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| 1 | +// Based on unstable std::sync::OnceLock. |
| 2 | +// |
| 3 | +// Source: https://github.com/rust-lang/rust/blob/8e9c93df464b7ada3fc7a1c8ccddd9dcb24ee0a0/library/std/src/sync/once_lock.rs |
| 4 | + |
| 5 | +use core::cell::UnsafeCell; |
| 6 | +use core::mem::MaybeUninit; |
| 7 | +use core::sync::atomic::{AtomicBool, Ordering}; |
| 8 | +use std::sync::Once; |
| 9 | + |
| 10 | +pub(crate) struct OnceLock<T> { |
| 11 | + once: Once, |
| 12 | + // Once::is_completed requires Rust 1.43, so use this to track of whether they have been initialized. |
| 13 | + is_initialized: AtomicBool, |
| 14 | + value: UnsafeCell<MaybeUninit<T>>, |
| 15 | + // Unlike std::sync::OnceLock, we don't need PhantomData here because |
| 16 | + // we don't use #[may_dangle]. |
| 17 | +} |
| 18 | + |
| 19 | +unsafe impl<T: Sync + Send> Sync for OnceLock<T> {} |
| 20 | +unsafe impl<T: Send> Send for OnceLock<T> {} |
| 21 | + |
| 22 | +impl<T> OnceLock<T> { |
| 23 | + /// Creates a new empty cell. |
| 24 | + #[must_use] |
| 25 | + pub(crate) const fn new() -> Self { |
| 26 | + Self { |
| 27 | + once: Once::new(), |
| 28 | + is_initialized: AtomicBool::new(false), |
| 29 | + value: UnsafeCell::new(MaybeUninit::uninit()), |
| 30 | + } |
| 31 | + } |
| 32 | + |
| 33 | + /// Gets the contents of the cell, initializing it with `f` if the cell |
| 34 | + /// was empty. |
| 35 | + /// |
| 36 | + /// Many threads may call `get_or_init` concurrently with different |
| 37 | + /// initializing functions, but it is guaranteed that only one function |
| 38 | + /// will be executed. |
| 39 | + /// |
| 40 | + /// # Panics |
| 41 | + /// |
| 42 | + /// If `f` panics, the panic is propagated to the caller, and the cell |
| 43 | + /// remains uninitialized. |
| 44 | + /// |
| 45 | + /// It is an error to reentrantly initialize the cell from `f`. The |
| 46 | + /// exact outcome is unspecified. Current implementation deadlocks, but |
| 47 | + /// this may be changed to a panic in the future. |
| 48 | + pub(crate) fn get_or_init<F>(&self, f: F) -> &T |
| 49 | + where |
| 50 | + F: FnOnce() -> T, |
| 51 | + { |
| 52 | + // Fast path check |
| 53 | + if self.is_initialized() { |
| 54 | + // SAFETY: The inner value has been initialized |
| 55 | + return unsafe { self.get_unchecked() }; |
| 56 | + } |
| 57 | + self.initialize(f); |
| 58 | + |
| 59 | + debug_assert!(self.is_initialized()); |
| 60 | + |
| 61 | + // SAFETY: The inner value has been initialized |
| 62 | + unsafe { self.get_unchecked() } |
| 63 | + } |
| 64 | + |
| 65 | + #[inline] |
| 66 | + fn is_initialized(&self) -> bool { |
| 67 | + self.is_initialized.load(Ordering::Acquire) |
| 68 | + } |
| 69 | + |
| 70 | + #[cold] |
| 71 | + fn initialize<F>(&self, f: F) |
| 72 | + where |
| 73 | + F: FnOnce() -> T, |
| 74 | + { |
| 75 | + let slot = self.value.get().cast::<T>(); |
| 76 | + let is_initialized = &self.is_initialized; |
| 77 | + |
| 78 | + self.once.call_once(|| { |
| 79 | + let value = f(); |
| 80 | + unsafe { |
| 81 | + slot.write(value); |
| 82 | + } |
| 83 | + is_initialized.store(true, Ordering::Release); |
| 84 | + }); |
| 85 | + } |
| 86 | + |
| 87 | + /// # Safety |
| 88 | + /// |
| 89 | + /// The value must be initialized |
| 90 | + unsafe fn get_unchecked(&self) -> &T { |
| 91 | + debug_assert!(self.is_initialized()); |
| 92 | + &*self.value.get().cast::<T>() |
| 93 | + } |
| 94 | +} |
| 95 | + |
| 96 | +impl<T> Drop for OnceLock<T> { |
| 97 | + fn drop(&mut self) { |
| 98 | + if self.is_initialized() { |
| 99 | + // SAFETY: The inner value has been initialized |
| 100 | + unsafe { self.value.get().cast::<T>().drop_in_place() }; |
| 101 | + } |
| 102 | + } |
| 103 | +} |
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