1use crate::cell::UnsafeCell;
2use crate::fmt;
3use crate::marker::PhantomData;
4use crate::mem::MaybeUninit;
5use crate::panic::{RefUnwindSafe, UnwindSafe};
6use crate::sync::Once;
7
8/// A synchronization primitive which can nominally be written to only once.
9///
10/// This type is a thread-safe [`OnceCell`], and can be used in statics.
11/// In many simple cases, you can use [`LazyLock<T, F>`] instead to get the benefits of this type
12/// with less effort: `LazyLock<T, F>` "looks like" `&T` because it initializes with `F` on deref!
13/// Where OnceLock shines is when LazyLock is too simple to support a given case, as LazyLock
14/// doesn't allow additional inputs to its function after you call [`LazyLock::new(|| ...)`].
15///
16/// A `OnceLock` can be thought of as a safe abstraction over uninitialized data that becomes
17/// initialized once written.
18///
19/// [`OnceCell`]: crate::cell::OnceCell
20/// [`LazyLock<T, F>`]: crate::sync::LazyLock
21/// [`LazyLock::new(|| ...)`]: crate::sync::LazyLock::new
22///
23/// # Examples
24///
25/// Writing to a `OnceLock` from a separate thread:
26///
27/// ```
28/// use std::sync::OnceLock;
29///
30/// static CELL: OnceLock<usize> = OnceLock::new();
31///
32/// // `OnceLock` has not been written to yet.
33/// assert!(CELL.get().is_none());
34///
35/// // Spawn a thread and write to `OnceLock`.
36/// std::thread::spawn(|| {
37/// let value = CELL.get_or_init(|| 12345);
38/// assert_eq!(value, &12345);
39/// })
40/// .join()
41/// .unwrap();
42///
43/// // `OnceLock` now contains the value.
44/// assert_eq!(
45/// CELL.get(),
46/// Some(&12345),
47/// );
48/// ```
49///
50/// You can use `OnceLock` to implement a type that requires "append-only" logic:
51///
52/// ```
53/// use std::sync::{OnceLock, atomic::{AtomicU32, Ordering}};
54/// use std::thread;
55///
56/// struct OnceList<T> {
57/// data: OnceLock<T>,
58/// next: OnceLock<Box<OnceList<T>>>,
59/// }
60/// impl<T> OnceList<T> {
61/// const fn new() -> OnceList<T> {
62/// OnceList { data: OnceLock::new(), next: OnceLock::new() }
63/// }
64/// fn push(&self, value: T) {
65/// // FIXME: this impl is concise, but is also slow for long lists or many threads.
66/// // as an exercise, consider how you might improve on it while preserving the behavior
67/// if let Err(value) = self.data.set(value) {
68/// let next = self.next.get_or_init(|| Box::new(OnceList::new()));
69/// next.push(value)
70/// };
71/// }
72/// fn contains(&self, example: &T) -> bool
73/// where
74/// T: PartialEq,
75/// {
76/// self.data.get().map(|item| item == example).filter(|v| *v).unwrap_or_else(|| {
77/// self.next.get().map(|next| next.contains(example)).unwrap_or(false)
78/// })
79/// }
80/// }
81///
82/// // Let's exercise this new Sync append-only list by doing a little counting
83/// static LIST: OnceList<u32> = OnceList::new();
84/// static COUNTER: AtomicU32 = AtomicU32::new(0);
85///
86/// # const LEN: u32 = if cfg!(miri) { 50 } else { 1000 };
87/// # /*
88/// const LEN: u32 = 1000;
89/// # */
90/// thread::scope(|s| {
91/// for _ in 0..thread::available_parallelism().unwrap().get() {
92/// s.spawn(|| {
93/// while let i @ 0..LEN = COUNTER.fetch_add(1, Ordering::Relaxed) {
94/// LIST.push(i);
95/// }
96/// });
97/// }
98/// });
99///
100/// for i in 0..LEN {
101/// assert!(LIST.contains(&i));
102/// }
103///
104/// ```
105#[stable(feature = "once_cell", since = "1.70.0")]
106pub struct OnceLock<T> {
107 // FIXME(nonpoison_once): switch to nonpoison version once it is available
108 once: Once,
109 // Whether or not the value is initialized is tracked by `once.is_completed()`.
