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    use crate::runtime::task::core::{Cell, Core, Header, Trailer};
    use crate::runtime::task::state::Snapshot;
    use crate::runtime::task::{JoinError, Notified, Schedule, Task};

    use std::future::Future;
    use std::mem;
    use std::panic;
    use std::ptr::NonNull;
    use std::task::{Poll, Waker};

    /// Typed raw task handle
    pub(super) struct Harness<T: Future, S: 'static> {
        cell: NonNull<Cell<T, S>>,
    }

    impl<T, S> Harness<T, S>
    where
        T: Future,
        S: 'static,
    {
        pub(super) unsafe fn from_raw(ptr: NonNull<Header>) -> Harness<T, S> {
            Harness {
                cell: ptr.cast::<Cell<T, S>>(),
            }
        }

        fn header(&self) -> &Header {
            unsafe { &self.cell.as_ref().header }
        }

        fn trailer(&self) -> &Trailer {
            unsafe { &self.cell.as_ref().trailer }
        }

        fn core(&self) -> &Core<T, S> {
            unsafe { &self.cell.as_ref().core }
        }
    }

    impl<T, S> Harness<T, S>
    where
        T: Future,
        S: Schedule,
    {
        /// Polls the inner future.
        ///
        /// All necessary state checks and transitions are performed.
        ///
        /// Panics raised while polling the future are handled.
        pub(super) fn poll(self) {
            // If this is the first time the task is polled, the task will be bound
            // to the scheduler, in which case the task ref count must be
            // incremented.
            let is_not_bound = !self.core().is_bound();

            // Transition the task to the running state.
            //
            // A failure to transition here indicates the task has been cancelled
            // while in the run queue pending execution.
            let snapshot = match self.header().state.transition_to_running(is_not_bound) {
                Ok(snapshot) => snapshot,
                Err(_) => {
                    // The task was shutdown while in the run queue. At this point,
                    // we just hold a ref counted reference. Drop it here.
                    self.drop_reference();
                    return;
                }
            };

            if is_not_bound {
                // Ensure the task is bound to a scheduler instance. Since this is
                // the first time polling the task, a scheduler instance is pulled
                // from the local context and assigned to the task.
                //
                // The scheduler maintains ownership of the task and responds to
                // `wake` calls.
                //
                // The task reference count has been incremented.
                //
                // Safety: Since we have unique access to the task so that we can
                // safely call `bind_scheduler`.
                self.core().bind_scheduler(self.to_task());
            }

            // The transition to `Running` done above ensures that a lock on the
            // future has been obtained. This also ensures the `*mut T` pointer
            // contains the future (as opposed to the output) and is initialized.

            let res = panic::catch_unwind(panic::AssertUnwindSafe(|| {
                struct Guard<'a, T: Future, S: Schedule> {
                    core: &'a Core<T, S>,
                }

                impl<T: Future, S: Schedule> Drop for Guard<'_, T, S> {
                    fn drop(&mut self) {
                        self.core.drop_future_or_output();
                    }
                }

                let guard = Guard { core: self.core() };

                // If the task is cancelled, avoid polling it, instead signalling it
                // is complete.
                if snapshot.is_cancelled() {
                    Poll::Ready(Err(JoinError::cancelled()))
                } else {
                    let res = guard.core.poll(self.header());

                    // prevent the guard from dropping the future
                    mem::forget(guard);

                    res.map(Ok)
                }
            }));

            match res {
                Ok(Poll::Ready(out)) => {
                    self.complete(out, snapshot.is_join_interested());
                }
                Ok(Poll::Pending) => {
                    match self.header().state.transition_to_idle() {
                        Ok(snapshot) => {
                            if snapshot.is_notified() {
                                // Signal yield
                                self.core().yield_now(Notified(self.to_task()));
                                // The ref-count was incremented as part of
                                // `transition_to_idle`.
                                self.drop_reference();
                            }
                        }
                        Err(_) => self.cancel_task(),
                    }
                }
                Err(err) => {
                    self.complete(Err(JoinError::panic(err)), snapshot.is_join_interested());
                }
            }
        }

        pub(super) fn dealloc(self) {
            // Release the join waker, if there is one.
            self.trailer().waker.with_mut(|_| ());

            // Check causality
            self.core().stage.with_mut(|_| {});
            self.core().scheduler.with_mut(|_| {});

            unsafe {
                drop(Box::from_raw(self.cell.as_ptr()));
            }
        }

        // ===== join handle =====

        /// Read the task output into `dst`.
        pub(super) fn try_read_output(self, dst: &mut Poll<super::Result<T::Output>>, waker: &Waker) {
            // Load a snapshot of the current task state
            let snapshot = self.header().state.load();

            debug_assert!(snapshot.is_join_interested());

            if !snapshot.is_complete() {
                // The waker must be stored in the task struct.
                let res = if snapshot.has_join_waker() {
                    // There already is a waker stored in the struct. If it matches
                    // the provided waker, then there is no further work to do.
                    // Otherwise, the waker must be swapped.
                    let will_wake = unsafe {
                        // Safety: when `JOIN_INTEREST` is set, only `JOIN_HANDLE`
                        // may mutate the `waker` field.
                        self.trailer()
                            .waker
                            .with(|ptr| (*ptr).as_ref().unwrap().will_wake(waker))
                    };

                    if will_wake {
                        // The task is not complete **and** the waker is up to date,
                        // there is nothing further that needs to be done.
                        return;
                    }

