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    // Copyright 2015 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 ptr;
    use sync::Arc;
    use sys_common;
    use sys_common::mutex::Mutex;

    pub struct Lazy<T> {
        lock: Mutex,
        ptr: Cell<*mut Arc<T>>,
        init: fn() -> Arc<T>,
    }

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

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

    impl<T: Send + Sync + 'static> Lazy<T> {
        pub const fn new(init: fn() -> Arc<T>) -> Lazy<T> {
            Lazy {
                lock: Mutex::new(),
                ptr: Cell::new(ptr::null_mut()),
                init,
            }
        }

        pub fn get(&'static self) -> Option<Arc<T>> {
            unsafe {
                let _guard = self.lock.lock();
                let ptr = self.ptr.get();
                if ptr.is_null() {
                    Some(self.init())
                } else if ptr == done() {
                    None
                } else {
                    Some((*ptr).clone())
                }
            }
        }

        unsafe fn init(&'static self) -> 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))
            });
            let ret = (self.init)();
            if registered.is_ok() {
                self.ptr.set(Box::into_raw(Box::new(ret.clone())));
            }
            ret
        }
    }

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