110 value: UnsafeCell<MaybeUninit<T>>,
111 /// `PhantomData` to make sure dropck understands we're dropping T in our Drop impl.
112 ///
113 /// ```compile_fail,E0597
114 /// use std::sync::OnceLock;
115 ///
116 /// struct A<'a>(&'a str);
117 ///
118 /// impl<'a> Drop for A<'a> {
119 /// fn drop(&mut self) {}
120 /// }
121 ///
122 /// let cell = OnceLock::new();
123 /// {
124 /// let s = String::new();
125 /// let _ = cell.set(A(&s));
126 /// }
127 /// ```
128 _marker: PhantomData<T>,
129}
130
131impl<T> OnceLock<T> {
132 /// Creates a new uninitialized cell.
133 #[inline]
134 #[must_use]
135 #[stable(feature = "once_cell", since = "1.70.0")]
136 #[rustc_const_stable(feature = "once_cell", since = "1.70.0")]
137 pub const fn new() -> OnceLock<T> {
138 OnceLock {
139 once: Once::new(),
140 value: UnsafeCell::new(MaybeUninit::uninit()),
141 _marker: PhantomData,
142 }
143 }
144
145 /// Gets the reference to the underlying value.
146 ///
147 /// Returns `None` if the cell is uninitialized, or being initialized.
148 /// This method never blocks.
149 #[inline]
150 #[stable(feature = "once_cell", since = "1.70.0")]
151 pub fn get(&self) -> Option<&T> {
152 if self.is_initialized() {
153 // Safe b/c checked is_initialized
154 Some(unsafe { self.get_unchecked() })
155 } else {
156 None
157 }
158 }
159
160 /// Gets the mutable reference to the underlying value.
161 ///
162 /// Returns `None` if the cell is uninitialized, or being initialized.
163 /// This method never blocks.
164 #[inline]
165 #[stable(feature = "once_cell", since = "1.70.0")]
166 pub fn get_mut(&mut self) -> Option<&mut T> {
167 if self.is_initialized() {
168 // Safe b/c checked is_initialized and we have a unique access
169 Some(unsafe { self.get_unchecked_mut() })
170 } else {
171 None
172 }
173 }
174
175 /// Blocks the current thread until the cell is initialized.
176 ///
177 /// # Example
178 ///
179 /// Waiting for a computation on another thread to finish:
180 /// ```rust
181 /// use std::thread;
182 /// use std::sync::OnceLock;
183 ///
184 /// let value = OnceLock::new();
185 ///
186 /// thread::scope(|s| {
187 /// s.spawn(|| value.set(1 + 1));
188 ///
189 /// let result = value.wait();
190 /// assert_eq!(result, &2);
191 /// })
192 /// ```
193 #[inline]
194 #[stable(feature = "once_wait", since = "1.86.0")]
195 pub fn wait(&self) -> &T {
196 self.once.wait_force();
197
198 unsafe { self.get_unchecked() }
199 }
200
201 /// Initializes the contents of the cell to `value`.
202 ///
203 /// May block if another thread is currently attempting to initialize the cell. The cell is
204 /// guaranteed to contain a value when `set` returns, though not necessarily the one provided.
205 ///
206 /// Returns `Ok(())` if the cell was uninitialized and
207 /// `Err(value)` if the cell was already initialized.
