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    //! Runs `!Send` futures on the current thread.
    use crate::runtime::task::{self, JoinHandle, Task};
    use crate::sync::AtomicWaker;
    use crate::util::linked_list::{Link, LinkedList};

    use std::cell::{Cell, RefCell};
    use std::collections::VecDeque;
    use std::fmt;
    use std::future::Future;
    use std::marker::PhantomData;
    use std::pin::Pin;
    use std::sync::{Arc, Mutex};
    use std::task::Poll;

    use pin_project_lite::pin_project;

    cfg_rt! {
        /// A set of tasks which are executed on the same thread.
        ///
        /// In some cases, it is necessary to run one or more futures that do not
        /// implement [`Send`] and thus are unsafe to send between threads. In these
        /// cases, a [local task set] may be used to schedule one or more `!Send`
        /// futures to run together on the same thread.
        ///
        /// For example, the following code will not compile:
        ///
        /// ```rust,compile_fail
        /// use std::rc::Rc;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     // `Rc` does not implement `Send`, and thus may not be sent between
        ///     // threads safely.
        ///     let unsend_data = Rc::new("my unsend data...");
        ///
        ///     let unsend_data = unsend_data.clone();
        ///     // Because the `async` block here moves `unsend_data`, the future is `!Send`.
        ///     // Since `tokio::spawn` requires the spawned future to implement `Send`, this
        ///     // will not compile.
        ///     tokio::spawn(async move {
        ///         println!("{}", unsend_data);
        ///         // ...
        ///     }).await.unwrap();
        /// }
        /// ```
        /// In order to spawn `!Send` futures, we can use a local task set to
        /// schedule them on the thread calling [`Runtime::block_on`]. When running
        /// inside of the local task set, we can use [`task::spawn_local`], which can
        /// spawn `!Send` futures. For example:
        ///
        /// ```rust
        /// use std::rc::Rc;
        /// use tokio::task;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let unsend_data = Rc::new("my unsend data...");
        ///
        ///     // Construct a local task set that can run `!Send` futures.
        ///     let local = task::LocalSet::new();
        ///
        ///     // Run the local task set.
        ///     local.run_until(async move {
        ///         let unsend_data = unsend_data.clone();
        ///         // `spawn_local` ensures that the future is spawned on the local
        ///         // task set.
        ///         task::spawn_local(async move {
        ///             println!("{}", unsend_data);
        ///             // ...
        ///         }).await.unwrap();
        ///     }).await;
        /// }
        /// ```
        ///
        /// ## Awaiting a `LocalSet`
        ///
        /// Additionally, a `LocalSet` itself implements `Future`, completing when
        /// *all* tasks spawned on the `LocalSet` complete. This can be used to run
        /// several futures on a `LocalSet` and drive the whole set until they
        /// complete. For example,
        ///
        /// ```rust
        /// use tokio::{task, time};
        /// use std::rc::Rc;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let unsend_data = Rc::new("world");
        ///     let local = task::LocalSet::new();
        ///
        ///     let unsend_data2 = unsend_data.clone();
        ///     local.spawn_local(async move {
        ///         // ...
        ///         println!("hello {}", unsend_data2)
        ///     });
        ///
        ///     local.spawn_local(async move {
        ///         time::sleep(time::Duration::from_millis(100)).await;
        ///         println!("goodbye {}", unsend_data)
        ///     });
        ///
        ///     // ...
        ///
        ///     local.await;
        /// }
        /// ```
        ///
        /// [`Send`]: trait@std::marker::Send
        /// [local task set]: struct@LocalSet
        /// [`Runtime::block_on`]: method@crate::runtime::Runtime::block_on
        /// [`task::spawn_local`]: fn@spawn_local
        pub struct LocalSet {
            /// Current scheduler tick
            tick: Cell<u8>,

            /// State available from thread-local
            context: Context,

            /// This type should not be Send.
            _not_send: PhantomData<*const ()>,
        }
    }

    /// State available from the thread-local
    struct Context {
        /// Owned task set and local run queue
        tasks: RefCell<Tasks>,

        /// State shared between threads.
        shared: Arc<Shared>,
    }

    struct Tasks {
        /// Collection of all active tasks spawned onto this executor.
        owned: LinkedList<Task<Arc<Shared>>, <Task<Arc<Shared>> as Link>::Target>,

        /// Local run queue sender and receiver.
        queue: VecDeque<task::Notified<Arc<Shared>>>,
    }

