89 lines
2.5 KiB
Rust
89 lines
2.5 KiB
Rust
// Based on unstable std::sync::OnceLock.
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//
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// Source: https://github.com/rust-lang/rust/blob/8e9c93df464b7ada3fc7a1c8ccddd9dcb24ee0a0/library/std/src/sync/once_lock.rs
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use core::cell::UnsafeCell;
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use core::mem::MaybeUninit;
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use std::sync::Once;
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pub(crate) struct OnceLock<T> {
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once: Once,
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value: UnsafeCell<MaybeUninit<T>>,
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// Unlike std::sync::OnceLock, we don't need PhantomData here because
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// we don't use #[may_dangle].
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}
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unsafe impl<T: Sync + Send> Sync for OnceLock<T> {}
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unsafe impl<T: Send> Send for OnceLock<T> {}
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impl<T> OnceLock<T> {
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/// Creates a new empty cell.
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#[must_use]
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pub(crate) const fn new() -> Self {
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Self {
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once: Once::new(),
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value: UnsafeCell::new(MaybeUninit::uninit()),
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}
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}
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/// Gets the contents of the cell, initializing it with `f` if the cell
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/// was empty.
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///
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/// Many threads may call `get_or_init` concurrently with different
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/// initializing functions, but it is guaranteed that only one function
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/// will be executed.
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///
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/// # Panics
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///
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/// If `f` panics, the panic is propagated to the caller, and the cell
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/// remains uninitialized.
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///
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/// It is an error to reentrantly initialize the cell from `f`. The
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/// exact outcome is unspecified. Current implementation deadlocks, but
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/// this may be changed to a panic in the future.
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pub(crate) fn get_or_init<F>(&self, f: F) -> &T
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where
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F: FnOnce() -> T,
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{
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// Fast path check
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if self.once.is_completed() {
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// SAFETY: The inner value has been initialized
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return unsafe { self.get_unchecked() };
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}
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self.initialize(f);
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// SAFETY: The inner value has been initialized
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unsafe { self.get_unchecked() }
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}
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#[cold]
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fn initialize<F>(&self, f: F)
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where
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F: FnOnce() -> T,
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{
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let slot = self.value.get();
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self.once.call_once(|| {
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let value = f();
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unsafe { slot.write(MaybeUninit::new(value)) }
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});
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}
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/// # Safety
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///
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/// The value must be initialized
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unsafe fn get_unchecked(&self) -> &T {
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debug_assert!(self.once.is_completed());
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&*self.value.get().cast::<T>()
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}
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}
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impl<T> Drop for OnceLock<T> {
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fn drop(&mut self) {
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if self.once.is_completed() {
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// SAFETY: The inner value has been initialized
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unsafe { (*self.value.get()).assume_init_drop() };
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}
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}
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}
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