208 ///
209 /// # Examples
210 ///
211 /// ```
212 /// use std::sync::OnceLock;
213 ///
214 /// static CELL: OnceLock<i32> = OnceLock::new();
215 ///
216 /// fn main() {
217 /// assert!(CELL.get().is_none());
218 ///
219 /// std::thread::spawn(|| {
220 /// assert_eq!(CELL.set(92), Ok(()));
221 /// }).join().unwrap();
222 ///
223 /// assert_eq!(CELL.set(62), Err(62));
224 /// assert_eq!(CELL.get(), Some(&92));
225 /// }
226 /// ```
227 #[inline]
228 #[stable(feature = "once_cell", since = "1.70.0")]
229 pub fn set(&self, value: T) -> Result<(), T> {
230 match self.try_insert(value) {
231 Ok(_) => Ok(()),
232 Err((_, value)) => Err(value),
233 }
234 }
235
236 /// Initializes the contents of the cell to `value` if the cell was uninitialized,
237 /// then returns a reference to it.
238 ///
239 /// May block if another thread is currently attempting to initialize the cell. The cell is
240 /// guaranteed to contain a value when `try_insert` returns, though not necessarily the
241 /// one provided.
242 ///
243 /// Returns `Ok(&value)` if the cell was uninitialized and
244 /// `Err((&current_value, value))` if it was already initialized.
245 ///
246 /// # Examples
247 ///
248 /// ```
249 /// #![feature(once_cell_try_insert)]
250 ///
251 /// use std::sync::OnceLock;
252 ///
253 /// static CELL: OnceLock<i32> = OnceLock::new();
254 ///
255 /// fn main() {
256 /// assert!(CELL.get().is_none());
257 ///
258 /// std::thread::spawn(|| {
259 /// assert_eq!(CELL.try_insert(92), Ok(&92));
260 /// }).join().unwrap();
261 ///
262 /// assert_eq!(CELL.try_insert(62), Err((&92, 62)));
263 /// assert_eq!(CELL.get(), Some(&92));
264 /// }
265 /// ```
266 #[inline]
267 #[unstable(feature = "once_cell_try_insert", issue = "116693")]
268 pub fn try_insert(&self, value: T) -> Result<&T, (&T, T)> {
269 let mut value = Some(value);
270 let res = self.get_or_init(|| value.take().unwrap());
271 match value {
272 None => Ok(res),
273 Some(value) => Err((res, value)),
274 }
275 }
276
277 /// Gets the contents of the cell, initializing it to `f()` if the cell
278 /// was uninitialized.
279 ///
280 /// Many threads may call `get_or_init` concurrently with different
281 /// initializing functions, but it is guaranteed that only one function
282 /// will be executed.
283 ///
284 /// # Panics
285 ///
286 /// If `f()` panics, the panic is propagated to the caller, and the cell
287 /// remains uninitialized.
288 ///
289 /// It is an error to reentrantly initialize the cell from `f`. The
290 /// exact outcome is unspecified. Current implementation deadlocks, but
291 /// this may be changed to a panic in the future.
292 ///
293 /// # Examples
294 ///
295 /// ```
296 /// use std::sync::OnceLock;
297 ///
298 /// let cell = OnceLock::new();
299 /// let value = cell.get_or_init(|| 92);
300 /// assert_eq!(value, &92);
301 /// let value = cell.get_or_init(|| unreachable!());
302 /// assert_eq!(value, &92);
303 /// ```
304 #[inline]
305 #[stable(feature = "once_cell", since = "1.70.0")]
306 pub fn get_or_init<F>(&self, f: F) -> &T
307 where
308 F: FnOnce() -> T,
309 {
310 match self.get_or_try_init(|| Ok::<T, !>(f())) {
311 Ok(val) => val,
312 }
313 }
314
315 /// Gets the mutable reference of the contents of the cell, initializing
316 /// it to `f()` if the cell was uninitialized.
317 ///
318 /// This method never blocks.
319 ///
320 /// # Panics
321 ///
322 /// If `f()` panics, the panic is propagated to the caller, and the cell
323 /// remains uninitialized.
