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 std::sync::Once;
8
9pub(crate) struct OnceLock<T> {
10 once: Once,
11 value: UnsafeCell<MaybeUninit<T>>,
12 // Unlike std::sync::OnceLock, we don't need PhantomData here because
13 // we don't use #[may_dangle].
14}
15
16unsafe impl<T: Sync + Send> Sync for OnceLock<T> {}
17unsafe impl<T: Send> Send for OnceLock<T> {}
18
19impl<T> OnceLock<T> {
20 /// Creates a new empty cell.
21 #[must_use]
22 pub(crate) const fn new() -> Self {
23 Self {
24 once: Once::new(),
25 value: UnsafeCell::new(MaybeUninit::uninit()),
26 }
27 }
28
29 /// Gets the contents of the cell, initializing it with `f` if the cell
30 /// was empty.
31 ///
32 /// Many threads may call `get_or_init` concurrently with different
33 /// initializing functions, but it is guaranteed that only one function
34 /// will be executed.
35 ///
36 /// # Panics
37 ///
38 /// If `f` panics, the panic is propagated to the caller, and the cell
39 /// remains uninitialized.
40 ///
41 /// It is an error to reentrantly initialize the cell from `f`. The
42 /// exact outcome is unspecified. Current implementation deadlocks, but
43 /// this may be changed to a panic in the future.
44 pub(crate) fn get_or_init<F>(&self, f: F) -> &T
45 where
46 F: FnOnce() -> T,
47 {
48 // Fast path check
49 if self.once.is_completed() {
50 // SAFETY: The inner value has been initialized
51 return unsafe { self.get_unchecked() };
52 }
53 self.initialize(f);
54
55 // SAFETY: The inner value has been initialized
56 unsafe { self.get_unchecked() }
57 }
58
59 #[cold]
60 fn initialize<F>(&self, f: F)
61 where
62 F: FnOnce() -> T,
63 {
64 let slot = self.value.get();
65
66 self.once.call_once(|| {
67 let value = f();
68 unsafe { slot.write(MaybeUninit::new(value)) }
69 });
70 }
71
72 /// # Safety
73 ///
74 /// The value must be initialized
75 unsafe fn get_unchecked(&self) -> &T {
76 debug_assert!(self.once.is_completed());
77 &*self.value.get().cast::<T>()
78 }
79}
80
81impl<T> Drop for OnceLock<T> {
82 fn drop(&mut self) {
83 if self.once.is_completed() {
84 // SAFETY: The inner value has been initialized
85 unsafe { (*self.value.get()).assume_init_drop() };
86 }
87 }
88}
89