sync: add blocking_recv method to UnboundedReceiver, similar to Receiver::blocking_recv (#3262)

tokio/src/sync/mpsc/unbounded.rs

@@ -122,6 +122,36 @@ impl<T> UnboundedReceiver<T> {
         poll_fn(|cx| self.poll_recv(cx)).await
     }
 
+    /// Blocking receive to call outside of asynchronous contexts.
+    ///
+    /// # Panics
+    ///
+    /// This function panics if called within an asynchronous execution
+    /// context.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::thread;
+    /// use tokio::sync::mpsc;
+    ///
+    /// #[tokio::main]
+    /// async fn main() {
+    ///     let (tx, mut rx) = mpsc::unbounded_channel::<u8>();
+    ///
+    ///     let sync_code = thread::spawn(move || {
+    ///         assert_eq!(Some(10), rx.blocking_recv());
+    ///     });
+    ///
+    ///     let _ = tx.send(10);
+    ///     sync_code.join().unwrap();
+    /// }
+    /// ```
+    #[cfg(feature = "sync")]
+    pub fn blocking_recv(&mut self) -> Option<T> {
+        crate::future::block_on(self.recv())
+    }
+
     /// Attempts to return a pending value on this receiver without blocking.
     ///
     /// This method will never block the caller in order to wait for data to