// 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::UnsafeCell;
use fmt;
use mem;
use ops::{Deref, DerefMut};
use ptr;
use sys_common::poison::{self, LockResult, TryLockError, TryLockResult};
use sys_common::rwlock as sys;

/// A reader-writer lock
///
/// This type of lock allows a number of readers or at most one writer at any
/// point in time. The write portion of this lock typically allows modification
/// of the underlying data (exclusive access) and the read portion of this lock
/// typically allows for read-only access (shared access).
///
/// In comparison, a [`Mutex`] does not distinguish between readers or writers
/// that acquire the lock, therefore blocking any threads waiting for the lock to
/// become available. An `RwLock` will allow any number of readers to acquire the
/// lock as long as a writer is not holding the lock.
///
/// The priority policy of the lock is dependent on the underlying operating
/// system's implementation, and this type does not guarantee that any
/// particular policy will be used.
///
/// The type parameter `T` represents the data that this lock protects. It is
/// required that `T` satisfies [`Send`] to be shared across threads and
/// [`Sync`] to allow concurrent access through readers. The RAII guards
/// returned from the locking methods implement [`Deref`][] (and [`DerefMut`]
/// for the `write` methods) to allow access to the content of the lock.
///
/// # Poisoning
///
/// An `RwLock`, like [`Mutex`], will become poisoned on a panic. Note, however,
/// that an `RwLock` may only be poisoned if a panic occurs while it is locked
/// exclusively (write mode). If a panic occurs in any reader, then the lock
/// will not be poisoned.
///
/// # Examples
///
/// ```
/// use std::sync::RwLock;
///
/// let lock = RwLock::new(5);
///
/// // many reader locks can be held at once
/// {
///     let r1 = lock.read().unwrap();
///     let r2 = lock.read().unwrap();
///     assert_eq!(*r1, 5);
///     assert_eq!(*r2, 5);
/// } // read locks are dropped at this point
///
/// // only one write lock may be held, however
/// {
///     let mut w = lock.write().unwrap();
///     *w += 1;
///     assert_eq!(*w, 6);
/// } // write lock is dropped here
/// ```
///
/// [`Deref`]: ../../std/ops/trait.Deref.html
/// [`DerefMut`]: ../../std/ops/trait.DerefMut.html
/// [`Send`]: ../../std/marker/trait.Send.html
/// [`Sync`]: ../../std/marker/trait.Sync.html
/// [`Mutex`]: struct.Mutex.html
#[stable(feature = "rust1", since = "1.0.0")]
pub struct RwLock<T: ?Sized> {
    inner: Box<sys::RWLock>,
    poison: poison::Flag,
    data: UnsafeCell<T>,
}

#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized + Send> Send for RwLock<T> {}
#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}

/// RAII structure used to release the shared read access of a lock when
/// dropped.
///
/// This structure is created by the [`read`] and [`try_read`] methods on
/// [`RwLock`].
///
/// [`read`]: struct.RwLock.html#method.read
/// [`try_read`]: struct.RwLock.html#method.try_read
/// [`RwLock`]: struct.RwLock.html
#[must_use]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct RwLockReadGuard<'a, T: ?Sized + 'a> {
    __lock: &'a RwLock<T>,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T: ?Sized> !Send for RwLockReadGuard<'a, T> {}

#[stable(feature = "rwlock_guard_sync", since = "1.23.0")]
unsafe impl<'a, T: ?Sized + Sync> Sync for RwLockReadGuard<'a, T> {}

/// RAII structure used to release the exclusive write access of a lock when
/// dropped.
///
/// This structure is created by the [`write`] and [`try_write`] methods
/// on [`RwLock`].
///
/// [`write`]: struct.RwLock.html#method.write
/// [`try_write`]: struct.RwLock.html#method.try_write
/// [`RwLock`]: struct.RwLock.html
#[must_use]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct RwLockWriteGuard<'a, T: ?Sized + 'a> {
    __lock: &'a RwLock<T>,
    __poison: poison::Guard,
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T: ?Sized> !Send for RwLockWriteGuard<'a, T> {}

