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    //! Lazy values and one-time initialization of static data.

    #[cfg(test)]
    mod tests;

    use crate::{
        cell::{Cell, UnsafeCell},
        fmt,
        marker::PhantomData,
        mem::MaybeUninit,
        ops::{Deref, Drop},
        panic::{RefUnwindSafe, UnwindSafe},
        pin::Pin,
        sync::Once,
    };

    #[doc(inline)]
    #[unstable(feature = "once_cell", issue = "74465")]
    pub use core::lazy::*;

    /// A synchronization primitive which can be written to only once.
    ///
    /// This type is a thread-safe `OnceCell`.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::lazy::SyncOnceCell;
    ///
    /// static CELL: SyncOnceCell<String> = SyncOnceCell::new();
    /// assert!(CELL.get().is_none());
    ///
    /// std::thread::spawn(|| {
    ///     let value: &String = CELL.get_or_init(|| {
    ///         "Hello, World!".to_string()
    ///     });
    ///     assert_eq!(value, "Hello, World!");
    /// }).join().unwrap();
    ///
    /// let value: Option<&String> = CELL.get();
    /// assert!(value.is_some());
    /// assert_eq!(value.unwrap().as_str(), "Hello, World!");
    /// ```
    #[unstable(feature = "once_cell", issue = "74465")]
    pub struct SyncOnceCell<T> {
        once: Once,
        // Whether or not the value is initialized is tracked by `state_and_queue`.
        value: UnsafeCell<MaybeUninit<T>>,
        /// `PhantomData` to make sure dropck understands we're dropping T in our Drop impl.
        ///
        /// ```compile_fail,E0597
        /// #![feature(once_cell)]
        ///
        /// use std::lazy::SyncOnceCell;
        ///
        /// struct A<'a>(&'a str);
        ///
        /// impl<'a> Drop for A<'a> {
        ///     fn drop(&mut self) {}
        /// }
        ///
        /// let cell = SyncOnceCell::new();
        /// {
        ///     let s = String::new();
        ///     let _ = cell.set(A(&s));
        /// }
        /// ```
        _marker: PhantomData<T>,
    }

    // Why do we need `T: Send`?
    // Thread A creates a `SyncOnceCell` and shares it with
    // scoped thread B, which fills the cell, which is
    // then destroyed by A. That is, destructor observes
    // a sent value.
    #[unstable(feature = "once_cell", issue = "74465")]
    unsafe impl<T: Sync + Send> Sync for SyncOnceCell<T> {}
    #[unstable(feature = "once_cell", issue = "74465")]
    unsafe impl<T: Send> Send for SyncOnceCell<T> {}

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for SyncOnceCell<T> {}
    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T: UnwindSafe> UnwindSafe for SyncOnceCell<T> {}

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T> Default for SyncOnceCell<T> {
        fn default() -> SyncOnceCell<T> {
            SyncOnceCell::new()
        }
    }

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T: fmt::Debug> fmt::Debug for SyncOnceCell<T> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            match self.get() {
                Some(v) => f.debug_tuple("Once").field(v).finish(),
                None => f.write_str("Once(Uninit)"),
            }
        }
    }

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T: Clone> Clone for SyncOnceCell<T> {
        fn clone(&self) -> SyncOnceCell<T> {
            let cell = Self::new();
            if let Some(value) = self.get() {
                match cell.set(value.clone()) {
                    Ok(()) => (),
                    Err(_) => unreachable!(),
                }
            }
            cell
        }
    }

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T> From<T> for SyncOnceCell<T> {
        fn from(value: T) -> Self {
            let cell = Self::new();
            match cell.set(value) {
                Ok(()) => cell,
                Err(_) => unreachable!(),
            }
        }
    }

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T: PartialEq> PartialEq for SyncOnceCell<T> {
        fn eq(&self, other: &SyncOnceCell<T>) -> bool {
            self.get() == other.get()
        }
    }

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T: Eq> Eq for SyncOnceCell<T> {}

    impl<T> SyncOnceCell<T> {
        /// Creates a new empty cell.
        #[unstable(feature = "once_cell", issue = "74465")]
        pub const fn new() -> SyncOnceCell<T> {
            SyncOnceCell {
                once: Once::new(),
                value: UnsafeCell::new(MaybeUninit::uninit()),
                _marker: PhantomData,
            }
        }

        /// Gets the reference to the underlying value.
        ///
        /// Returns `None` if the cell is empty, or being initialized. This
        /// method never blocks.
        #[unstable(feature = "once_cell", issue = "74465")]
        pub fn get(&self) -> Option<&T> {
            if self.is_initialized() {
                // Safe b/c checked is_initialized
                Some(unsafe { self.get_unchecked() })
            } else {
                None
            }
        }

