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use super::UnsafeCell;
use crate::ops::Deref;
use crate::{fmt, mem};
enum State<T, F> {
Uninit(F),
Init(T),
Poisoned,
}
/// A value which is initialized on the first access.
///
/// For a thread-safe version of this struct, see [`std::sync::LazyLock`].
///
/// [`std::sync::LazyLock`]: ../../std/sync/struct.LazyLock.html
///
/// # Examples
///
/// ```
/// use std::cell::LazyCell;
///
/// let lazy: LazyCell<i32> = LazyCell::new(|| {
/// println!("initializing");
/// 92
/// });
/// println!("ready");
/// println!("{}", *lazy);
/// println!("{}", *lazy);
///
/// // Prints:
/// // ready
/// // initializing
/// // 92
/// // 92
/// ```
#[stable(feature = "lazy_cell", since = "1.80.0")]
pub struct LazyCell<T, F = fn() -> T> {
state: UnsafeCell<State<T, F>>,
}
impl<T, F: FnOnce() -> T> LazyCell<T, F> {
/// Creates a new lazy value with the given initializing function.
///
/// # Examples
///
/// ```
/// use std::cell::LazyCell;
///
/// let hello = "Hello, World!".to_string();
///
/// let lazy = LazyCell::new(|| hello.to_uppercase());
///
/// assert_eq!(&*lazy, "HELLO, WORLD!");
/// ```
#[inline]
#[stable(feature = "lazy_cell", since = "1.80.0")]
#[rustc_const_stable(feature = "lazy_cell", since = "1.80.0")]
pub const fn new(f: F) -> LazyCell<T, F> {
LazyCell { state: UnsafeCell::new(State::Uninit(f)) }
}
/// Consumes this `LazyCell` returning the stored value.
///
/// Returns `Ok(value)` if `Lazy` is initialized and `Err(f)` otherwise.
///
/// # Examples
///
/// ```
/// #![feature(lazy_cell_into_inner)]
///
/// use std::cell::LazyCell;
///
/// let hello = "Hello, World!".to_string();
///
/// let lazy = LazyCell::new(|| hello.to_uppercase());
///
/// assert_eq!(&*lazy, "HELLO, WORLD!");
/// assert_eq!(LazyCell::into_inner(lazy).ok(), Some("HELLO, WORLD!".to_string()));
/// ```
#[unstable(feature = "lazy_cell_into_inner", issue = "125623")]
pub fn into_inner(this: Self) -> Result<T, F> {
match this.state.into_inner() {
State::Init(data) => Ok(data),
State::Uninit(f) => Err(f),
State::Poisoned => panic!("LazyCell instance has previously been poisoned"),
}
}
/// 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
///
/// ```
/// use std::cell::LazyCell;
///
/// let lazy = LazyCell::new(|| 92);
///
/// assert_eq!(LazyCell::force(&lazy), &92);
/// assert_eq!(&*lazy, &92);
/// ```
#[inline]
#[stable(feature = "lazy_cell", since = "1.80.0")]
pub fn force(this: &LazyCell<T, F>) -> &T {
// SAFETY:
// This invalidates any mutable references to the data. The resulting
// reference lives either until the end of the borrow of `this` (in the
// initialized case) or is invalidated in `really_init` (in the
// uninitialized case; `really_init` will create and return a fresh reference).
let state = unsafe { &*this.state.get() };
match state {
State::Init(data) => data,
// SAFETY: The state is uninitialized.
State::Uninit(_) => unsafe { LazyCell::really_init(this) },
State::Poisoned => panic!("LazyCell has previously been poisoned"),
}
}
/// # Safety
/// May only be called when the state is `Uninit`.
#[cold]
unsafe fn really_init(this: &LazyCell<T, F>) -> &T {
// SAFETY:
// This function is only called when the state is uninitialized,
// so no references to `state` can exist except for the reference
// in `force`, which is invalidated here and not accessed again.
let state = unsafe { &mut *this.state.get() };
// Temporarily mark the state as poisoned. This prevents reentrant
// accesses and correctly poisons the cell if the closure panicked.
let State::Uninit(f) = mem::replace(state, State::Poisoned) else { unreachable!() };
let data = f();
// SAFETY:
// If the closure accessed the cell through something like a reentrant
// mutex, but caught the panic resulting from the state being poisoned,
// the mutable borrow for `state` will be invalidated, so we need to
// go through the `UnsafeCell` pointer here. The state can only be
// poisoned at this point, so using `write` to skip the destructor
// of `State` should help the optimizer.
unsafe { this.state.get().write(State::Init(data)) };
// SAFETY:
// The previous references were invalidated by the `write` call above,
// so do a new shared borrow of the state instead.
let state = unsafe { &*this.state.get() };
let State::Init(data) = state else { unreachable!() };
data
}
}
impl<T, F> LazyCell<T, F> {
#[inline]
fn get(&self) -> Option<&T> {
// SAFETY:
// This is sound for the same reason as in `force`: once the state is
// initialized, it will not be mutably accessed again, so this reference
// will stay valid for the duration of the borrow to `self`.
let state = unsafe { &*self.state.get() };
match state {
State::Init(data) => Some(data),
_ => None,
}
}
}
#[stable(feature = "lazy_cell", since = "1.80.0")]
impl<T, F: FnOnce() -> T> Deref for LazyCell<T, F> {
type Target = T;
#[inline]
fn deref(&self) -> &T {
LazyCell::force(self)
}
}
#[stable(feature = "lazy_cell", since = "1.80.0")]
impl<T: Default> Default for LazyCell<T> {
/// Creates a new lazy value using `Default` as the initializing function.
#[inline]
fn default() -> LazyCell<T> {
LazyCell::new(T::default)
}
}
#[stable(feature = "lazy_cell", since = "1.80.0")]
impl<T: fmt::Debug, F> fmt::Debug for LazyCell<T, F> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let mut d = f.debug_tuple("LazyCell");
match self.get() {
Some(data) => d.field(data),
None => d.field(&format_args!("<uninit>")),
};
d.finish()
}
}