#[stable(feature = "rwlock_guard_sync", since = "1.23.0")]
unsafe impl<'a, T: ?Sized + Sync> Sync for RwLockWriteGuard<'a, T> {}

impl<T> RwLock<T> {
    /// Creates a new instance of an `RwLock<T>` which is unlocked.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::RwLock;
    ///
    /// let lock = RwLock::new(5);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn new(t: T) -> RwLock<T> {
        RwLock {
            inner: box sys::RWLock::new(),
            poison: poison::Flag::new(),
            data: UnsafeCell::new(t),
        }
    }
}

impl<T: ?Sized> RwLock<T> {
    /// Locks this rwlock with shared read access, blocking the current thread
    /// until it can be acquired.
    ///
    /// The calling thread will be blocked until there are no more writers which
    /// hold the lock. There may be other readers currently inside the lock when
    /// this method returns. This method does not provide any guarantees with
    /// respect to the ordering of whether contentious readers or writers will
    /// acquire the lock first.
    ///
    /// Returns an RAII guard which will release this thread's shared access
    /// once it is dropped.
    ///
    /// # Errors
    ///
    /// This function will return an error if the RwLock is poisoned. An RwLock
    /// is poisoned whenever a writer panics while holding an exclusive lock.
    /// The failure will occur immediately after the lock has been acquired.
    ///
    /// # Panics
    ///
    /// This function might panic when called if the lock is already held by the current thread.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Arc, RwLock};
    /// use std::thread;
    ///
    /// let lock = Arc::new(RwLock::new(1));
    /// let c_lock = lock.clone();
    ///
    /// let n = lock.read().unwrap();
    /// assert_eq!(*n, 1);
    ///
    /// thread::spawn(move || {
    ///     let r = c_lock.read();
    ///     assert!(r.is_ok());
    /// }).join().unwrap();
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn read(&self) -> LockResult<RwLockReadGuard<T>> {
        unsafe {
            self.inner.read();
            RwLockReadGuard::new(self)
        }
    }

    /// Attempts to acquire this rwlock with shared read access.
    ///
    /// If the access could not be granted at this time, then `Err` is returned.
    /// Otherwise, an RAII guard is returned which will release the shared access
    /// when it is dropped.
    ///
    /// This function does not block.
    ///
    /// This function does not provide any guarantees with respect to the ordering
    /// of whether contentious readers or writers will acquire the lock first.
    ///
    /// # Errors
    ///
    /// This function will return an error if the RwLock is poisoned. An RwLock
    /// is poisoned whenever a writer panics while holding an exclusive lock. An
    /// error will only be returned if the lock would have otherwise been
    /// acquired.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::RwLock;
    ///
    /// let lock = RwLock::new(1);
    ///
    /// match lock.try_read() {
    ///     Ok(n) => assert_eq!(*n, 1),
    ///     Err(_) => unreachable!(),
    /// };
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn try_read(&self) -> TryLockResult<RwLockReadGuard<T>> {
        unsafe {
            if self.inner.try_read() {
                Ok(RwLockReadGuard::new(self)?)
            } else {
                Err(TryLockError::WouldBlock)
            }
        }
    }

    /// Locks this rwlock with exclusive write access, blocking the current
    /// thread until it can be acquired.
    ///
    /// This function will not return while other writers or other readers
    /// currently have access to the lock.
    ///
    /// Returns an RAII guard which will drop the write access of this rwlock
    /// when dropped.
    ///
    /// # Errors
    ///
    /// This function will return an error if the RwLock is poisoned. An RwLock
    /// is poisoned whenever a writer panics while holding an exclusive lock.
    /// An error will be returned when the lock is acquired.
    ///
    /// # Panics
    ///
    /// This function might panic when called if the lock is already held by the current thread.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::RwLock;
    ///
    /// let lock = RwLock::new(1);
    ///
    /// let mut n = lock.write().unwrap();
    /// *n = 2;
    ///
    /// assert!(lock.try_read().is_err());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn write(&self) -> LockResult<RwLockWriteGuard<T>> {
        unsafe {
            self.inner.write();
            RwLockWriteGuard::new(self)
        }
    }