    /// LocalSet state shared between threads.
    struct Shared {
        /// Remote run queue sender
        queue: Mutex<VecDeque<task::Notified<Arc<Shared>>>>,

        /// Wake the `LocalSet` task
        waker: AtomicWaker,
    }

    pin_project! {
        #[derive(Debug)]
        struct RunUntil<'a, F> {
            local_set: &'a LocalSet,
            #[pin]
            future: F,
        }
    }

    scoped_thread_local!(static CURRENT: Context);

    cfg_rt! {
        /// Spawns a `!Send` future on the local task set.
        ///
        /// The spawned future will be run on the same thread that called `spawn_local.`
        /// This may only be called from the context of a local task set.
        ///
        /// # Panics
        ///
        /// - This function panics if called outside of a local task set.
        ///
        /// # Examples
        ///
        /// ```rust
        /// use std::rc::Rc;
        /// use tokio::task;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let unsend_data = Rc::new("my unsend data...");
        ///
        ///     let local = task::LocalSet::new();
        ///
        ///     // Run the local task set.
        ///     local.run_until(async move {
        ///         let unsend_data = unsend_data.clone();
        ///         task::spawn_local(async move {
        ///             println!("{}", unsend_data);
        ///             // ...
        ///         }).await.unwrap();
        ///     }).await;
        /// }
        /// ```
        #[cfg_attr(tokio_track_caller, track_caller)]
        pub fn spawn_local<F>(future: F) -> JoinHandle<F::Output>
        where
            F: Future + 'static,
            F::Output: 'static,
        {
            let future = crate::util::trace::task(future, "local");
            CURRENT.with(|maybe_cx| {
                let cx = maybe_cx
                    .expect("`spawn_local` called from outside of a `task::LocalSet`");

                // Safety: Tasks are only polled and dropped from the thread that
                // spawns them.
                let (task, handle) = unsafe { task::joinable_local(future) };
                cx.tasks.borrow_mut().queue.push_back(task);
                handle
            })
        }
    }

    /// Initial queue capacity
    const INITIAL_CAPACITY: usize = 64;

    /// Max number of tasks to poll per tick.
    const MAX_TASKS_PER_TICK: usize = 61;

    /// How often it check the remote queue first
    const REMOTE_FIRST_INTERVAL: u8 = 31;

    impl LocalSet {
        /// Returns a new local task set.
        pub fn new() -> LocalSet {
            LocalSet {
                tick: Cell::new(0),
                context: Context {
                    tasks: RefCell::new(Tasks {
                        owned: LinkedList::new(),
                        queue: VecDeque::with_capacity(INITIAL_CAPACITY),
                    }),
                    shared: Arc::new(Shared {
                        queue: Mutex::new(VecDeque::with_capacity(INITIAL_CAPACITY)),
                        waker: AtomicWaker::new(),
                    }),
                },
                _not_send: PhantomData,
            }
        }

        /// Spawns a `!Send` task onto the local task set.
        ///
        /// This task is guaranteed to be run on the current thread.
        ///
        /// Unlike the free function [`spawn_local`], this method may be used to
        /// spawn local tasks when the task set is _not_ running. For example:
        /// ```rust
        /// use tokio::task;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let local = task::LocalSet::new();
        ///
        ///     // Spawn a future on the local set. This future will be run when
        ///     // we call `run_until` to drive the task set.
        ///     local.spawn_local(async {
        ///        // ...
        ///     });
        ///
        ///     // Run the local task set.
        ///     local.run_until(async move {
        ///         // ...
        ///     }).await;
        ///
        ///     // When `run` finishes, we can spawn _more_ futures, which will
        ///     // run in subsequent calls to `run_until`.
        ///     local.spawn_local(async {
        ///        // ...
        ///     });
        ///
        ///     local.run_until(async move {
        ///         // ...
        ///     }).await;
        /// }
        /// ```
        /// [`spawn_local`]: fn@spawn_local
        #[cfg_attr(tokio_track_caller, track_caller)]
        pub fn spawn_local<F>(&self, future: F) -> JoinHandle<F::Output>
        where
            F: Future + 'static,
            F::Output: 'static,
        {
            let future = crate::util::trace::task(future, "local");
            let (task, handle) = unsafe { task::joinable_local(future) };
            self.context.tasks.borrow_mut().queue.push_back(task);
            handle
        }

