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    use crate::error::Error;
    use crate::fmt;
    use crate::sync::atomic::{AtomicBool, Ordering};
    use crate::thread;

    pub struct Flag {
        failed: AtomicBool,
    }

    // Note that the Ordering uses to access the `failed` field of `Flag` below is
    // always `Relaxed`, and that's because this isn't actually protecting any data,
    // it's just a flag whether we've panicked or not.
    //
    // The actual location that this matters is when a mutex is **locked** which is
    // where we have external synchronization ensuring that we see memory
    // reads/writes to this flag.
    //
    // As a result, if it matters, we should see the correct value for `failed` in
    // all cases.

    impl Flag {
        pub const fn new() -> Flag {
            Flag { failed: AtomicBool::new(false) }
        }

        #[inline]
        pub fn borrow(&self) -> LockResult<Guard> {
            let ret = Guard { panicking: thread::panicking() };
            if self.get() { Err(PoisonError::new(ret)) } else { Ok(ret) }
        }

        #[inline]
        pub fn done(&self, guard: &Guard) {
            if !guard.panicking && thread::panicking() {
                self.failed.store(true, Ordering::Relaxed);
            }
        }

        #[inline]
        pub fn get(&self) -> bool {
            self.failed.load(Ordering::Relaxed)
        }
    }

    pub struct Guard {
        panicking: bool,
    }

    /// A type of error which can be returned whenever a lock is acquired.
    ///
    /// Both [`Mutex`]es and [`RwLock`]s are poisoned whenever a thread fails while the lock
    /// is held. The precise semantics for when a lock is poisoned is documented on
    /// each lock, but once a lock is poisoned then all future acquisitions will
    /// return this error.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Arc, Mutex};
    /// use std::thread;
    ///
    /// let mutex = Arc::new(Mutex::new(1));
    ///
    /// // poison the mutex
    /// let c_mutex = mutex.clone();
    /// let _ = thread::spawn(move || {
    ///     let mut data = c_mutex.lock().unwrap();
    ///     *data = 2;
    ///     panic!();
    /// }).join();
    ///
    /// match mutex.lock() {
    ///     Ok(_) => unreachable!(),
    ///     Err(p_err) => {
    ///         let data = p_err.get_ref();
    ///         println!("recovered: {}", data);
    ///     }
    /// };
    /// ```
    ///
    /// [`Mutex`]: ../../std/sync/struct.Mutex.html
    /// [`RwLock`]: ../../std/sync/struct.RwLock.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct PoisonError<T> {
        guard: T,
    }

    /// An enumeration of possible errors associated with a [`TryLockResult`] which
    /// can occur while trying to acquire a lock, from the [`try_lock`] method on a
    /// [`Mutex`] or the [`try_read`] and [`try_write`] methods on an [`RwLock`].
    ///
    /// [`Mutex`]: struct.Mutex.html
    /// [`RwLock`]: struct.RwLock.html
    /// [`TryLockResult`]: type.TryLockResult.html
    /// [`try_lock`]: struct.Mutex.html#method.try_lock
    /// [`try_read`]: struct.RwLock.html#method.try_read
    /// [`try_write`]: struct.RwLock.html#method.try_write
    #[stable(feature = "rust1", since = "1.0.0")]
    pub enum TryLockError<T> {
        /// The lock could not be acquired because another thread failed while holding
        /// the lock.
        #[stable(feature = "rust1", since = "1.0.0")]
        Poisoned(#[stable(feature = "rust1", since = "1.0.0")] PoisonError<T>),
        /// The lock could not be acquired at this time because the operation would
        /// otherwise block.
        #[stable(feature = "rust1", since = "1.0.0")]
        WouldBlock,
    }

    /// A type alias for the result of a lock method which can be poisoned.
    ///
    /// The [`Ok`] variant of this result indicates that the primitive was not
    /// poisoned, and the `Guard` is contained within. The [`Err`] variant indicates
    /// that the primitive was poisoned. Note that the [`Err`] variant *also* carries
    /// the associated guard, and it can be acquired through the [`into_inner`]
    /// method.
    ///
    /// [`Ok`]: ../../std/result/enum.Result.html#variant.Ok
    /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
    /// [`into_inner`]: ../../std/sync/struct.PoisonError.html#method.into_inner
    #[stable(feature = "rust1", since = "1.0.0")]
    pub type LockResult<Guard> = Result<Guard, PoisonError<Guard>>;