    /// Attempts to lock this rwlock with exclusive write access.
    ///
    /// If the lock could not be acquired at this time, then `Err` is returned.
    /// Otherwise, an RAII guard is returned which will release the lock when
    /// it is dropped.
    ///
    /// This function does not block.
    ///
    /// This function does not provide any guarantees with respect to the ordering
    /// of whether contentious readers or writers will acquire the lock first.
    ///
    /// # Errors
    ///
    /// This function will return an error if the RwLock is poisoned. An RwLock
    /// is poisoned whenever a writer panics while holding an exclusive lock. An
    /// error will only be returned if the lock would have otherwise been
    /// acquired.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::RwLock;
    ///
    /// let lock = RwLock::new(1);
    ///
    /// let n = lock.read().unwrap();
    /// assert_eq!(*n, 1);
    ///
    /// assert!(lock.try_write().is_err());
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn try_write(&self) -> TryLockResult<RwLockWriteGuard<T>> {
        unsafe {
            if self.inner.try_write() {
                Ok(RwLockWriteGuard::new(self)?)
            } else {
                Err(TryLockError::WouldBlock)
            }
        }
    }

    /// Determines whether the lock is poisoned.
    ///
    /// If another thread is active, the lock can still become poisoned at any
    /// time.  You should not trust a `false` value for program correctness
    /// without additional synchronization.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::{Arc, RwLock};
    /// use std::thread;
    ///
    /// let lock = Arc::new(RwLock::new(0));
    /// let c_lock = lock.clone();
    ///
    /// let _ = thread::spawn(move || {
    ///     let _lock = c_lock.write().unwrap();
    ///     panic!(); // the lock gets poisoned
    /// }).join();
    /// assert_eq!(lock.is_poisoned(), true);
    /// ```
    #[inline]
    #[stable(feature = "sync_poison", since = "1.2.0")]
    pub fn is_poisoned(&self) -> bool {
        self.poison.get()
    }

    /// Consumes this `RwLock`, returning the underlying data.
    ///
    /// # Errors
    ///
    /// This function will return an error if the RwLock is poisoned. An RwLock
    /// is poisoned whenever a writer panics while holding an exclusive lock. An
    /// error will only be returned if the lock would have otherwise been
    /// acquired.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::RwLock;
    ///
    /// let lock = RwLock::new(String::new());
    /// {
    ///     let mut s = lock.write().unwrap();
    ///     *s = "modified".to_owned();
    /// }
    /// assert_eq!(lock.into_inner().unwrap(), "modified");
    /// ```
    #[stable(feature = "rwlock_into_inner", since = "1.6.0")]
    pub fn into_inner(self) -> LockResult<T> where T: Sized {
        // We know statically that there are no outstanding references to
        // `self` so there's no need to lock the inner lock.
        //
        // To get the inner value, we'd like to call `data.into_inner()`,
        // but because `RwLock` impl-s `Drop`, we can't move out of it, so
        // we'll have to destructure it manually instead.
        unsafe {
            // Like `let RwLock { inner, poison, data } = self`.
            let (inner, poison, data) = {
                let RwLock { ref inner, ref poison, ref data } = self;
                (ptr::read(inner), ptr::read(poison), ptr::read(data))
            };
            mem::forget(self);
            inner.destroy(); // Keep in sync with the `Drop` impl.
            drop(inner);

