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

    use crate::cell::{Cell, UnsafeCell};
    use crate::fmt;
    use crate::mem;
    use crate::ops::Deref;

    /// A cell which can be written to only once.
    ///
    /// Unlike `RefCell`, a `OnceCell` only provides shared `&T` references to its value.
    /// Unlike `Cell`, a `OnceCell` doesn't require copying or replacing the value to access it.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::lazy::OnceCell;
    ///
    /// let cell = OnceCell::new();
    /// assert!(cell.get().is_none());
    ///
    /// let value: &String = cell.get_or_init(|| {
    ///     "Hello, World!".to_string()
    /// });
    /// assert_eq!(value, "Hello, World!");
    /// assert!(cell.get().is_some());
    /// ```
    #[unstable(feature = "once_cell", issue = "74465")]
    pub struct OnceCell<T> {
        // Invariant: written to at most once.
        inner: UnsafeCell<Option<T>>,
    }

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

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

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

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

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

    #[unstable(feature = "once_cell", issue = "74465")]
    impl<T> From<T> for OnceCell<T> {
        fn from(value: T) -> Self {
            OnceCell { inner: UnsafeCell::new(Some(value)) }
        }
    }

    impl<T> OnceCell<T> {
        /// Creates a new empty cell.
        #[unstable(feature = "once_cell", issue = "74465")]
        pub const fn new() -> OnceCell<T> {
            OnceCell { inner: UnsafeCell::new(None) }
        }

        /// Gets the reference to the underlying value.
        ///
        /// Returns `None` if the cell is empty.
        #[unstable(feature = "once_cell", issue = "74465")]
        pub fn get(&self) -> Option<&T> {
            // SAFETY: Safe due to `inner`'s invariant
            unsafe { &*self.inner.get() }.as_ref()
        }

        /// Gets the mutable reference to the underlying value.
        ///
        /// Returns `None` if the cell is empty.
        #[unstable(feature = "once_cell", issue = "74465")]
        pub fn get_mut(&mut self) -> Option<&mut T> {
            // SAFETY: Safe because we have unique access
            unsafe { &mut *self.inner.get() }.as_mut()
        }

        /// Sets the contents of the cell to `value`.
        ///
        /// # Errors
        ///
        /// This method returns `Ok(())` if the cell was empty and `Err(value)` if
        /// it was full.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_cell)]
        ///
        /// use std::lazy::OnceCell;
        ///
        /// let cell = OnceCell::new();
        /// assert!(cell.get().is_none());
        ///
        /// assert_eq!(cell.set(92), Ok(()));
        /// assert_eq!(cell.set(62), Err(62));
        ///
        /// assert!(cell.get().is_some());
        /// ```
        #[unstable(feature = "once_cell", issue = "74465")]
        pub fn set(&self, value: T) -> Result<(), T> {
            // SAFETY: Safe because we cannot have overlapping mutable borrows
            let slot = unsafe { &*self.inner.get() };
            if slot.is_some() {
                return Err(value);
            }

            // SAFETY: This is the only place where we set the slot, no races
            // due to reentrancy/concurrency are possible, and we've
            // checked that slot is currently `None`, so this write
            // maintains the `inner`'s invariant.
            let slot = unsafe { &mut *self.inner.get() };
            *slot = Some(value);
            Ok(())
        }

        /// Gets the contents of the cell, initializing it with `f`
        /// if the cell was empty.
        ///
        /// # 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`. Doing
        /// so results in a panic.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_cell)]
        ///
        /// use std::lazy::OnceCell;
        ///
        /// let cell = OnceCell::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`. Doing
        /// so results in a panic.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_cell)]
        ///
        /// use std::lazy::OnceCell;
        ///
        /// let cell = OnceCell::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>,
        {
            if let Some(val) = self.get() {
                return Ok(val);
            }
            let val = f()?;
            // Note that *some* forms of reentrant initialization might lead to
            // UB (see `reentrant_init` test). I believe that just removing this
            // `assert`, while keeping `set/get` would be sound, but it seems
            // better to panic, rather than to silently use an old value.
            assert!(self.set(val).is_ok(), "reentrant init");
            Ok(self.get().unwrap())
        }

        /// Consumes the cell, returning the wrapped value.
        ///
        /// Returns `None` if the cell was empty.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_cell)]
        ///
        /// use std::lazy::OnceCell;
        ///
        /// let cell: OnceCell<String> = OnceCell::new();
        /// assert_eq!(cell.into_inner(), None);
        ///
        /// let cell = OnceCell::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(self) -> Option<T> {
            // Because `into_inner` takes `self` by value, the compiler statically verifies
            // that it is not currently borrowed. So it is safe to move out `Option<T>`.
            self.inner.into_inner()
        }

        /// Takes the value out of this `OnceCell`, moving it back to an uninitialized state.
        ///
        /// Has no effect and returns `None` if the `OnceCell` hasn't been initialized.
        ///
        /// Safety is guaranteed by requiring a mutable reference.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_cell)]
        ///
        /// use std::lazy::OnceCell;
        ///
        /// let mut cell: OnceCell<String> = OnceCell::new();
        /// assert_eq!(cell.take(), None);
        ///
        /// let mut cell = OnceCell::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> {
            mem::take(self).into_inner()
        }
    }

    /// A value which is initialized on the first access.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::lazy::Lazy;
    ///
    /// let lazy: Lazy<i32> = Lazy::new(|| {
    ///     println!("initializing");
    ///     92
    /// });
    /// println!("ready");
    /// println!("{}", *lazy);
    /// println!("{}", *lazy);
    ///
    /// // Prints:
    /// //   ready
    /// //   initializing
    /// //   92
    /// //   92
    /// ```
    #[unstable(feature = "once_cell", issue = "74465")]
    pub struct Lazy<T, F = fn() -> T> {
        cell: OnceCell<T>,
        init: Cell<Option<F>>,
    }

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

    impl<T, F> Lazy<T, F> {
        /// Creates a new lazy value with the given initializing function.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_cell)]
        ///
        /// # fn main() {
        /// use std::lazy::Lazy;
        ///
        /// let hello = "Hello, World!".to_string();
        ///
        /// let lazy = Lazy::new(|| hello.to_uppercase());
        ///
        /// assert_eq!(&*lazy, "HELLO, WORLD!");
        /// # }
        /// ```
        #[unstable(feature = "once_cell", issue = "74465")]
        pub const fn new(init: F) -> Lazy<T, F> {
            Lazy { cell: OnceCell::new(), init: Cell::new(Some(init)) }
        }
    }

    impl<T, F: FnOnce() -> T> Lazy<T, F> {
        /// Forces the evaluation of this lazy value and returns a reference to
        /// the result.
        ///
        /// This is equivalent to the `Deref` impl, but is explicit.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(once_cell)]
        ///
        /// use std::lazy::Lazy;
        ///
        /// let lazy = Lazy::new(|| 92);
        ///
        /// assert_eq!(Lazy::force(&lazy), &92);
        /// assert_eq!(&*lazy, &92);
        /// ```
        #[unstable(feature = "once_cell", issue = "74465")]
        pub fn force(this: &Lazy<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 Lazy<T, F> {
        type Target = T;
        fn deref(&self) -> &T {
            Lazy::force(self)
        }
    }

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