rt: add a method to determine if the runtime is multi-threaded.

tokio/src/runtime/handle.rs

@@ -101,6 +101,33 @@ impl Handle {
         }
     }
 
+    /// Returns true if the current `Runtime` is multi-threaded and false
+    /// otherwise.
+    ///
+    /// ```
+    /// use tokio::runtime::Handle;
+    ///
+    /// #[tokio::main(flavor = "current_thread")]
+    /// async fn main() {
+    ///   assert!(!Handle::current().is_multi_threaded());
+    /// }
+    /// ```
+    ///
+    /// ```
+    /// use tokio::runtime::Handle;
+    ///
+    /// #[tokio::main(flavor = "multi_thread", worker_threads = 4)]
+    /// async fn main() {
+    ///   assert!(Handle::current().is_multi_threaded());
+    /// }
+    /// ```
+    pub fn is_multi_threaded(&self) -> bool {
+        match self.inner {
+            scheduler::Handle::CurrentThread(_) => false,
+            scheduler::Handle::MultiThread(_) => true,
+        }
+    }
+
     /// Returns a Handle view over the currently running Runtime
     ///
     /// Returns an error if no Runtime has been started