            poison::map_result(poison.borrow(), |_| data.into_inner())
        }
    }

    /// Returns a mutable reference to the underlying data.
    ///
    /// Since this call borrows the `RwLock` mutably, no actual locking needs to
    /// take place---the mutable borrow statically guarantees no locks exist.
    ///
    /// # Errors
    ///
    /// This function will return an error if the RwLock is poisoned. An RwLock
    /// is poisoned whenever a writer panics while holding an exclusive lock. An
    /// error will only be returned if the lock would have otherwise been
    /// acquired.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::sync::RwLock;
    ///
    /// let mut lock = RwLock::new(0);
    /// *lock.get_mut().unwrap() = 10;
    /// assert_eq!(*lock.read().unwrap(), 10);
    /// ```
    #[stable(feature = "rwlock_get_mut", since = "1.6.0")]
    pub fn get_mut(&mut self) -> LockResult<&mut T> {
        // We know statically that there are no other references to `self`, so
        // there's no need to lock the inner lock.
        let data = unsafe { &mut *self.data.get() };
        poison::map_result(self.poison.borrow(), |_| data)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
unsafe impl<#[may_dangle] T: ?Sized> Drop for RwLock<T> {
    fn drop(&mut self) {
        // IMPORTANT: This code needs to be kept in sync with `RwLock::into_inner`.
        unsafe { self.inner.destroy() }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLock<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match self.try_read() {
            Ok(guard) => f.debug_struct("RwLock").field("data", &&*guard).finish(),
            Err(TryLockError::Poisoned(err)) => {
                f.debug_struct("RwLock").field("data", &&**err.get_ref()).finish()
            },
            Err(TryLockError::WouldBlock) => {
                struct LockedPlaceholder;
                impl fmt::Debug for LockedPlaceholder {
                    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { f.write_str("<locked>") }
                }

                f.debug_struct("RwLock").field("data", &LockedPlaceholder).finish()
            }
        }
    }
}

#[stable(feature = "rw_lock_default", since = "1.10.0")]
impl<T: Default> Default for RwLock<T> {
    /// Creates a new `RwLock<T>`, with the `Default` value for T.
    fn default() -> RwLock<T> {
        RwLock::new(Default::default())
    }
}

#[stable(feature = "rw_lock_from", since = "1.24.0")]
impl<T> From<T> for RwLock<T> {
    /// Creates a new instance of an `RwLock<T>` which is unlocked.
    /// This is equivalent to [`RwLock::new`].
    ///
    /// [`RwLock::new`]: #method.new
    fn from(t: T) -> Self {
        RwLock::new(t)
    }
}

impl<'rwlock, T: ?Sized> RwLockReadGuard<'rwlock, T> {
    unsafe fn new(lock: &'rwlock RwLock<T>)
                  -> LockResult<RwLockReadGuard<'rwlock, T>> {
        poison::map_result(lock.poison.borrow(), |_| {
            RwLockReadGuard {
                __lock: lock,
            }
        })
    }
}

impl<'rwlock, T: ?Sized> RwLockWriteGuard<'rwlock, T> {
    unsafe fn new(lock: &'rwlock RwLock<T>)
                  -> LockResult<RwLockWriteGuard<'rwlock, T>> {
        poison::map_result(lock.poison.borrow(), |guard| {
            RwLockWriteGuard {
                __lock: lock,
                __poison: guard,
            }
        })
    }
}

#[stable(feature = "std_debug", since = "1.16.0")]
impl<'a, T: fmt::Debug> fmt::Debug for RwLockReadGuard<'a, T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("RwLockReadGuard")
            .field("lock", &self.__lock)
            .finish()
    }
}

#[stable(feature = "std_guard_impls", since = "1.20.0")]
impl<'a, T: ?Sized + fmt::Display> fmt::Display for RwLockReadGuard<'a, T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        (**self).fmt(f)
    }
}

#[stable(feature = "std_debug", since = "1.16.0")]
impl<'a, T: fmt::Debug> fmt::Debug for RwLockWriteGuard<'a, T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("RwLockWriteGuard")
            .field("lock", &self.__lock)
            .finish()
    }
}

#[stable(feature = "std_guard_impls", since = "1.20.0")]
impl<'a, T: ?Sized + fmt::Display> fmt::Display for RwLockWriteGuard<'a, T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        (**self).fmt(f)
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'rwlock, T: ?Sized> Deref for RwLockReadGuard<'rwlock, T> {
    type Target = T;

    fn deref(&self) -> &T {
        unsafe { &*self.__lock.data.get() }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'rwlock, T: ?Sized> Deref for RwLockWriteGuard<'rwlock, T> {
    type Target = T;

    fn deref(&self) -> &T {
        unsafe { &*self.__lock.data.get() }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'rwlock, T: ?Sized> DerefMut for RwLockWriteGuard<'rwlock, T> {
    fn deref_mut(&mut self) -> &mut T {
        unsafe { &mut *self.__lock.data.get() }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T: ?Sized> Drop for RwLockReadGuard<'a, T> {
    fn drop(&mut self) {
        unsafe { self.__lock.inner.read_unlock(); }
    }
}