        /// Runs a future to completion on the provided runtime, driving any local
        /// futures spawned on this task set on the current thread.
        ///
        /// This runs the given future on the runtime, blocking until it is
        /// complete, and yielding its resolved result. Any tasks or timers which
        /// the future spawns internally will be executed on the runtime. The future
        /// may also call [`spawn_local`] to spawn_local additional local futures on the
        /// current thread.
        ///
        /// This method should not be called from an asynchronous context.
        ///
        /// # Panics
        ///
        /// This function panics if the executor is at capacity, if the provided
        /// future panics, or if called within an asynchronous execution context.
        ///
        /// # Notes
        ///
        /// Since this function internally calls [`Runtime::block_on`], and drives
        /// futures in the local task set inside that call to `block_on`, the local
        /// futures may not use [in-place blocking]. If a blocking call needs to be
        /// issued from a local task, the [`spawn_blocking`] API may be used instead.
        ///
        /// For example, this will panic:
        /// ```should_panic
        /// use tokio::runtime::Runtime;
        /// use tokio::task;
        ///
        /// let rt  = Runtime::new().unwrap();
        /// let local = task::LocalSet::new();
        /// local.block_on(&rt, async {
        ///     let join = task::spawn_local(async {
        ///         let blocking_result = task::block_in_place(|| {
        ///             // ...
        ///         });
        ///         // ...
        ///     });
        ///     join.await.unwrap();
        /// })
        /// ```
        /// This, however, will not panic:
        /// ```
        /// use tokio::runtime::Runtime;
        /// use tokio::task;
        ///
        /// let rt  = Runtime::new().unwrap();
        /// let local = task::LocalSet::new();
        /// local.block_on(&rt, async {
        ///     let join = task::spawn_local(async {
        ///         let blocking_result = task::spawn_blocking(|| {
        ///             // ...
        ///         }).await;
        ///         // ...
        ///     });
        ///     join.await.unwrap();
        /// })
        /// ```
        ///
        /// [`spawn_local`]: fn@spawn_local
        /// [`Runtime::block_on`]: method@crate::runtime::Runtime::block_on
        /// [in-place blocking]: fn@crate::task::block_in_place
        /// [`spawn_blocking`]: fn@crate::task::spawn_blocking
        #[cfg(feature = "rt")]
        #[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
        pub fn block_on<F>(&self, rt: &crate::runtime::Runtime, future: F) -> F::Output
        where
            F: Future,
        {
            rt.block_on(self.run_until(future))
        }

        /// Run a future to completion on the local set, returning its output.
        ///
        /// This returns a future that runs the given future with a local set,
        /// allowing it to call [`spawn_local`] to spawn additional `!Send` futures.
        /// Any local futures spawned on the local set will be driven in the
        /// background until the future passed to `run_until` completes. When the future
        /// passed to `run` finishes, any local futures which have not completed
        /// will remain on the local set, and will be driven on subsequent calls to
        /// `run_until` or when [awaiting the local set] itself.
        ///
        /// # Examples
        ///
        /// ```rust
        /// use tokio::task;
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     task::LocalSet::new().run_until(async {
        ///         task::spawn_local(async move {
        ///             // ...
        ///         }).await.unwrap();
        ///         // ...
        ///     }).await;
        /// }
        /// ```
        ///
        /// [`spawn_local`]: fn@spawn_local
        /// [awaiting the local set]: #awaiting-a-localset
        pub async fn run_until<F>(&self, future: F) -> F::Output
        where
            F: Future,
        {
            let run_until = RunUntil {
                future,
                local_set: self,
            };
            run_until.await
        }

        /// Tick the scheduler, returning whether the local future needs to be
        /// notified again.
        fn tick(&self) -> bool {
            for _ in 0..MAX_TASKS_PER_TICK {
                match self.next_task() {
                    // Run the task
                    //
                    // Safety: As spawned tasks are `!Send`, `run_unchecked` must be
                    // used. We are responsible for maintaining the invariant that
                    // `run_unchecked` is only called on threads that spawned the
                    // task initially. Because `LocalSet` itself is `!Send`, and
                    // `spawn_local` spawns into the `LocalSet` on the current
                    // thread, the invariant is maintained.
                    Some(task) => crate::coop::budget(|| task.run()),
                    // We have fully drained the queue of notified tasks, so the
                    // local future doesn't need to be notified again — it can wait
                    // until something else wakes a task in the local set.
                    None => return false,
                }
            }

            true
        }

        fn next_task(&self) -> Option<task::Notified<Arc<Shared>>> {
            let tick = self.tick.get();
            self.tick.set(tick.wrapping_add(1));

            if tick % REMOTE_FIRST_INTERVAL == 0 {
                self.context
                    .shared
                    .queue
                    .lock()
                    .unwrap()
                    .pop_front()
                    .or_else(|| self.context.tasks.borrow_mut().queue.pop_front())
            } else {
                self.context
                    .tasks
                    .borrow_mut()
                    .queue
                    .pop_front()
                    .or_else(|| self.context.shared.queue.lock().unwrap().pop_front())
            }
        }

        fn with<T>(&self, f: impl FnOnce() -> T) -> T {
            CURRENT.set(&self.context, f)
        }
    }