        /// Gets the mutable reference to the underlying value.
        ///
        /// Returns `None` if the cell is empty. This method never blocks.
        #[unstable(feature = "once_cell", issue = "74465")]
        pub fn get_mut(&mut self) -> Option<&mut T> {
            if self.is_initialized() {
                // Safe b/c checked is_initialized and we have a unique access
                Some(unsafe { self.get_unchecked_mut() })
            } else {
                None
            }
        }

        /// Sets the contents of this cell to `value`.
        ///
        /// May block if another thread is currently attempting to initialize the cell. The cell is
        /// guaranteed to contain a value when set returns, though not necessarily the one provided.
        ///
        /// Returns `Ok(())` if the cell's value was set by this call.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_cell)]
        ///
        /// use std::lazy::SyncOnceCell;
        ///
        /// static CELL: SyncOnceCell<i32> = SyncOnceCell::new();
        ///
        /// fn main() {
        ///     assert!(CELL.get().is_none());
        ///
        ///     std::thread::spawn(|| {
        ///         assert_eq!(CELL.set(92), Ok(()));
        ///     }).join().unwrap();
        ///
        ///     assert_eq!(CELL.set(62), Err(62));
        ///     assert_eq!(CELL.get(), Some(&92));
        /// }
        /// ```
        #[unstable(feature = "once_cell", issue = "74465")]
        pub fn set(&self, value: T) -> Result<(), T> {
            let mut value = Some(value);
            self.get_or_init(|| value.take().unwrap());
            match value {
                None => Ok(()),
                Some(value) => Err(value),
            }
        }

        /// Gets the contents of the cell, initializing it with `f` if the cell
        /// was empty.
        ///
        /// Many threads may call `get_or_init` concurrently with different
        /// initializing functions, but it is guaranteed that only one function
        /// will be executed.
        ///
        /// # Panics
        ///
        /// If `f` panics, the panic is propagated to the caller, and the cell
        /// remains uninitialized.
        ///
        /// It is an error to reentrantly initialize the cell from `f`. The
        /// exact outcome is unspecified. Current implementation deadlocks, but
        /// this may be changed to a panic in the future.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_cell)]
        ///
        /// use std::lazy::SyncOnceCell;
        ///
        /// let cell = SyncOnceCell::new();
        /// let value = cell.get_or_init(|| 92);
        /// assert_eq!(value, &92);
        /// let value = cell.get_or_init(|| unreachable!());
        /// assert_eq!(value, &92);
        /// ```
        #[unstable(feature = "once_cell", issue = "74465")]
        pub fn get_or_init<F>(&self, f: F) -> &T
        where
            F: FnOnce() -> T,
        {
            match self.get_or_try_init(|| Ok::<T, !>(f())) {
                Ok(val) => val,
            }
        }

        /// Gets the contents of the cell, initializing it with `f` if
        /// the cell was empty. If the cell was empty and `f` failed, an
        /// error is returned.
        ///
        /// # Panics
        ///
        /// If `f` panics, the panic is propagated to the caller, and
        /// the cell remains uninitialized.
        ///
        /// It is an error to reentrantly initialize the cell from `f`.
        /// The exact outcome is unspecified. Current implementation
        /// deadlocks, but this may be changed to a panic in the future.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_cell)]
        ///
        /// use std::lazy::SyncOnceCell;
        ///
        /// let cell = SyncOnceCell::new();
        /// assert_eq!(cell.get_or_try_init(|| Err(())), Err(()));
        /// assert!(cell.get().is_none());
        /// let value = cell.get_or_try_init(|| -> Result<i32, ()> {
        ///     Ok(92)
        /// });
        /// assert_eq!(value, Ok(&92));
        /// assert_eq!(cell.get(), Some(&92))
        /// ```
        #[unstable(feature = "once_cell", issue = "74465")]
        pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
        where
            F: FnOnce() -> Result<T, E>,
        {
            // Fast path check
            // NOTE: We need to perform an acquire on the state in this method
            // in order to correctly synchronize `SyncLazy::force`. This is
            // currently done by calling `self.get()`, which in turn calls
            // `self.is_initialized()`, which in turn performs the acquire.
            if let Some(value) = self.get() {
                return Ok(value);
            }
            self.initialize(f)?;

            debug_assert!(self.is_initialized());

            // SAFETY: The inner value has been initialized
            Ok(unsafe { self.get_unchecked() })
        }