#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T: ?Sized> Drop for RwLockWriteGuard<'a, T> {
    fn drop(&mut self) {
        self.__lock.poison.done(&self.__poison);
        unsafe { self.__lock.inner.write_unlock(); }
    }
}

#[cfg(all(test, not(target_os = "emscripten")))]
mod tests {
    use rand::{self, Rng};
    use sync::mpsc::channel;
    use thread;
    use sync::{Arc, RwLock, TryLockError};
    use sync::atomic::{AtomicUsize, Ordering};

    #[derive(Eq, PartialEq, Debug)]
    struct NonCopy(i32);

    #[test]
    fn smoke() {
        let l = RwLock::new(());
        drop(l.read().unwrap());
        drop(l.write().unwrap());
        drop((l.read().unwrap(), l.read().unwrap()));
        drop(l.write().unwrap());
    }

    #[test]
    fn frob() {
        const N: u32 = 10;
        const M: usize = 1000;

        let r = Arc::new(RwLock::new(()));

        let (tx, rx) = channel::<()>();
        for _ in 0..N {
            let tx = tx.clone();
            let r = r.clone();
            thread::spawn(move || {
                let mut rng = rand::thread_rng();
                for _ in 0..M {
                    if rng.gen_weighted_bool(N) {
                        drop(r.write().unwrap());
                    } else {
                        drop(r.read().unwrap());
                    }
                }
                drop(tx);
            });
        }
        drop(tx);
        let _ = rx.recv();
    }

    #[test]
    fn test_rw_arc_poison_wr() {
        let arc = Arc::new(RwLock::new(1));
        let arc2 = arc.clone();
        let _: Result<(), _> = thread::spawn(move || {
            let _lock = arc2.write().unwrap();
            panic!();
        }).join();
        assert!(arc.read().is_err());
    }

    #[test]
    fn test_rw_arc_poison_ww() {
        let arc = Arc::new(RwLock::new(1));
        assert!(!arc.is_poisoned());
        let arc2 = arc.clone();
        let _: Result<(), _> = thread::spawn(move || {
            let _lock = arc2.write().unwrap();
            panic!();
        }).join();
        assert!(arc.write().is_err());
        assert!(arc.is_poisoned());
    }

    #[test]
    fn test_rw_arc_no_poison_rr() {
        let arc = Arc::new(RwLock::new(1));
        let arc2 = arc.clone();
        let _: Result<(), _> = thread::spawn(move || {
            let _lock = arc2.read().unwrap();
            panic!();
        }).join();
        let lock = arc.read().unwrap();
        assert_eq!(*lock, 1);
    }
    #[test]
    fn test_rw_arc_no_poison_rw() {
        let arc = Arc::new(RwLock::new(1));
        let arc2 = arc.clone();
        let _: Result<(), _> = thread::spawn(move || {
            let _lock = arc2.read().unwrap();
            panic!()
        }).join();
        let lock = arc.write().unwrap();
        assert_eq!(*lock, 1);
    }

    #[test]
    fn test_rw_arc() {
        let arc = Arc::new(RwLock::new(0));
        let arc2 = arc.clone();
        let (tx, rx) = channel();

        thread::spawn(move || {
            let mut lock = arc2.write().unwrap();
            for _ in 0..10 {
                let tmp = *lock;
                *lock = -1;
                thread::yield_now();
                *lock = tmp + 1;
            }
            tx.send(()).unwrap();
        });

        // Readers try to catch the writer in the act
        let mut children = Vec::new();
        for _ in 0..5 {
            let arc3 = arc.clone();
            children.push(thread::spawn(move || {
                let lock = arc3.read().unwrap();
                assert!(*lock >= 0);
            }));
        }

