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    // Copyright 2016 lazy-static.rs Developers
    //
    // Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
    // http://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.

    extern crate core;
    extern crate std;

    use self::std::prelude::v1::*;
    use self::std::cell::Cell;
    use self::std::sync::Once;
    pub use self::std::sync::ONCE_INIT;

    // FIXME: Replace Option<T> with MaybeInitialized<T>
    pub struct Lazy<T: Sync>(Cell<Option<T>>, Once);

    impl<T: Sync> Lazy<T> {
        pub const INIT: Self = Lazy(Cell::new(None), ONCE_INIT);

        #[inline(always)]
        pub fn get<F>(&'static self, f: F) -> &T
        where
            F: FnOnce() -> T,
        {
            self.1.call_once(|| {
                self.0.set(Some(f()));
            });

            // `self.0` is guaranteed to be `Some` by this point
            // The `Once` will catch and propegate panics
            unsafe {
                match *self.0.as_ptr() {
                    Some(ref x) => x,
                    None => {
                        debug_assert!(false, "attempted to derefence an uninitialized lazy static. This is a bug");

                        unreachable_unchecked()
                    },
                }
            }
        }
    }

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

    #[macro_export]
    #[doc(hidden)]
    macro_rules! __lazy_static_create {
        ($NAME:ident, $T:ty) => {
            static $NAME: $crate::lazy::Lazy<$T> = $crate::lazy::Lazy::INIT;
        };
    }

    /// Polyfill for std::hint::unreachable_unchecked. There currently exists a
    /// [crate](https://docs.rs/unreachable) for an equivalent to std::hint::unreachable_unchecked, but
    /// lazy_static currently doesn't include any runtime dependencies and we've chosen to include this
    /// short polyfill rather than include a new crate in every consumer's build.
    ///
    /// This should be replaced by std's version when lazy_static starts to require at least Rust 1.27.
    unsafe fn unreachable_unchecked() -> ! {
        enum Void {}
        match std::mem::uninitialized::<Void>() {}
    }

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