diff options
Diffstat (limited to 'library/std/src/sync/lazy_lock.rs')
-rw-r--r-- | library/std/src/sync/lazy_lock.rs | 86 |
1 files changed, 70 insertions, 16 deletions
diff --git a/library/std/src/sync/lazy_lock.rs b/library/std/src/sync/lazy_lock.rs index 4a1530530..7e85d6a06 100644 --- a/library/std/src/sync/lazy_lock.rs +++ b/library/std/src/sync/lazy_lock.rs @@ -1,8 +1,21 @@ -use crate::cell::Cell; +use crate::cell::UnsafeCell; use crate::fmt; +use crate::mem::ManuallyDrop; use crate::ops::Deref; use crate::panic::{RefUnwindSafe, UnwindSafe}; -use crate::sync::OnceLock; +use crate::sync::Once; + +use super::once::ExclusiveState; + +// We use the state of a Once as discriminant value. Upon creation, the state is +// "incomplete" and `f` contains the initialization closure. In the first call to +// `call_once`, `f` is taken and run. If it succeeds, `value` is set and the state +// is changed to "complete". If it panics, the Once is poisoned, so none of the +// two fields is initialized. +union Data<T, F> { + value: ManuallyDrop<T>, + f: ManuallyDrop<F>, +} /// A value which is initialized on the first access. /// @@ -43,16 +56,17 @@ use crate::sync::OnceLock; /// ``` #[unstable(feature = "once_cell", issue = "74465")] pub struct LazyLock<T, F = fn() -> T> { - cell: OnceLock<T>, - init: Cell<Option<F>>, + once: Once, + data: UnsafeCell<Data<T, F>>, } + impl<T, F: FnOnce() -> T> LazyLock<T, F> { /// Creates a new lazy value with the given initializing /// function. #[inline] #[unstable(feature = "once_cell", issue = "74465")] pub const fn new(f: F) -> LazyLock<T, F> { - LazyLock { cell: OnceLock::new(), init: Cell::new(Some(f)) } + LazyLock { once: Once::new(), data: UnsafeCell::new(Data { f: ManuallyDrop::new(f) }) } } /// Forces the evaluation of this lazy value and @@ -74,10 +88,50 @@ impl<T, F: FnOnce() -> T> LazyLock<T, F> { #[inline] #[unstable(feature = "once_cell", issue = "74465")] pub fn force(this: &LazyLock<T, F>) -> &T { - this.cell.get_or_init(|| match this.init.take() { - Some(f) => f(), - None => panic!("Lazy instance has previously been poisoned"), - }) + this.once.call_once(|| { + // SAFETY: `call_once` only runs this closure once, ever. + let data = unsafe { &mut *this.data.get() }; + let f = unsafe { ManuallyDrop::take(&mut data.f) }; + let value = f(); + data.value = ManuallyDrop::new(value); + }); + + // SAFETY: + // There are four possible scenarios: + // * the closure was called and initialized `value`. + // * the closure was called and panicked, so this point is never reached. + // * the closure was not called, but a previous call initialized `value`. + // * the closure was not called because the Once is poisoned, so this point + // is never reached. + // So `value` has definitely been initialized and will not be modified again. + unsafe { &*(*this.data.get()).value } + } +} + +impl<T, F> LazyLock<T, F> { + /// Get the inner value if it has already been initialized. + fn get(&self) -> Option<&T> { + if self.once.is_completed() { + // SAFETY: + // The closure has been run successfully, so `value` has been initialized + // and will not be modified again. + Some(unsafe { &*(*self.data.get()).value }) + } else { + None + } + } +} + +#[unstable(feature = "once_cell", issue = "74465")] +impl<T, F> Drop for LazyLock<T, F> { + fn drop(&mut self) { + match self.once.state() { + ExclusiveState::Incomplete => unsafe { ManuallyDrop::drop(&mut self.data.get_mut().f) }, + ExclusiveState::Complete => unsafe { + ManuallyDrop::drop(&mut self.data.get_mut().value) + }, + ExclusiveState::Poisoned => {} + } } } @@ -103,23 +157,23 @@ impl<T: Default> Default for LazyLock<T> { #[unstable(feature = "once_cell", issue = "74465")] impl<T: fmt::Debug, F> fmt::Debug for LazyLock<T, F> { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { - f.debug_struct("Lazy").field("cell", &self.cell).finish_non_exhaustive() + match self.get() { + Some(v) => f.debug_tuple("LazyLock").field(v).finish(), + None => f.write_str("LazyLock(Uninit)"), + } } } // We never create a `&F` from a `&LazyLock<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 LazyLock<T, F> where OnceLock<T>: Sync {} +unsafe impl<T: Sync + Send, F: Send> Sync for LazyLock<T, F> {} // auto-derived `Send` impl is OK. #[unstable(feature = "once_cell", issue = "74465")] -impl<T, F: UnwindSafe> RefUnwindSafe for LazyLock<T, F> where OnceLock<T>: RefUnwindSafe {} +impl<T: RefUnwindSafe + UnwindSafe, F: UnwindSafe> RefUnwindSafe for LazyLock<T, F> {} #[unstable(feature = "once_cell", issue = "74465")] -impl<T, F: UnwindSafe> UnwindSafe for LazyLock<T, F> where OnceLock<T>: UnwindSafe {} +impl<T: UnwindSafe, F: UnwindSafe> UnwindSafe for LazyLock<T, F> {} #[cfg(test)] mod tests; |