From d5d68a22bcd8d2e3709e468fe1c6fe77056c3c05 Mon Sep 17 00:00:00 2001
From: Alice Ryhl <aliceryhl@google.com>
Date: Tue, 26 Oct 2021 13:36:27 +0000
Subject: [PATCH 1/3] sync: make `watch::send_replace` infallible

---
 tokio/src/sync/watch.rs | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/tokio/src/sync/watch.rs b/tokio/src/sync/watch.rs
index 74c5b40ecee..f41c5de5671 100644
--- a/tokio/src/sync/watch.rs
+++ b/tokio/src/sync/watch.rs
@@ -428,6 +428,11 @@ impl<T> Sender<T> {
     /// This method fails if the channel has been closed, which happens when
     /// every receiver has been dropped.
     pub fn send(&self, value: T) -> Result<(), error::SendError<T>> {
+        // This is pretty much only useful as a hint anyway, so synchronization isn't critical.
+        if 0 == self.receiver_count() {
+            return Err(error::SendError(value));
+        }
+
         self.send_replace(value)?;
         Ok(())
     }
@@ -436,6 +441,8 @@ impl<T> Sender<T> {
     /// the previous value in the channel.
     ///
     /// This can be useful for reusing the buffers inside a watched value.
+    /// Additionally, this method permits sending values even when there are no
+    /// receivers.
     ///
     /// # Examples
     ///
@@ -446,12 +453,7 @@ impl<T> Sender<T> {
     /// assert_eq!(tx.send_replace(2).unwrap(), 1);
     /// assert_eq!(tx.send_replace(3).unwrap(), 2);
     /// ```
-    pub fn send_replace(&self, value: T) -> Result<T, error::SendError<T>> {
-        // This is pretty much only useful as a hint anyway, so synchronization isn't critical.
-        if 0 == self.receiver_count() {
-            return Err(error::SendError(value));
-        }
-
+    pub fn send_replace(&self, value: T) -> T {
         let old = {
             // Acquire the write lock and update the value.
             let mut lock = self.shared.value.write().unwrap();
@@ -472,7 +474,7 @@ impl<T> Sender<T> {
         // Notify all watchers
         self.shared.notify_rx.notify_waiters();
 
-        Ok(old)
+        old
     }
 
     /// Returns a reference to the most recently sent value

From 9fb859d3e6c6e0ffd8b573cb8f99478f6683a28c Mon Sep 17 00:00:00 2001
From: Alice Ryhl <aliceryhl@google.com>
Date: Tue, 26 Oct 2021 13:42:52 +0000
Subject: [PATCH 2/3] Remove question mark

---
 tokio/src/sync/watch.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tokio/src/sync/watch.rs b/tokio/src/sync/watch.rs
index f41c5de5671..e33977cae3f 100644
--- a/tokio/src/sync/watch.rs
+++ b/tokio/src/sync/watch.rs
@@ -433,7 +433,7 @@ impl<T> Sender<T> {
             return Err(error::SendError(value));
         }
 
-        self.send_replace(value)?;
+        self.send_replace(value);
         Ok(())
     }
 

From 966e4bb9afa8ce95d9bf0386fcc726fa993f15e9 Mon Sep 17 00:00:00 2001
From: Alice Ryhl <alice@ryhl.io>
Date: Tue, 26 Oct 2021 16:18:40 +0200
Subject: [PATCH 3/3] Update tokio/src/sync/watch.rs

Co-authored-by: Taiki Endo <te316e89@gmail.com>
---
 tokio/src/sync/watch.rs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tokio/src/sync/watch.rs b/tokio/src/sync/watch.rs
index e33977cae3f..7e45c116c82 100644
--- a/tokio/src/sync/watch.rs
+++ b/tokio/src/sync/watch.rs
@@ -450,8 +450,8 @@ impl<T> Sender<T> {
     /// use tokio::sync::watch;
     ///
     /// let (tx, _rx) = watch::channel(1);
-    /// assert_eq!(tx.send_replace(2).unwrap(), 1);
-    /// assert_eq!(tx.send_replace(3).unwrap(), 2);
+    /// assert_eq!(tx.send_replace(2), 1);
+    /// assert_eq!(tx.send_replace(3), 2);
     /// ```
     pub fn send_replace(&self, value: T) -> T {
         let old = {