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    use std::marker::PhantomData;
    use std::mem;
    use std::sync::Arc;
    use std::sync::atomic::{AtomicUsize, Ordering};

    /// A type providing atomic storage and retrieval of an `Arc<T>`.
    #[derive(Debug)]
    pub struct ArcCell<T>(AtomicUsize, PhantomData<Arc<T>>);

    impl<T> Drop for ArcCell<T> {
        fn drop(&mut self) {
            self.take();
        }
    }

    impl<T> ArcCell<T> {
        /// Creates a new `ArcCell`.
        pub fn new(t: Arc<T>) -> ArcCell<T> {
            ArcCell(AtomicUsize::new(unsafe { mem::transmute(t) }), PhantomData)
        }

        fn take(&self) -> Arc<T> {
            loop {
                match self.0.swap(0, Ordering::Acquire) {
                    0 => {}
                    n => return unsafe { mem::transmute(n) }
                }
            }
        }

        fn put(&self, t: Arc<T>) {
            debug_assert_eq!(self.0.load(Ordering::SeqCst), 0);
            self.0.store(unsafe { mem::transmute(t) }, Ordering::Release);
        }

        /// Stores a new value in the `ArcCell`, returning the previous
        /// value.
        pub fn set(&self, t: Arc<T>) -> Arc<T> {
            let old = self.take();
            self.put(t);
            old
        }

        /// Returns a copy of the value stored by the `ArcCell`.
        pub fn get(&self) -> Arc<T> {
            let t = self.take();
            // NB: correctness here depends on Arc's clone impl not panicking
            let out = t.clone();
            self.put(t);
            out
        }
    }

    #[cfg(test)]
    mod test {
        use std::sync::Arc;
        use std::sync::atomic::{ATOMIC_USIZE_INIT, AtomicUsize, Ordering};

        use super::*;

        #[test]
        fn basic() {
            let r = ArcCell::new(Arc::new(0));
            assert_eq!(*r.get(), 0);
            assert_eq!(*r.set(Arc::new(1)), 0);
            assert_eq!(*r.get(), 1);
        }

        #[test]
        fn drop_runs() {
            static DROPS: AtomicUsize = ATOMIC_USIZE_INIT;

            struct Foo;

            impl Drop for Foo {
                fn drop(&mut self) {
                    DROPS.fetch_add(1, Ordering::SeqCst);
                }
            }

            let r = ArcCell::new(Arc::new(Foo));
            let _f = r.get();
            r.get();
            r.set(Arc::new(Foo));
            drop(_f);
            assert_eq!(DROPS.load(Ordering::SeqCst), 1);
            drop(r);
            assert_eq!(DROPS.load(Ordering::SeqCst), 2);
        }
    }

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