324 ///
325 /// # Examples
326 ///
327 /// ```
328 /// #![feature(once_cell_get_mut)]
329 ///
330 /// use std::sync::OnceLock;
331 ///
332 /// let mut cell = OnceLock::new();
333 /// let value = cell.get_mut_or_init(|| 92);
334 /// assert_eq!(*value, 92);
335 ///
336 /// *value += 2;
337 /// assert_eq!(*value, 94);
338 ///
339 /// let value = cell.get_mut_or_init(|| unreachable!());
340 /// assert_eq!(*value, 94);
341 /// ```
342 #[inline]
343 #[unstable(feature = "once_cell_get_mut", issue = "121641")]
344 pub fn get_mut_or_init<F>(&mut self, f: F) -> &mut T
345 where
346 F: FnOnce() -> T,
347 {
348 match self.get_mut_or_try_init(|| Ok::<T, !>(f())) {
349 Ok(val) => val,
350 }
351 }
352
353 /// Gets the contents of the cell, initializing it to `f()` if
354 /// the cell was uninitialized. If the cell was uninitialized
355 /// and `f()` failed, an error is returned.
356 ///
357 /// # Panics
358 ///
359 /// If `f()` panics, the panic is propagated to the caller, and
360 /// the cell remains uninitialized.
361 ///
362 /// It is an error to reentrantly initialize the cell from `f`.
363 /// The exact outcome is unspecified. Current implementation
364 /// deadlocks, but this may be changed to a panic in the future.
365 ///
366 /// # Examples
367 ///
368 /// ```
369 /// #![feature(once_cell_try)]
370 ///
371 /// use std::sync::OnceLock;
372 ///
373 /// let cell = OnceLock::new();
374 /// assert_eq!(cell.get_or_try_init(|| Err(())), Err(()));
375 /// assert!(cell.get().is_none());
376 /// let value = cell.get_or_try_init(|| -> Result<i32, ()> {
377 /// Ok(92)
378 /// });
379 /// assert_eq!(value, Ok(&92));
380 /// assert_eq!(cell.get(), Some(&92))
381 /// ```
382 #[inline]
383 #[unstable(feature = "once_cell_try", issue = "109737")]
384 pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
385 where
386 F: FnOnce() -> Result<T, E>,
387 {
388 // Fast path check
389 // NOTE: We need to perform an acquire on the state in this method
390 // in order to correctly synchronize `LazyLock::force`. This is
391 // currently done by calling `self.get()`, which in turn calls
392 // `self.is_initialized()`, which in turn performs the acquire.
393 if let Some(value) = self.get() {
394 return Ok(value);
395 }
396 self.initialize(f)?;
397
398 debug_assert!(self.is_initialized());
399
400 // SAFETY: The inner value has been initialized
401 Ok(unsafe { self.get_unchecked() })
402 }
403
404 /// Gets the mutable reference of the contents of the cell, initializing
405 /// it to `f()` if the cell was uninitialized. If the cell was uninitialized
406 /// and `f()` failed, an error is returned.
407 ///
408 /// This method never blocks.
409 ///
410 /// # Panics
411 ///
412 /// If `f()` panics, the panic is propagated to the caller, and
413 /// the cell remains uninitialized.
414 ///
415 /// # Examples
416 ///
417 /// ```
418 /// #![feature(once_cell_get_mut)]
419 ///
420 /// use std::sync::OnceLock;
421 ///
422 /// let mut cell: OnceLock<u32> = OnceLock::new();
423 ///
424 /// // Failed attempts to initialize the cell do not change its contents
425 /// assert!(cell.get_mut_or_try_init(|| "not a number!".parse()).is_err());
426 /// assert!(cell.get().is_none());
427 ///
428 /// let value = cell.get_mut_or_try_init(|| "1234".parse());
429 /// assert_eq!(value, Ok(&mut 1234));
430 /// *value.unwrap() += 2;
431 /// assert_eq!(cell.get(), Some(&1236))
432 /// ```
433 #[inline]
434 #[unstable(feature = "once_cell_get_mut", issue = "121641")]
435 pub fn get_mut_or_try_init<F, E>(&mut self, f: F) -> Result<&mut T, E>
436 where
437 F: FnOnce() -> Result<T, E>,
438 {
439 if self.get().is_none() {
440 self.initialize(f)?;
441 }
442 debug_assert!(self.is_initialized());
443 // SAFETY: The inner value has been initialized
444 Ok(unsafe { self.get_unchecked_mut() })
445 }
446
447 /// Consumes the `OnceLock`, returning the wrapped value. Returns
448 /// `None` if the cell was uninitialized.
