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use crate::cell::{Cell, OnceCell};
use crate::fmt;
use crate::ops::Deref;
/// A value which is initialized on the first access.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::cell::LazyCell;
///
/// let lazy: LazyCell<i32> = LazyCell::new(|| {
/// println!("initializing");
/// 92
/// });
/// println!("ready");
/// println!("{}", *lazy);
/// println!("{}", *lazy);
///
/// // Prints:
/// // ready
/// // initializing
/// // 92
/// // 92
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub struct LazyCell<T, F = fn() -> T> {
 cell: OnceCell<T>,
 init: Cell<Option<F>>,
}
impl<T, F> LazyCell<T, F> {
 /// Creates a new lazy value with the given initializing function.
 ///
 /// # Examples
 ///
 /// ```
 /// #![feature(once_cell)]
 ///
 /// use std::cell::LazyCell;
 ///
 /// let hello = "Hello, World!".to_string();
 ///
 /// let lazy = LazyCell::new(|| hello.to_uppercase());
 ///
 /// assert_eq!(&*lazy, "HELLO, WORLD!");
 /// ```
 #[unstable(feature = "once_cell", issue = "74465")]
 pub const fn new(init: F) -> LazyCell<T, F> {
 LazyCell { cell: OnceCell::new(), init: Cell::new(Some(init)) }
 }
}
impl<T, F: FnOnce() -> T> LazyCell<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::cell::LazyCell;
 ///
 /// let lazy = LazyCell::new(|| 92);
 ///
 /// assert_eq!(LazyCell::force(&lazy), &92);
 /// assert_eq!(&*lazy, &92);
 /// ```
 #[unstable(feature = "once_cell", issue = "74465")]
 pub fn force(this: &LazyCell<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 LazyCell<T, F> {
 type Target = T;
 fn deref(&self) -> &T {
 LazyCell::force(self)
 }
}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T: Default> Default for LazyCell<T> {
 /// Creates a new lazy value using `Default` as the initializing function.
 fn default() -> LazyCell<T> {
 LazyCell::new(T::default)
 }
}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T: fmt::Debug, F> fmt::Debug for LazyCell<T, F> {
 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 f.debug_struct("Lazy").field("cell", &self.cell).field("init", &"..").finish()
 }
}