    /// A type alias for the result of a nonblocking locking method.
    ///
    /// For more information, see [`LockResult`]. A `TryLockResult` doesn't
    /// necessarily hold the associated guard in the [`Err`] type as the lock may not
    /// have been acquired for other reasons.
    ///
    /// [`LockResult`]: ../../std/sync/type.LockResult.html
    /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
    #[stable(feature = "rust1", since = "1.0.0")]
    pub type TryLockResult<Guard> = Result<Guard, TryLockError<Guard>>;

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> fmt::Debug for PoisonError<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            "PoisonError { inner: .. }".fmt(f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> fmt::Display for PoisonError<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            "poisoned lock: another task failed inside".fmt(f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Error for PoisonError<T> {
        fn description(&self) -> &str {
            "poisoned lock: another task failed inside"
        }
    }

    impl<T> PoisonError<T> {
        /// Creates a `PoisonError`.
        ///
        /// This is generally created by methods like [`Mutex::lock`] or [`RwLock::read`].
        ///
        /// [`Mutex::lock`]: ../../std/sync/struct.Mutex.html#method.lock
        /// [`RwLock::read`]: ../../std/sync/struct.RwLock.html#method.read
        #[stable(feature = "sync_poison", since = "1.2.0")]
        pub fn new(guard: T) -> PoisonError<T> {
            PoisonError { guard }
        }

        /// Consumes this error indicating that a lock is poisoned, returning the
        /// underlying guard to allow access regardless.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::collections::HashSet;
        /// use std::sync::{Arc, Mutex};
        /// use std::thread;
        ///
        /// let mutex = Arc::new(Mutex::new(HashSet::new()));
        ///
        /// // poison the mutex
        /// let c_mutex = mutex.clone();
        /// let _ = thread::spawn(move || {
        ///     let mut data = c_mutex.lock().unwrap();
        ///     data.insert(10);
        ///     panic!();
        /// }).join();
        ///
        /// let p_err = mutex.lock().unwrap_err();
        /// let data = p_err.into_inner();
        /// println!("recovered {} items", data.len());
        /// ```
        #[stable(feature = "sync_poison", since = "1.2.0")]
        pub fn into_inner(self) -> T {
            self.guard
        }

        /// Reaches into this error indicating that a lock is poisoned, returning a
        /// reference to the underlying guard to allow access regardless.
        #[stable(feature = "sync_poison", since = "1.2.0")]
        pub fn get_ref(&self) -> &T {
            &self.guard
        }

        /// Reaches into this error indicating that a lock is poisoned, returning a
        /// mutable reference to the underlying guard to allow access regardless.
        #[stable(feature = "sync_poison", since = "1.2.0")]
        pub fn get_mut(&mut self) -> &mut T {
            &mut self.guard
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> From<PoisonError<T>> for TryLockError<T> {
        fn from(err: PoisonError<T>) -> TryLockError<T> {
            TryLockError::Poisoned(err)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> fmt::Debug for TryLockError<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            match *self {
                TryLockError::Poisoned(..) => "Poisoned(..)".fmt(f),
                TryLockError::WouldBlock => "WouldBlock".fmt(f),
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> fmt::Display for TryLockError<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            match *self {
                TryLockError::Poisoned(..) => "poisoned lock: another task failed inside",
                TryLockError::WouldBlock => "try_lock failed because the operation would block",
            }
            .fmt(f)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T> Error for TryLockError<T> {
        fn description(&self) -> &str {
            match *self {
                TryLockError::Poisoned(ref p) => p.description(),
                TryLockError::WouldBlock => "try_lock failed because the operation would block",
            }
        }

        fn cause(&self) -> Option<&dyn Error> {
            match *self {
                TryLockError::Poisoned(ref p) => Some(p),
                _ => None,
            }
        }
    }

    pub fn map_result<T, U, F>(result: LockResult<T>, f: F) -> LockResult<U>
    where
        F: FnOnce(T) -> U,
    {
        match result {
            Ok(t) => Ok(f(t)),
            Err(PoisonError { guard }) => Err(PoisonError::new(f(guard))),
        }
    }