449 ///
450 /// # Examples
451 ///
452 /// ```
453 /// use std::sync::OnceLock;
454 ///
455 /// let cell: OnceLock<String> = OnceLock::new();
456 /// assert_eq!(cell.into_inner(), None);
457 ///
458 /// let cell = OnceLock::new();
459 /// cell.set("hello".to_string()).unwrap();
460 /// assert_eq!(cell.into_inner(), Some("hello".to_string()));
461 /// ```
462 #[inline]
463 #[stable(feature = "once_cell", since = "1.70.0")]
464 pub fn into_inner(mut self) -> Option<T> {
465 self.take()
466 }
467
468 /// Takes the value out of this `OnceLock`, moving it back to an uninitialized state.
469 ///
470 /// Has no effect and returns `None` if the `OnceLock` was uninitialized.
471 ///
472 /// Safety is guaranteed by requiring a mutable reference.
473 ///
474 /// # Examples
475 ///
476 /// ```
477 /// use std::sync::OnceLock;
478 ///
479 /// let mut cell: OnceLock<String> = OnceLock::new();
480 /// assert_eq!(cell.take(), None);
481 ///
482 /// let mut cell = OnceLock::new();
483 /// cell.set("hello".to_string()).unwrap();
484 /// assert_eq!(cell.take(), Some("hello".to_string()));
485 /// assert_eq!(cell.get(), None);
486 /// ```
487 #[inline]
488 #[stable(feature = "once_cell", since = "1.70.0")]
489 pub fn take(&mut self) -> Option<T> {
490 if self.is_initialized() {
491 self.once = Once::new();
492 // SAFETY: `self.value` is initialized and contains a valid `T`.
493 // `self.once` is reset, so `is_initialized()` will be false again
494 // which prevents the value from being read twice.
495 unsafe { Some((&mut *self.value.get()).assume_init_read()) }
496 } else {
497 None
498 }
499 }
500
501 #[inline]
502 fn is_initialized(&self) -> bool {
503 self.once.is_completed()
504 }
505
506 #[cold]
507 #[optimize(size)]
508 fn initialize<F, E>(&self, f: F) -> Result<(), E>
509 where
510 F: FnOnce() -> Result<T, E>,
511 {
512 let mut res: Result<(), E> = Ok(());
513 let slot = &self.value;
514
515 // Ignore poisoning from other threads
516 // If another thread panics, then we'll be able to run our closure
517 self.once.call_once_force(|p| {
518 match f() {
519 Ok(value) => {
520 unsafe { (&mut *slot.get()).write(value) };
521 }
522 Err(e) => {
523 res = Err(e);
524
525 // Treat the underlying `Once` as poisoned since we
526 // failed to initialize our value.
527 p.poison();
528 }
529 }
530 });
531 res
532 }
533
534 /// # Safety
535 ///
536 /// The cell must be initialized
537 #[inline]
538 unsafe fn get_unchecked(&self) -> &T {
539 debug_assert!(self.is_initialized());
540 unsafe { (&*self.value.get()).assume_init_ref() }
541 }
542
543 /// # Safety
544 ///
545 /// The cell must be initialized
546 #[inline]
547 unsafe fn get_unchecked_mut(&mut self) -> &mut T {
548 debug_assert!(self.is_initialized());
549 unsafe { (&mut *self.value.get()).assume_init_mut() }
550 }
551}
552
553// Why do we need `T: Send`?
554// Thread A creates a `OnceLock` and shares it with
555// scoped thread B, which fills the cell, which is
556// then destroyed by A. That is, destructor observes
557// a sent value.
