Add JoinHandle::is_running.

library/std/src/thread/mod.rs

@@ -1402,6 +1402,15 @@ impl<T> JoinHandle<T> {
     pub fn join(mut self) -> Result<T> {
         self.0.join()
     }
+
+    /// Checks if the the associated thread is still running its main function.
+    ///
+    /// This might return `false` for a brief moment after the thread's main
+    /// function has returned, but before the thread itself has stopped running.
+    #[unstable(feature = "thread_is_running", issue = "none")]
+    pub fn is_running(&self) -> bool {
+        Arc::strong_count(&self.0.packet.0) > 1
+    }
 }
 
 impl<T> AsInner<imp::Thread> for JoinHandle<T> {