytesCodec(());
 
 impl BytesCodec {
     /// Creates a new `BytesCodec` for shipping around raw bytes.
     pub fn new() -> BytesCodec {
-        BytesCodec
+        BytesCodec(())
     }
 }
 

tokio-util/src/codec/framed.rs

@@ -219,6 +219,7 @@ 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: (),
         }
     }
 }
@@ -281,7 +282,7 @@ where
 ///
 /// [`Framed`]: crate::codec::Framed
 #[derive(Debug)]
-#[non_exhaustive]
+#[allow(clippy::manual_non_exhaustive)]
 pub struct FramedParts<T, U> {
     /// The inner transport used to read bytes to and write bytes to
     pub io: T,
@@ -294,6 +295,10 @@ 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> {
@@ -307,6 +312,7 @@ impl<T, U> FramedParts<T, U> {
             codec,
             read_buf: BytesMut::new(),
             write_buf: BytesMut::new(),
+            _priv: (),
         }
     }
 }

tokio-util/src/codec/length_delimited.rs

@@ -409,8 +409,9 @@ pub struct Builder {
 }
 
 /// An error when the number of bytes read is more than max frame length.
-#[non_exhaustive]
-pub struct LengthDelimitedCodecError;
+pub struct LengthDelimitedCodecError {
+    _priv: (),
+}
 
 /// A codec for frames delimited by a frame head specifying their lengths.
 ///
@@ -495,7 +496,7 @@ impl LengthDelimitedCodec {
             if n > self.builder.max_frame_len as u64 {
                 return Err(io::Error::new(
                     io::ErrorKind::InvalidData,
-                    LengthDelimitedCodecError,
+                    LengthDelimitedCodecError { _priv: () },
                 ));
             }
 
@@ -585,7 +586,7 @@ impl Encoder<Bytes> for LengthDelimitedCodec {
         if n > self.builder.max_frame_len {
             return Err(io::Error::new(
                 io::ErrorKind::InvalidInput,
-                LengthDelimitedCodecError,
+                LengthDelimitedCodecError { _priv: () },
             ));
         }