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    use cell::Cell;
    use ptr;
    use sync::Arc;
    use sys_common;
    use sys_common::mutex::Mutex;

    pub struct Lazy<T> {
        // We never call `lock.init()`, so it is UB to attempt to acquire this mutex reentrantly!
        lock: Mutex,
        ptr: Cell<*mut Arc<T>>,
    }

    #[inline]
    const fn done<T>() -> *mut Arc<T> { 1_usize as *mut _ }

    unsafe impl<T> Sync for Lazy<T> {}

    impl<T> Lazy<T> {
        pub const fn new() -> Lazy<T> {
            Lazy {
                lock: Mutex::new(),
                ptr: Cell::new(ptr::null_mut()),
            }
        }
    }

    impl<T: Send + Sync + 'static> Lazy<T> {
        /// Safety: `init` must not call `get` on the variable that is being
        /// initialized.
        pub unsafe fn get(&'static self, init: fn() -> Arc<T>) -> Option<Arc<T>> {
            let _guard = self.lock.lock();
            let ptr = self.ptr.get();
            if ptr.is_null() {
                Some(self.init(init))
            } else if ptr == done() {
                None
            } else {
                Some((*ptr).clone())
            }
        }

        // Must only be called with `lock` held
        unsafe fn init(&'static self, init: fn() -> Arc<T>) -> Arc<T> {
            // If we successfully register an at exit handler, then we cache the
            // `Arc` allocation in our own internal box (it will get deallocated by
            // the at exit handler). Otherwise we just return the freshly allocated
            // `Arc`.
            let registered = sys_common::at_exit(move || {
                let ptr = {
                    let _guard = self.lock.lock();
                    self.ptr.replace(done())
                };
                drop(Box::from_raw(ptr))
            });
            // This could reentrantly call `init` again, which is a problem
            // because our `lock` allows reentrancy!
            // That's why `get` is unsafe and requires the caller to ensure no reentrancy happens.
            let ret = init();
            if registered.is_ok() {
                self.ptr.set(Box::into_raw(Box::new(ret.clone())));
            }
            ret
        }
    }

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