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

    impl Future for LocalSet {
        type Output = ();

        fn poll(self: Pin<&mut Self>, cx: &mut std::task::Context<'_>) -> Poll<Self::Output> {
            // Register the waker before starting to work
            self.context.shared.waker.register_by_ref(cx.waker());

            if self.with(|| self.tick()) {
                // If `tick` returns true, we need to notify the local future again:
                // there are still tasks remaining in the run queue.
                cx.waker().wake_by_ref();
                Poll::Pending
            } else if self.context.tasks.borrow().owned.is_empty() {
                // If the scheduler has no remaining futures, we're done!
                Poll::Ready(())
            } else {
                // There are still futures in the local set, but we've polled all the
                // futures in the run queue. Therefore, we can just return Pending
                // since the remaining futures will be woken from somewhere else.
                Poll::Pending
            }
        }
    }

    impl Default for LocalSet {
        fn default() -> LocalSet {
            LocalSet::new()
        }
    }

    impl Drop for LocalSet {
        fn drop(&mut self) {
            self.with(|| {
                // Loop required here to ensure borrow is dropped between iterations
                #[allow(clippy::while_let_loop)]
                loop {
                    let task = match self.context.tasks.borrow_mut().owned.pop_back() {
                        Some(task) => task,
                        None => break,
                    };

                    // Safety: same as `run_unchecked`.
                    task.shutdown();
                }

                for task in self.context.tasks.borrow_mut().queue.drain(..) {
                    task.shutdown();
                }

                for task in self.context.shared.queue.lock().unwrap().drain(..) {
                    task.shutdown();
                }

                assert!(self.context.tasks.borrow().owned.is_empty());
            });
        }
    }

    // === impl LocalFuture ===

    impl<T: Future> Future for RunUntil<'_, T> {
        type Output = T::Output;

        fn poll(self: Pin<&mut Self>, cx: &mut std::task::Context<'_>) -> Poll<Self::Output> {
            let me = self.project();

            me.local_set.with(|| {
                me.local_set
                    .context
                    .shared
                    .waker
                    .register_by_ref(cx.waker());

                let _no_blocking = crate::runtime::enter::disallow_blocking();
                let f = me.future;

                if let Poll::Ready(output) = crate::coop::budget(|| f.poll(cx)) {
                    return Poll::Ready(output);
                }

                if me.local_set.tick() {
                    // If `tick` returns `true`, we need to notify the local future again:
                    // there are still tasks remaining in the run queue.
                    cx.waker().wake_by_ref();
                }

                Poll::Pending
            })
        }
    }

    impl Shared {
        /// Schedule the provided task on the scheduler.
        fn schedule(&self, task: task::Notified<Arc<Self>>) {
            CURRENT.with(|maybe_cx| match maybe_cx {
                Some(cx) if cx.shared.ptr_eq(self) => {
                    cx.tasks.borrow_mut().queue.push_back(task);
                }
                _ => {
                    self.queue.lock().unwrap().push_back(task);
                    self.waker.wake();
                }
            });
        }

        fn ptr_eq(&self, other: &Shared) -> bool {
            self as *const _ == other as *const _
        }
    }

    impl task::Schedule for Arc<Shared> {
        fn bind(task: Task<Self>) -> Arc<Shared> {
            CURRENT.with(|maybe_cx| {
                let cx = maybe_cx.expect("scheduler context missing");
                cx.tasks.borrow_mut().owned.push_front(task);
                cx.shared.clone()
            })
        }

        fn release(&self, task: &Task<Self>) -> Option<Task<Self>> {
            use std::ptr::NonNull;

            CURRENT.with(|maybe_cx| {
                let cx = maybe_cx.expect("scheduler context missing");

                assert!(cx.shared.ptr_eq(self));

                let ptr = NonNull::from(task.header());
                // safety: task must be contained by list. It is inserted into the
                // list in `bind`.
                unsafe { cx.tasks.borrow_mut().owned.remove(ptr) }
            })
        }

        fn schedule(&self, task: task::Notified<Self>) {
            Shared::schedule(self, task);
        }
    }