tokio: add Runtime::spawn_blocking (#2980)

This allows writing

rt.spawn_blocking(f);

instead of

let _enter = rt.enter();
tokio::task::spawn_blocking(f);

Signed-off-by: Marc-Antoine Perennou <[email protected]>

tokio/src/runtime/mod.rs

@@ -187,6 +187,7 @@ cfg_rt! {
 
     mod blocking;
     use blocking::BlockingPool;
+    use blocking::task::BlockingTask;
     pub(crate) use blocking::spawn_blocking;
 
     mod builder;
@@ -371,6 +372,32 @@ cfg_rt! {
             }
         }
 
+        /// Run the provided function on an executor dedicated to blocking operations.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// use tokio::runtime::Runtime;
+        ///
+        /// # fn dox() {
+        /// // Create the runtime
+        /// let rt = Runtime::new().unwrap();
+        ///
+        /// // Spawn a blocking function onto the runtime
+        /// rt.spawn_blocking(|| {
+        ///     println!("now running on a worker thread");
+        /// });
+        /// # }
+        #[cfg(feature = "rt")]
+        pub fn spawn_blocking<F, R>(&self, func: F) -> JoinHandle<R>
+        where
+            F: FnOnce() -> R + Send + 'static,
+        {
+            let (task, handle) = task::joinable(BlockingTask::new(func));
+            let _ = self.handle.blocking_spawner.spawn(task, &self.handle);
+            handle
+        }
+
         /// Run a future to completion on the Tokio runtime. This is the
         /// runtime's entry point.
         ///