        /// Internal-only API that gets the contents of the cell, initializing it
        /// in two steps with `f` and `g` if the cell was empty.
        ///
        /// `f` is called to construct the value, which is then moved into the cell
        /// and given as a (pinned) mutable reference to `g` to finish
        /// initialization.
        ///
        /// This allows `g` to inspect an manipulate the value after it has been
        /// moved into its final place in the cell, but before the cell is
        /// considered initialized.
        ///
        /// # Panics
        ///
        /// If `f` or `g` panics, the panic is propagated to the caller, and the
        /// cell remains uninitialized.
        ///
        /// With the current implementation, if `g` panics, the value from `f` will
        /// not be dropped. This should probably be fixed if this is ever used for
        /// a type where this matters.
        ///
        /// It is an error to reentrantly initialize the cell from `f`. The exact
        /// outcome is unspecified. Current implementation deadlocks, but this may
        /// be changed to a panic in the future.
        pub(crate) fn get_or_init_pin<F, G>(self: Pin<&Self>, f: F, g: G) -> Pin<&T>
        where
            F: FnOnce() -> T,
            G: FnOnce(Pin<&mut T>),
        {
            if let Some(value) = self.get_ref().get() {
                // SAFETY: The inner value was already initialized, and will not be
                // moved anymore.
                return unsafe { Pin::new_unchecked(value) };
            }

            let slot = &self.value;

            // Ignore poisoning from other threads
            // If another thread panics, then we'll be able to run our closure
            self.once.call_once_force(|_| {
                let value = f();
                // SAFETY: We use the Once (self.once) to guarantee unique access
                // to the UnsafeCell (slot).
                let value: &mut T = unsafe { (&mut *slot.get()).write(value) };
                // SAFETY: The value has been written to its final place in
                // self.value. We do not to move it anymore, which we promise here
                // with a Pin<&mut T>.
                g(unsafe { Pin::new_unchecked(value) });
            });

            // SAFETY: The inner value has been initialized, and will not be moved
            // anymore.
            unsafe { Pin::new_unchecked(self.get_ref().get_unchecked()) }
        }

        /// Consumes the `SyncOnceCell`, returning the wrapped value. Returns
        /// `None` if the cell was empty.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_cell)]
        ///
        /// use std::lazy::SyncOnceCell;
        ///
        /// let cell: SyncOnceCell<String> = SyncOnceCell::new();
        /// assert_eq!(cell.into_inner(), None);
        ///
        /// let cell = SyncOnceCell::new();
        /// cell.set("hello".to_string()).unwrap();
        /// assert_eq!(cell.into_inner(), Some("hello".to_string()));
        /// ```
        #[unstable(feature = "once_cell", issue = "74465")]
        pub fn into_inner(mut self) -> Option<T> {
            self.take()
        }

        /// Takes the value out of this `SyncOnceCell`, moving it back to an uninitialized state.
        ///
        /// Has no effect and returns `None` if the `SyncOnceCell` hasn't been initialized.
        ///
        /// Safety is guaranteed by requiring a mutable reference.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_cell)]
        ///
        /// use std::lazy::SyncOnceCell;
        ///
        /// let mut cell: SyncOnceCell<String> = SyncOnceCell::new();
        /// assert_eq!(cell.take(), None);
        ///
        /// let mut cell = SyncOnceCell::new();
        /// cell.set("hello".to_string()).unwrap();
        /// assert_eq!(cell.take(), Some("hello".to_string()));
        /// assert_eq!(cell.get(), None);
        /// ```
        #[unstable(feature = "once_cell", issue = "74465")]
        pub fn take(&mut self) -> Option<T> {
            if self.is_initialized() {
                self.once = Once::new();
                // SAFETY: `self.value` is initialized and contains a valid `T`.
                // `self.once` is reset, so `is_initialized()` will be false again
                // which prevents the value from being read twice.
                unsafe { Some((&mut *self.value.get()).assume_init_read()) }
            } else {
                None
            }
        }

        #[inline]
        fn is_initialized(&self) -> bool {
            self.once.is_completed()
        }

        #[cold]
        fn initialize<F, E>(&self, f: F) -> Result<(), E>
        where
            F: FnOnce() -> Result<T, E>,
        {
            let mut res: Result<(), E> = Ok(());
            let slot = &self.value;

            // Ignore poisoning from other threads
            // If another thread panics, then we'll be able to run our closure
            self.once.call_once_force(|p| {
                match f() {
                    Ok(value) => {
                        unsafe { (&mut *slot.get()).write(value) };
                    }
                    Err(e) => {
                        res = Err(e);

                        // Treat the underlying `Once` as poisoned since we
                        // failed to initialize our value. Calls
                        p.poison();
                    }
                }
            });
            res
        }

