channel: add max_capacity method

tests-build/tests/fail/macros_dead_code.stderr

@@ -1,11 +1,11 @@
-error: function is never used: `f`
- --> $DIR/macros_dead_code.rs:6:10
+error: function `f` is never used
+ --> tests/fail/macros_dead_code.rs:6:10
   |
 6 | async fn f() {}
   |          ^
   |
 note: the lint level is defined here
- --> $DIR/macros_dead_code.rs:1:9
+ --> tests/fail/macros_dead_code.rs:1:9
   |
 1 | #![deny(dead_code)]
   |         ^^^^^^^^^

tokio/src/sync/mpsc/bounded.rs

@@ -1036,6 +1036,36 @@ impl<T> Sender<T> {
             chan: self.chan.downgrade(),
         }
     }
+
+    /// Returns the max buffer capacity of the channel.
+    ///
+    /// The max capacity is the buffer capacity you initially specified with [`channel`]
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use tokio::sync::mpsc;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, rx) = mpsc::channel::<()>(5);
+    ///      
+    ///     // both max capacity and capacity are the same at first
+    ///     assert_eq!(tx.max_capacity(), 5);
+    ///     assert_eq!(tx.capacity(), 5);
+    ///
+    ///     // Making a reservation doesn't change the max capacity.
+    ///     let permit = tx.reserve().await.unwrap();
+    ///     assert_eq!(tx.max_capacity(), 5);
+    ///     // but drops the capacity by one
+    ///     assert_eq!(tx.capacity(), 4);
+    /// }
+    /// ```
+    ///
+    /// [`channel`]: channel
+    pub fn max_capacity(&self) -> usize {
+        self.chan.semaphore().1
+    }
 }
 
 impl<T> Clone for Sender<T> {