1#![allow(dead_code)]
2use std::cell::UnsafeCell;
3use std::mem::MaybeUninit;
4use std::sync::Once;
5
6pub(crate) struct OnceCell<T> {
7 once: Once,
8 value: UnsafeCell<MaybeUninit<T>>,
9}
10
11unsafe impl<T: Send + Sync> Send for OnceCell<T> {}
12unsafe impl<T: Send + Sync> Sync for OnceCell<T> {}
13
14impl<T> OnceCell<T> {
15 pub(crate) const fn new() -> Self {
16 Self {
17 once: Once::new(),
18 value: UnsafeCell::new(MaybeUninit::uninit()),
19 }
20 }
21
22 /// Get the value inside this cell, initializing it using the provided
23 /// function if necessary.
24 ///
25 /// If the `init` closure panics, then the `OnceCell` is poisoned and all
26 /// future calls to `get` will panic.
27 #[inline]
28 pub(crate) fn get(&self, init: impl FnOnce() -> T) -> &T {
29 if !self.once.is_completed() {
30 self.do_init(init);
31 }
32
33 // Safety: The `std::sync::Once` guarantees that we can only reach this
34 // line if a `call_once` closure has been run exactly once and without
35 // panicking. Thus, the value is not uninitialized.
36 //
37 // There is also no race because the only `&self` method that modifies
38 // `value` is `do_init`, but if the `call_once` closure is still
39 // running, then no thread has gotten past the `call_once`.
40 unsafe { &*(self.value.get() as *const T) }
41 }
42
43 #[cold]
44 fn do_init(&self, init: impl FnOnce() -> T) {
45 let value_ptr = self.value.get() as *mut T;
46
47 self.once.call_once(|| {
48 let set_to = init();
49
50 // Safety: The `std::sync::Once` guarantees that this initialization
51 // will run at most once, and that no thread can get past the
52 // `call_once` until it has run exactly once. Thus, we have
53 // exclusive access to `value`.
54 unsafe {
55 std::ptr::write(value_ptr, set_to);
56 }
57 });
58 }
59}
60
61impl<T> Drop for OnceCell<T> {
62 fn drop(&mut self) {
63 if self.once.is_completed() {
64 let value_ptr = self.value.get() as *mut T;
65 unsafe {
66 std::ptr::drop_in_place(value_ptr);
67 }
68 }
69 }
70}
71