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    use pin_project_lite::pin_project;
    use std::cell::RefCell;
    use std::error::Error;
    use std::future::Future;
    use std::pin::Pin;
    use std::task::{Context, Poll};
    use std::{fmt, thread};

    /// Declares a new task-local key of type [`tokio::task::LocalKey`].
    ///
    /// # Syntax
    ///
    /// The macro wraps any number of static declarations and makes them local to the current task.
    /// Publicity and attributes for each static is preserved. For example:
    ///
    /// # Examples
    ///
    /// ```
    /// # use tokio::task_local;
    /// task_local! {
    ///     pub static ONE: u32;
    ///
    ///     #[allow(unused)]
    ///     static TWO: f32;
    /// }
    /// # fn main() {}
    /// ```
    ///
    /// See [LocalKey documentation][`tokio::task::LocalKey`] for more
    /// information.
    ///
    /// [`tokio::task::LocalKey`]: struct@crate::task::LocalKey
    #[macro_export]
    #[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
    macro_rules! task_local {
         // empty (base case for the recursion)
        () => {};

        ($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty; $($rest:tt)*) => {
            $crate::__task_local_inner!($(#[$attr])* $vis $name, $t);
            $crate::task_local!($($rest)*);
        };

        ($(#[$attr:meta])* $vis:vis static $name:ident: $t:ty) => {
            $crate::__task_local_inner!($(#[$attr])* $vis $name, $t);
        }
    }

    #[doc(hidden)]
    #[macro_export]
    macro_rules! __task_local_inner {
        ($(#[$attr:meta])* $vis:vis $name:ident, $t:ty) => {
            $vis static $name: $crate::task::LocalKey<$t> = {
                std::thread_local! {
                    static __KEY: std::cell::RefCell<Option<$t>> = std::cell::RefCell::new(None);
                }

                $crate::task::LocalKey { inner: __KEY }
            };
        };
    }

    /// A key for task-local data.
    ///
    /// This type is generated by the `task_local!` macro.
    ///
    /// Unlike [`std::thread::LocalKey`], `tokio::task::LocalKey` will
    /// _not_ lazily initialize the value on first access. Instead, the
    /// value is first initialized when the future containing
    /// the task-local is first polled by a futures executor, like Tokio.
    ///
    /// # Examples
    ///
    /// ```
    /// # async fn dox() {
    /// tokio::task_local! {
    ///     static NUMBER: u32;
    /// }
    ///
    /// NUMBER.scope(1, async move {
    ///     assert_eq!(NUMBER.get(), 1);
    /// }).await;
    ///
    /// NUMBER.scope(2, async move {
    ///     assert_eq!(NUMBER.get(), 2);
    ///
    ///     NUMBER.scope(3, async move {
    ///         assert_eq!(NUMBER.get(), 3);
    ///     }).await;
    /// }).await;
    /// # }
    /// ```
    /// [`std::thread::LocalKey`]: struct@std::thread::LocalKey
    #[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
    pub struct LocalKey<T: 'static> {
        #[doc(hidden)]
        pub inner: thread::LocalKey<RefCell<Option<T>>>,
    }

    impl<T: 'static> LocalKey<T> {
        /// Sets a value `T` as the task-local value for the future `F`.
        ///
        /// On completion of `scope`, the task-local will be dropped.
        ///
        /// ### Examples
        ///
        /// ```
        /// # async fn dox() {
        /// tokio::task_local! {
        ///     static NUMBER: u32;
        /// }
        ///
        /// NUMBER.scope(1, async move {
        ///     println!("task local value: {}", NUMBER.get());
        /// }).await;
        /// # }
        /// ```
        pub async fn scope<F>(&'static self, value: T, f: F) -> F::Output
        where
            F: Future,
        {
            TaskLocalFuture {
                local: &self,
                slot: Some(value),
                future: f,
            }
            .await
        }

        /// Accesses the current task-local and runs the provided closure.
        ///
        /// # Panics
        ///
        /// This function will panic if not called within the context
        /// of a future containing a task-local with the corresponding key.
        pub fn with<F, R>(&'static self, f: F) -> R
        where
            F: FnOnce(&T) -> R,
        {
            self.try_with(f).expect(
                "cannot access a Task Local Storage value \
                 without setting it via `LocalKey::set`",
            )
        }

        /// Accesses the current task-local and runs the provided closure.
        ///
        /// If the task-local with the accociated key is not present, this
        /// method will return an `AccessError`. For a panicking variant,
        /// see `with`.
        pub fn try_with<F, R>(&'static self, f: F) -> Result<R, AccessError>
        where
            F: FnOnce(&T) -> R,
        {
            self.inner.with(|v| {
                if let Some(val) = v.borrow().as_ref() {
                    Ok(f(val))
                } else {
                    Err(AccessError { _private: () })
                }
            })
        }
    }

    impl<T: Copy + 'static> LocalKey<T> {
        /// Returns a copy of the task-local value
        /// if the task-local value implements `Copy`.
        pub fn get(&'static self) -> T {
            self.with(|v| *v)
        }
    }

    impl<T: 'static> fmt::Debug for LocalKey<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.pad("LocalKey { .. }")
        }
    }

    pin_project! {
        struct TaskLocalFuture<T: StaticLifetime, F> {
            local: &'static LocalKey<T>,
            slot: Option<T>,
            #[pin]
            future: F,
        }
    }

    impl<T: 'static, F: Future> Future for TaskLocalFuture<T, F> {
        type Output = F::Output;

        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            struct Guard<'a, T: 'static> {
                local: &'static LocalKey<T>,
                slot: &'a mut Option<T>,
                prev: Option<T>,
            }

            impl<T> Drop for Guard<'_, T> {
                fn drop(&mut self) {
                    let value = self.local.inner.with(|c| c.replace(self.prev.take()));
                    *self.slot = value;
                }
            }

            let mut project = self.project();
            let val = project.slot.take();

            let prev = project.local.inner.with(|c| c.replace(val));

            let _guard = Guard {
                prev,
                slot: &mut project.slot,
                local: *project.local,
            };

            project.future.poll(cx)
        }
    }

    // Required to make `pin_project` happy.
    trait StaticLifetime: 'static {}
    impl<T: 'static> StaticLifetime for T {}

    /// An error returned by [`LocalKey::try_with`](method@LocalKey::try_with).
    #[derive(Clone, Copy, Eq, PartialEq)]
    pub struct AccessError {
        _private: (),
    }

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

    impl fmt::Display for AccessError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt::Display::fmt("task-local value not set", f)
        }
    }

    impl Error for AccessError {}