558#[stable(feature = "once_cell", since = "1.70.0")]
559unsafe impl<T: Sync + Send> Sync for OnceLock<T> {}
560#[stable(feature = "once_cell", since = "1.70.0")]
561unsafe impl<T: Send> Send for OnceLock<T> {}
562
563#[stable(feature = "once_cell", since = "1.70.0")]
564impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for OnceLock<T> {}
565#[stable(feature = "once_cell", since = "1.70.0")]
566impl<T: UnwindSafe> UnwindSafe for OnceLock<T> {}
567
568#[stable(feature = "once_cell", since = "1.70.0")]
569impl<T> Default for OnceLock<T> {
570 /// Creates a new uninitialized cell.
571 ///
572 /// # Example
573 ///
574 /// ```
575 /// use std::sync::OnceLock;
576 ///
577 /// fn main() {
578 /// assert_eq!(OnceLock::<()>::new(), OnceLock::default());
579 /// }
580 /// ```
581 #[inline]
582 fn default() -> OnceLock<T> {
583 OnceLock::new()
584 }
585}
586
587#[stable(feature = "once_cell", since = "1.70.0")]
588impl<T: fmt::Debug> fmt::Debug for OnceLock<T> {
589 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
590 let mut d: DebugTuple<'_, '_> = f.debug_tuple(name:"OnceLock");
591 match self.get() {
592 Some(v: &T) => d.field(v),
593 None => d.field(&format_args!("<uninit>")),
594 };
595 d.finish()
596 }
597}
598
599#[stable(feature = "once_cell", since = "1.70.0")]
600impl<T: Clone> Clone for OnceLock<T> {
601 #[inline]
602 fn clone(&self) -> OnceLock<T> {
603 let cell: OnceLock = Self::new();
604 if let Some(value: &T) = self.get() {
605 match cell.set(value.clone()) {
606 Ok(()) => (),
607 Err(_) => unreachable!(),
608 }
609 }
610 cell
611 }
612}
613
614#[stable(feature = "once_cell", since = "1.70.0")]
615impl<T> From<T> for OnceLock<T> {
616 /// Creates a new cell with its contents set to `value`.
617 ///
618 /// # Example
619 ///
620 /// ```
621 /// use std::sync::OnceLock;
622 ///
623 /// # fn main() -> Result<(), i32> {
624 /// let a = OnceLock::from(3);
625 /// let b = OnceLock::new();
626 /// b.set(3)?;
627 /// assert_eq!(a, b);
628 /// Ok(())
629 /// # }
630 /// ```
631 #[inline]
632 fn from(value: T) -> Self {
633 let cell = Self::new();
634 match cell.set(value) {
635 Ok(()) => cell,
636 Err(_) => unreachable!(),
637 }
638 }
639}
640
641#[stable(feature = "once_cell", since = "1.70.0")]
642impl<T: PartialEq> PartialEq for OnceLock<T> {
643 /// Equality for two `OnceLock`s.
644 ///
645 /// Two `OnceLock`s are equal if they either both contain values and their
646 /// values are equal, or if neither contains a value.
647 ///
648 /// # Examples
649 ///
650 /// ```
651 /// use std::sync::OnceLock;
652 ///
653 /// let five = OnceLock::new();
654 /// five.set(5).unwrap();
655 ///
656 /// let also_five = OnceLock::new();
657 /// also_five.set(5).unwrap();
658 ///
659 /// assert!(five == also_five);
660 ///
661 /// assert!(OnceLock::<u32>::new() == OnceLock::<u32>::new());
662 /// ```
663 #[inline]
664 fn eq(&self, other: &OnceLock<T>) -> bool {
665 self.get() == other.get()
666 }
667}
668
669#[stable(feature = "once_cell", since = "1.70.0")]
670impl<T: Eq> Eq for OnceLock<T> {}
671
672#[stable(feature = "once_cell", since = "1.70.0")]
673unsafe impl<#[may_dangle] T> Drop for OnceLock<T> {
674 #[inline]
675 fn drop(&mut self) {
676 if self.is_initialized() {
677 // SAFETY: The cell is initialized and being dropped, so it can't
678 // be accessed again. We also don't touch the `T` other than
679 // dropping it, which validates our usage of #[may_dangle].
680 unsafe { (&mut *self.value.get()).assume_init_drop() };
681 }
682 }
683}
684