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    #![cfg_attr(feature = "use_std", allow(dead_code))]

    use core::marker;
    use core::mem;
    use core::sync::atomic::{AtomicUsize, ATOMIC_USIZE_INIT};
    use core::sync::atomic::Ordering::{SeqCst, Relaxed};

    use super::{BorrowedTask, NotifyHandle};

    pub struct LocalKey;
    pub struct LocalMap;
    pub fn local_map() -> LocalMap { LocalMap }

    #[derive(Copy, Clone)]
    pub struct BorrowedEvents<'a>(marker::PhantomData<&'a ()>);

    #[derive(Copy, Clone)]
    pub struct BorrowedUnpark<'a> {
        f: &'a Fn() -> NotifyHandle,
        id: usize,
    }

    pub struct TaskUnpark {
        handle: NotifyHandle,
        id: usize,
    }

    #[derive(Clone)]
    pub struct UnparkEvents;

    impl<'a> BorrowedEvents<'a> {
        pub fn new() -> BorrowedEvents<'a> {
            BorrowedEvents(marker::PhantomData)
        }

        pub fn to_owned(&self) -> UnparkEvents {
            UnparkEvents
        }
    }

    impl<'a> BorrowedUnpark<'a> {
        #[inline]
        pub fn new(f: &'a Fn() -> NotifyHandle, id: usize) -> BorrowedUnpark<'a> {
            BorrowedUnpark { f: f, id: id }
        }

        #[inline]
        pub fn to_owned(&self) -> TaskUnpark {
            let handle = (self.f)();
            let id = handle.clone_id(self.id);
            TaskUnpark { handle: handle, id: id }
        }
    }

    impl UnparkEvents {
        pub fn notify(&self) {}

        pub fn will_notify(&self, _other: &BorrowedEvents) -> bool {
            true
        }
    }

    impl TaskUnpark {
        pub fn notify(&self) {
            self.handle.notify(self.id);
        }

        pub fn will_notify(&self, other: &BorrowedUnpark) -> bool {
            self.id == other.id && self.handle.inner == (other.f)().inner
        }
    }

    impl Clone for TaskUnpark {
        fn clone(&self) -> TaskUnpark {
            let handle = self.handle.clone();
            let id = handle.clone_id(self.id);
            TaskUnpark { handle: handle, id: id }
        }
    }

    impl Drop for TaskUnpark {
        fn drop(&mut self) {
            self.handle.drop_id(self.id);
        }
    }

    static GET: AtomicUsize = ATOMIC_USIZE_INIT;
    static SET: AtomicUsize = ATOMIC_USIZE_INIT;

    /// Initialize the `futures` task system.
    ///
    /// This function is an unsafe low-level implementation detail typically only
    /// used by crates using `futures` in `no_std` context. Users of this crate
    /// who also use the standard library never need to invoke this function.
    ///
    /// The task system in the `futures` crate relies on some notion of "local
    /// storage" for the running thread and/or context. The `task::current` function
    /// can get invoked in any context, for example, and needs to be able to return
    /// a `Task`. Typically with the standard library this is supported with
    /// thread-local-storage, but this is not available in `no_std` contexts!
    ///
    /// This function is provided to allow `no_std` contexts to continue to be able
    /// to use the standard task system in this crate. The functions provided here
    /// will be used as-if they were thread-local-storage getters/setters. The `get`
    /// function provided is used to retrieve the current thread-local value of the
    /// task system's pointer, returning null if not initialized. The `set` function
    /// updates the value of the pointer.
    ///
    /// # Return value
    ///
    /// This function will return whether initialization succeeded or not. This
    /// function can be called concurrently and only the first invocation will
    /// succeed. If `false` is returned then the `get` and `set` pointers provided
    /// were *not* registered for use with the task system, but if `true` was
    /// provided then they will be called when the task system is used.
    ///
    /// Note that while safe to call concurrently it's recommended to still perform
    /// external synchronization when calling this function. This task system is
    /// not guaranteed to be ready to go until a call to this function returns
    /// `true`. In other words, if you call this function and see `false`, the
    /// task system may not be ready to go as another thread may still be calling
    /// `init`.
    ///
    /// # Unsafety
    ///
    /// This function is unsafe due to the requirements on the behavior of the
    /// `get` and `set` functions. The pointers returned from these functions must
    /// reflect the semantics specified above and must also be thread-local,
    /// depending on the definition of a "thread" in the calling context.
    pub unsafe fn init(get: fn() -> *mut u8, set: fn(*mut u8)) -> bool {
        if GET.compare_exchange(0, get as usize, SeqCst, SeqCst).is_ok() {
            SET.store(set as usize, SeqCst);
            true
        } else {
            false
        }
    }

    #[inline]
    pub fn get_ptr() -> Option<*mut u8> {
        match GET.load(Relaxed) {
            0 => None,
            n => Some(unsafe { mem::transmute::<usize, fn() -> *mut u8>(n)() }),
        }
    }

    #[cfg(feature = "use_std")]
    #[inline]
    pub fn is_get_ptr(f: usize) -> bool {
        GET.load(Relaxed) == f
    }

    pub fn set<'a, F, R>(task: &BorrowedTask<'a>, f: F) -> R
        where F: FnOnce() -> R
    {
        let set = match SET.load(Relaxed) {
            0 => panic!("not initialized"),
            n => unsafe { mem::transmute::<usize, fn(*mut u8)>(n) },
        };

        struct Reset(fn(*mut u8), *mut u8);

        impl Drop for Reset {
            #[inline]
            fn drop(&mut self) {
                (self.0)(self.1);
            }
        }

        let _reset = Reset(set, get_ptr().unwrap());
        set(task as *const _ as *mut u8);
        f()
    }

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