                    // Unset the `JOIN_WAKER` to gain mutable access to the `waker`
                    // field then update the field with the new join worker.
                    //
                    // This requires two atomic operations, unsetting the bit and
                    // then resetting it. If the task transitions to complete
                    // concurrently to either one of those operations, then setting
                    // the join waker fails and we proceed to reading the task
                    // output.
                    self.header()
                        .state
                        .unset_waker()
                        .and_then(|snapshot| self.set_join_waker(waker.clone(), snapshot))
                } else {
                    self.set_join_waker(waker.clone(), snapshot)
                };

                match res {
                    Ok(_) => return,
                    Err(snapshot) => {
                        assert!(snapshot.is_complete());
                    }
                }
            }

            *dst = Poll::Ready(self.core().take_output());
        }

        fn set_join_waker(&self, waker: Waker, snapshot: Snapshot) -> Result<Snapshot, Snapshot> {
            assert!(snapshot.is_join_interested());
            assert!(!snapshot.has_join_waker());

            // Safety: Only the `JoinHandle` may set the `waker` field. When
            // `JOIN_INTEREST` is **not** set, nothing else will touch the field.
            unsafe {
                self.trailer().waker.with_mut(|ptr| {
                    *ptr = Some(waker);
                });
            }

            // Update the `JoinWaker` state accordingly
            let res = self.header().state.set_join_waker();

            // If the state could not be updated, then clear the join waker
            if res.is_err() {
                unsafe {
                    self.trailer().waker.with_mut(|ptr| {
                        *ptr = None;
                    });
                }
            }

            res
        }

        pub(super) fn drop_join_handle_slow(self) {
            // Try to unset `JOIN_INTEREST`. This must be done as a first step in
            // case the task concurrently completed.
            if self.header().state.unset_join_interested().is_err() {
                // It is our responsibility to drop the output. This is critical as
                // the task output may not be `Send` and as such must remain with
                // the scheduler or `JoinHandle`. i.e. if the output remains in the
                // task structure until the task is deallocated, it may be dropped
                // by a Waker on any arbitrary thread.
                self.core().drop_future_or_output();
            }

            // Drop the `JoinHandle` reference, possibly deallocating the task
            self.drop_reference();
        }

        // ===== waker behavior =====

        pub(super) fn wake_by_val(self) {
            self.wake_by_ref();
            self.drop_reference();
        }

        pub(super) fn wake_by_ref(&self) {
            if self.header().state.transition_to_notified() {
                self.core().schedule(Notified(self.to_task()));
            }
        }

        pub(super) fn drop_reference(self) {
            if self.header().state.ref_dec() {
                self.dealloc();
            }
        }

        /// Forcibly shutdown the task
        ///
        /// Attempt to transition to `Running` in order to forcibly shutdown the
        /// task. If the task is currently running or in a state of completion, then
        /// there is nothing further to do. When the task completes running, it will
        /// notice the `CANCELLED` bit and finalize the task.
        pub(super) fn shutdown(self) {
            if !self.header().state.transition_to_shutdown() {
                // The task is concurrently running. No further work needed.
                return;
            }

            // By transitioning the lifcycle to `Running`, we have permission to
            // drop the future.
            self.cancel_task();
        }

        // ====== internal ======

        fn cancel_task(self) {
            // Drop the future from a panic guard.
            let res = panic::catch_unwind(panic::AssertUnwindSafe(|| {
                self.core().drop_future_or_output();
            }));

            if let Err(err) = res {
                // Dropping the future panicked, complete the join
                // handle with the panic to avoid dropping the panic
                // on the ground.
                self.complete(Err(JoinError::panic(err)), true);
            } else {
                self.complete(Err(JoinError::cancelled()), true);
            }
        }

        fn complete(mut self, output: super::Result<T::Output>, is_join_interested: bool) {
            if is_join_interested {
                // Store the output. The future has already been dropped
                //
                // Safety: Mutual exclusion is obtained by having transitioned the task
                // state -> Running
                self.core().store_output(output);

                // Transition to `Complete`, notifying the `JoinHandle` if necessary.
                self.transition_to_complete();
            }

            // The task has completed execution and will no longer be scheduled.
            //
            // Attempts to batch a ref-dec with the state transition below.
            let ref_dec = if self.core().is_bound() {
                if let Some(task) = self.core().release(self.to_task()) {
                    mem::forget(task);
                    true
                } else {
                    false
                }
            } else {
                false
            };

            // This might deallocate
            let snapshot = self
                .header()
                .state
                .transition_to_terminal(!is_join_interested, ref_dec);

            if snapshot.ref_count() == 0 {
                self.dealloc()
            }
        }

        /// Transitions the task's lifecycle to `Complete`. Notifies the
        /// `JoinHandle` if it still has interest in the completion.
        fn transition_to_complete(&mut self) {
            // Transition the task's lifecycle to `Complete` and get a snapshot of
            // the task's sate.
            let snapshot = self.header().state.transition_to_complete();

            if !snapshot.is_join_interested() {
                // The `JoinHandle` is not interested in the output of this task. It
                // is our responsibility to drop the output.
                self.core().drop_future_or_output();
            } else if snapshot.has_join_waker() {
                // Notify the join handle. The previous transition obtains the
                // lock on the waker cell.
                self.wake_join();
            }
        }

        fn wake_join(&self) {
            self.trailer().waker.with(|ptr| match unsafe { &*ptr } {
                Some(waker) => waker.wake_by_ref(),
                None => panic!("waker missing"),
            });
        }

        fn to_task(&self) -> Task<S> {
            unsafe { Task::from_raw(self.header().into()) }
        }
    }