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    // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    use cell::Cell;
    use error::{Error};
    use fmt;
    use marker::Reflect;
    use thread;

    pub struct Flag { failed: Cell<bool> }

    // This flag is only ever accessed with a lock previously held. Note that this
    // a totally private structure.
    unsafe impl Send for Flag {}
    unsafe impl Sync for Flag {}

    impl Flag {
        pub const fn new() -> Flag {
            Flag { failed: Cell::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.set(true);
            }
        }

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

    pub struct Guard {
        panicking: bool,
    }

    /// A type of error which can be returned whenever a lock is acquired.
    ///
    /// Both Mutexes and RwLocks 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.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct PoisonError<T> {
        guard: T,
    }

    /// An enumeration of possible errors which can occur while calling the
    /// `try_lock` method.
    #[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.
    #[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.
    #[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: Reflect> Error for PoisonError<T> {
        fn description(&self) -> &str {
            "poisoned lock: another task failed inside"
        }
    }

    impl<T> PoisonError<T> {
        /// Creates a `PoisonError`.
        #[stable(feature = "sync_poison", since = "1.2.0")]
        pub fn new(guard: T) -> PoisonError<T> {
            PoisonError { guard: guard }
        }

        /// Consumes this error indicating that a lock is poisoned, returning the
        /// underlying guard to allow access regardless.
        #[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: Reflect> 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<&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)))
        }
    }

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