        /// # Safety
        ///
        /// The value must be initialized
        unsafe fn get_unchecked(&self) -> &T {
            debug_assert!(self.is_initialized());
            (&*self.value.get()).assume_init_ref()
        }

        /// # Safety
        ///
        /// The value must be initialized
        unsafe fn get_unchecked_mut(&mut self) -> &mut T {
            debug_assert!(self.is_initialized());
            (&mut *self.value.get()).assume_init_mut()
        }
    }

    unsafe impl<#[may_dangle] T> Drop for SyncOnceCell<T> {
        fn drop(&mut self) {
            if self.is_initialized() {
                // SAFETY: The cell is initialized and being dropped, so it can't
                // be accessed again. We also don't touch the `T` other than
                // dropping it, which validates our usage of #[may_dangle].
                unsafe { (&mut *self.value.get()).assume_init_drop() };
            }
        }
    }

    /// A value which is initialized on the first access.
    ///
    /// This type is a thread-safe `Lazy`, and can be used in statics.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::collections::HashMap;
    ///
    /// use std::lazy::SyncLazy;
    ///
    /// static HASHMAP: SyncLazy<HashMap<i32, String>> = SyncLazy::new(|| {
    ///     println!("initializing");
    ///     let mut m = HashMap::new();
    ///     m.insert(13, "Spica".to_string());
    ///     m.insert(74, "Hoyten".to_string());
    ///     m
    /// });
    ///
    /// fn main() {
    ///     println!("ready");
    ///     std::thread::spawn(|| {
    ///         println!("{:?}", HASHMAP.get(&13));
    ///     }).join().unwrap();
    ///     println!("{:?}", HASHMAP.get(&74));
    ///
    ///     // Prints:
    ///     //   ready
    ///     //   initializing
    ///     //   Some("Spica")
    ///     //   Some("Hoyten")
    /// }
    /// ```
    #[unstable(feature = "once_cell", issue = "74465")]
    pub struct SyncLazy<T, F = fn() -> T> {
        cell: SyncOnceCell<T>,
        init: Cell<Option<F>>,
    }

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T: fmt::Debug, F> fmt::Debug for SyncLazy<T, F> {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("Lazy").field("cell", &self.cell).field("init", &"..").finish()
        }
    }

    // We never create a `&F` from a `&SyncLazy<T, F>` so it is fine
    // to not impl `Sync` for `F`
    // we do create a `&mut Option<F>` in `force`, but this is
    // properly synchronized, so it only happens once
    // so it also does not contribute to this impl.
    #[unstable(feature = "once_cell", issue = "74465")]
    unsafe impl<T, F: Send> Sync for SyncLazy<T, F> where SyncOnceCell<T>: Sync {}
    // auto-derived `Send` impl is OK.

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T, F: UnwindSafe> RefUnwindSafe for SyncLazy<T, F> where SyncOnceCell<T>: RefUnwindSafe {}
    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T, F: UnwindSafe> UnwindSafe for SyncLazy<T, F> where SyncOnceCell<T>: UnwindSafe {}

    impl<T, F> SyncLazy<T, F> {
        /// Creates a new lazy value with the given initializing
        /// function.
        #[unstable(feature = "once_cell", issue = "74465")]
        pub const fn new(f: F) -> SyncLazy<T, F> {
            SyncLazy { cell: SyncOnceCell::new(), init: Cell::new(Some(f)) }
        }
    }

    impl<T, F: FnOnce() -> T> SyncLazy<T, F> {
        /// Forces the evaluation of this lazy value and
        /// returns a reference to result. This is equivalent
        /// to the `Deref` impl, but is explicit.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_cell)]
        ///
        /// use std::lazy::SyncLazy;
        ///
        /// let lazy = SyncLazy::new(|| 92);
        ///
        /// assert_eq!(SyncLazy::force(&lazy), &92);
        /// assert_eq!(&*lazy, &92);
        /// ```
        #[unstable(feature = "once_cell", issue = "74465")]
        pub fn force(this: &SyncLazy<T, F>) -> &T {
            this.cell.get_or_init(|| match this.init.take() {
                Some(f) => f(),
                None => panic!("Lazy instance has previously been poisoned"),
            })
        }
    }

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T, F: FnOnce() -> T> Deref for SyncLazy<T, F> {
        type Target = T;
        fn deref(&self) -> &T {
            SyncLazy::force(self)
        }
    }

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T: Default> Default for SyncLazy<T> {
        /// Creates a new lazy value using `Default` as the initializing function.
        fn default() -> SyncLazy<T> {
            SyncLazy::new(T::default)
        }
    }