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

tokio-util/src/codec/length_delimited.rs

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