        // Wait for children to pass their asserts
        for r in children {
            assert!(r.join().is_ok());
        }

        // Wait for writer to finish
        rx.recv().unwrap();
        let lock = arc.read().unwrap();
        assert_eq!(*lock, 10);
    }

    #[test]
    fn test_rw_arc_access_in_unwind() {
        let arc = Arc::new(RwLock::new(1));
        let arc2 = arc.clone();
        let _ = thread::spawn(move || -> () {
            struct Unwinder {
                i: Arc<RwLock<isize>>,
            }
            impl Drop for Unwinder {
                fn drop(&mut self) {
                    let mut lock = self.i.write().unwrap();
                    *lock += 1;
                }
            }
            let _u = Unwinder { i: arc2 };
            panic!();
        }).join();
        let lock = arc.read().unwrap();
        assert_eq!(*lock, 2);
    }

    #[test]
    fn test_rwlock_unsized() {
        let rw: &RwLock<[i32]> = &RwLock::new([1, 2, 3]);
        {
            let b = &mut *rw.write().unwrap();
            b[0] = 4;
            b[2] = 5;
        }
        let comp: &[i32] = &[4, 2, 5];
        assert_eq!(&*rw.read().unwrap(), comp);
    }

    #[test]
    fn test_rwlock_try_write() {
        let lock = RwLock::new(0isize);
        let read_guard = lock.read().unwrap();

        let write_result = lock.try_write();
        match write_result {
            Err(TryLockError::WouldBlock) => (),
            Ok(_) => assert!(false, "try_write should not succeed while read_guard is in scope"),
            Err(_) => assert!(false, "unexpected error"),
        }

        drop(read_guard);
    }

    #[test]
    fn test_into_inner() {
        let m = RwLock::new(NonCopy(10));
        assert_eq!(m.into_inner().unwrap(), NonCopy(10));
    }

    #[test]
    fn test_into_inner_drop() {
        struct Foo(Arc<AtomicUsize>);
        impl Drop for Foo {
            fn drop(&mut self) {
                self.0.fetch_add(1, Ordering::SeqCst);
            }
        }
        let num_drops = Arc::new(AtomicUsize::new(0));
        let m = RwLock::new(Foo(num_drops.clone()));
        assert_eq!(num_drops.load(Ordering::SeqCst), 0);
        {
            let _inner = m.into_inner().unwrap();
            assert_eq!(num_drops.load(Ordering::SeqCst), 0);
        }
        assert_eq!(num_drops.load(Ordering::SeqCst), 1);
    }

    #[test]
    fn test_into_inner_poison() {
        let m = Arc::new(RwLock::new(NonCopy(10)));
        let m2 = m.clone();
        let _ = thread::spawn(move || {
            let _lock = m2.write().unwrap();
            panic!("test panic in inner thread to poison RwLock");
        }).join();

        assert!(m.is_poisoned());
        match Arc::try_unwrap(m).unwrap().into_inner() {
            Err(e) => assert_eq!(e.into_inner(), NonCopy(10)),
            Ok(x) => panic!("into_inner of poisoned RwLock is Ok: {:?}", x),
        }
    }

    #[test]
    fn test_get_mut() {
        let mut m = RwLock::new(NonCopy(10));
        *m.get_mut().unwrap() = NonCopy(20);
        assert_eq!(m.into_inner().unwrap(), NonCopy(20));
    }

    #[test]
    fn test_get_mut_poison() {
        let m = Arc::new(RwLock::new(NonCopy(10)));
        let m2 = m.clone();
        let _ = thread::spawn(move || {
            let _lock = m2.write().unwrap();
            panic!("test panic in inner thread to poison RwLock");
        }).join();

        assert!(m.is_poisoned());
        match Arc::try_unwrap(m).unwrap().get_mut() {
            Err(e) => assert_eq!(*e.into_inner(), NonCopy(10)),
            Ok(x) => panic!("get_mut of poisoned RwLock is Ok: {:?}", x),
        }
    }
}