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
5use core::cell::UnsafeCell;
6use core::mem::MaybeUninit;
7use core::sync::atomic::{AtomicBool, Ordering};
8use std::sync::Once;
9
10pub(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
19unsafe impl<T: Sync + Send> Sync for OnceLock<T> {}
20unsafe impl<T: Send> Send for OnceLock<T> {}
21
22impl<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
96impl<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}
104