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

This reverts commit 575938d457.

tokio-stream/src/timeout.rs

@@ -24,8 +24,7 @@ pin_project! {
 
 /// Error returned by `Timeout`.
 #[derive(Debug, PartialEq)]
-#[non_exhaustive]
-pub struct Elapsed;
+pub struct Elapsed(());
 
 impl<S: Stream> Timeout<S> {
     pub(super) fn new(stream: S, duration: Duration) -> Self {
@@ -62,7 +61,7 @@ impl<S: Stream> Stream for Timeout<S> {
         if *me.poll_deadline {
             ready!(me.deadline.poll(cx));
             *me.poll_deadline = false;
-            return Poll::Ready(Some(Err(Elapsed)));
+            return Poll::Ready(Some(Err(Elapsed::new())));
         }
 
         Poll::Pending
@@ -75,6 +74,12 @@ impl<S: Stream> Stream for Timeout<S> {
 
 // ===== impl Elapsed =====
 
+impl Elapsed {
+    pub(crate) fn new() -> Self {
+        Elapsed(())
+    }
+}
+
 impl fmt::Display for Elapsed {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
         "deadline has elapsed".fmt(fmt)