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    use crate::runtime::blocking::task::BlockingTask;
    use crate::runtime::task::{self, JoinHandle};
    use crate::runtime::{blocking, context, driver, Spawner};

    use std::future::Future;
    use std::{error, fmt};

    /// Handle to the runtime.
    ///
    /// The handle is internally reference-counted and can be freely cloned. A handle can be
    /// obtained using the [`Runtime::handle`] method.
    ///
    /// [`Runtime::handle`]: crate::runtime::Runtime::handle()
    #[derive(Debug, Clone)]
    pub struct Handle {
        pub(super) spawner: Spawner,

        /// Handles to the I/O drivers
        pub(super) io_handle: driver::IoHandle,

        /// Handles to the signal drivers
        pub(super) signal_handle: driver::SignalHandle,

        /// Handles to the time drivers
        pub(super) time_handle: driver::TimeHandle,

        /// Source of `Instant::now()`
        pub(super) clock: driver::Clock,

        /// Blocking pool spawner
        pub(super) blocking_spawner: blocking::Spawner,
    }

    /// Runtime context guard.
    ///
    /// Returned by [`Runtime::enter`] and [`Handle::enter`], the context guard exits
    /// the runtime context on drop.
    ///
    /// [`Runtime::enter`]: fn@crate::runtime::Runtime::enter
    #[derive(Debug)]
    pub struct EnterGuard<'a> {
        handle: &'a Handle,
        guard: context::EnterGuard,
    }

    impl Handle {
        /// Enter the runtime context. This allows you to construct types that must
        /// have an executor available on creation such as [`Sleep`] or [`TcpStream`].
        /// It will also allow you to call methods such as [`tokio::spawn`].
        ///
        /// [`Sleep`]: struct@crate::time::Sleep
        /// [`TcpStream`]: struct@crate::net::TcpStream
        /// [`tokio::spawn`]: fn@crate::spawn
        pub fn enter(&self) -> EnterGuard<'_> {
            EnterGuard {
                handle: self,
                guard: context::enter(self.clone()),
            }
        }

        /// Returns a `Handle` view over the currently running `Runtime`
        ///
        /// # Panic
        ///
        /// This will panic if called outside the context of a Tokio runtime. That means that you must
        /// call this on one of the threads **being run by the runtime**. Calling this from within a
        /// thread created by `std::thread::spawn` (for example) will cause a panic.
        ///
        /// # Examples
        ///
        /// This can be used to obtain the handle of the surrounding runtime from an async
        /// block or function running on that runtime.
        ///
        /// ```
        /// # use std::thread;
        /// # use tokio::runtime::Runtime;
        /// # fn dox() {
        /// # let rt = Runtime::new().unwrap();
        /// # rt.spawn(async {
        /// use tokio::runtime::Handle;
        ///
        /// // Inside an async block or function.
        /// let handle = Handle::current();
        /// handle.spawn(async {
        ///     println!("now running in the existing Runtime");
        /// });
        ///
        /// # let handle =
        /// thread::spawn(move || {
        ///     // Notice that the handle is created outside of this thread and then moved in
        ///     handle.spawn(async { /* ... */ })
        ///     // This next line would cause a panic
        ///     // let handle2 = Handle::current();
        /// });
        /// # handle.join().unwrap();
        /// # });
        /// # }
        /// ```
        pub fn current() -> Self {
            context::current().expect("not currently running on the Tokio runtime.")
        }

        /// Returns a Handle view over the currently running Runtime
        ///
        /// Returns an error if no Runtime has been started
        ///
        /// Contrary to `current`, this never panics
        pub fn try_current() -> Result<Self, TryCurrentError> {
            context::current().ok_or(TryCurrentError(()))
        }

        /// Spawn a future onto the Tokio runtime.
        ///
        /// This spawns the given future onto the runtime's executor, usually a
        /// thread pool. The thread pool is then responsible for polling the future
        /// until it completes.
        ///
        /// See [module level][mod] documentation for more details.
        ///
        /// [mod]: index.html
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio::runtime::Runtime;
        ///
        /// # fn dox() {
        /// // Create the runtime
        /// let rt = Runtime::new().unwrap();
        /// // Get a handle from this runtime
        /// let handle = rt.handle();
        ///
        /// // Spawn a future onto the runtime using the handle
        /// handle.spawn(async {
        ///     println!("now running on a worker thread");
        /// });
        /// # }
        /// ```
        #[cfg_attr(tokio_track_caller, track_caller)]
        pub fn spawn<F>(&self, future: F) -> JoinHandle<F::Output>
        where
            F: Future + Send + 'static,
            F::Output: Send + 'static,
        {
            #[cfg(all(tokio_unstable, feature = "tracing"))]
            let future = crate::util::trace::task(future, "task");
            self.spawner.spawn(future)
        }

        /// 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();
        /// // Get a handle from this runtime
        /// let handle = rt.handle();
        ///
        /// // Spawn a blocking function onto the runtime using the handle
        /// handle.spawn_blocking(|| {
        ///     println!("now running on a worker thread");
        /// });
        /// # }
        #[cfg_attr(tokio_track_caller, track_caller)]
        pub fn spawn_blocking<F, R>(&self, func: F) -> JoinHandle<R>
        where
            F: FnOnce() -> R + Send + 'static,
            R: Send + 'static,
        {
            #[cfg(all(tokio_unstable, feature = "tracing"))]
            let func = {
                #[cfg(tokio_track_caller)]
                let location = std::panic::Location::caller();
                #[cfg(tokio_track_caller)]
                let span = tracing::trace_span!(
                    target: "tokio::task",
                    "task",
                    kind = %"blocking",
                    function = %std::any::type_name::<F>(),
                    spawn.location = %format_args!("{}:{}:{}", location.file(), location.line(), location.column()),
                );
                #[cfg(not(tokio_track_caller))]
                let span = tracing::trace_span!(
                    target: "tokio::task",
                    "task",
                    kind = %"blocking",
                    function = %std::any::type_name::<F>(),
                );
                move || {
                    let _g = span.enter();
                    func()
                }
            };
            let (task, handle) = task::joinable(BlockingTask::new(func));
            let _ = self.blocking_spawner.spawn(task, &self);
            handle
        }
    }

    /// Error returned by `try_current` when no Runtime has been started
    pub struct TryCurrentError(());

    impl fmt::Debug for TryCurrentError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("TryCurrentError").finish()
        }
    }

    impl fmt::Display for TryCurrentError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.write_str("no tokio Runtime has been initialized")
        }
    }

    impl error::Error for TryCurrentError {}