-use crate::cell::{Cell, OnceCell};
-use crate::fmt;
use crate::ops::Deref;
+use crate::{fmt, mem};
+
+use super::UnsafeCell;
+
+enum State<T, F> {
+ Uninit(F),
+ Init(T),
+ Poisoned,
+}
/// A value which is initialized on the first access.
///
/// # Examples
///
/// ```
-/// #![feature(once_cell)]
+/// #![feature(lazy_cell)]
///
/// use std::cell::LazyCell;
///
/// // 92
/// // 92
/// ```
-#[unstable(feature = "once_cell", issue = "74465")]
+#[unstable(feature = "lazy_cell", issue = "109736")]
pub struct LazyCell<T, F = fn() -> T> {
- cell: OnceCell<T>,
- init: Cell<Option<F>>,
+ state: UnsafeCell<State<T, F>>,
}
impl<T, F: FnOnce() -> T> LazyCell<T, F> {
/// # Examples
///
/// ```
- /// #![feature(once_cell)]
+ /// #![feature(lazy_cell)]
///
/// use std::cell::LazyCell;
///
/// assert_eq!(&*lazy, "HELLO, WORLD!");
/// ```
#[inline]
- #[unstable(feature = "once_cell", issue = "74465")]
- pub const fn new(init: F) -> LazyCell<T, F> {
- LazyCell { cell: OnceCell::new(), init: Cell::new(Some(init)) }
+ #[unstable(feature = "lazy_cell", issue = "109736")]
+ pub const fn new(f: F) -> LazyCell<T, F> {
+ LazyCell { state: UnsafeCell::new(State::Uninit(f)) }
}
/// Forces the evaluation of this lazy value and returns a reference to
/// # Examples
///
/// ```
- /// #![feature(once_cell)]
+ /// #![feature(lazy_cell)]
///
/// use std::cell::LazyCell;
///
/// assert_eq!(&*lazy, &92);
/// ```
#[inline]
- #[unstable(feature = "once_cell", issue = "74465")]
+ #[unstable(feature = "lazy_cell", issue = "109736")]
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"),
- })
+ // 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,
+ }
}
}
-#[unstable(feature = "once_cell", issue = "74465")]
+#[unstable(feature = "lazy_cell", issue = "109736")]
impl<T, F: FnOnce() -> T> Deref for LazyCell<T, F> {
type Target = T;
#[inline]
}
}
-#[unstable(feature = "once_cell", issue = "74465")]
+#[unstable(feature = "lazy_cell", issue = "109736")]
impl<T: Default> Default for LazyCell<T> {
/// Creates a new lazy value using `Default` as the initializing function.
#[inline]
}
}
-#[unstable(feature = "once_cell", issue = "74465")]
+#[unstable(feature = "lazy_cell", issue = "109736")]
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()
+ let mut d = f.debug_tuple("LazyCell");
+ match self.get() {
+ Some(data) => d.field(data),
+ None => d.field(&format_args!("<uninit>")),
+ };
+ d.finish()
}
}
projects / rustc.git / blobdiff