1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
use crate::cell::UnsafeCell;
use crate::fmt;
use crate::marker::PhantomData;
use crate::mem::MaybeUninit;
use crate::panic::{RefUnwindSafe, UnwindSafe};
use crate::sync::Once;

/// A synchronization primitive which can be written to only once.
///
/// This type is a thread-safe [`OnceCell`], and can be used in statics.
///
/// [`OnceCell`]: crate::cell::OnceCell
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::sync::OnceLock;
///
/// static CELL: OnceLock<String> = OnceLock::new();
/// assert!(CELL.get().is_none());
///
/// std::thread::spawn(|| {
///     let value: &String = CELL.get_or_init(|| {
///         "Hello, World!".to_string()
///     });
///     assert_eq!(value, "Hello, World!");
/// }).join().unwrap();
///
/// let value: Option<&String> = CELL.get();
/// assert!(value.is_some());
/// assert_eq!(value.unwrap().as_str(), "Hello, World!");
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub struct OnceLock<T> {
    once: Once,
    // Whether or not the value is initialized is tracked by `once.is_completed()`.
    value: UnsafeCell<MaybeUninit<T>>,
    /// `PhantomData` to make sure dropck understands we're dropping T in our Drop impl.
    ///
    /// ```compile_fail,E0597
    /// #![feature(once_cell)]
    ///
    /// use std::sync::OnceLock;
    ///
    /// struct A<'a>(&'a str);
    ///
    /// impl<'a> Drop for A<'a> {
    ///     fn drop(&mut self) {}
    /// }
    ///
    /// let cell = OnceLock::new();
    /// {
    ///     let s = String::new();
    ///     let _ = cell.set(A(&s));
    /// }
    /// ```
    _marker: PhantomData<T>,
}

impl<T> OnceLock<T> {
    /// Creates a new empty cell.
    #[inline]
    #[must_use]
    #[unstable(feature = "once_cell", issue = "74465")]
    pub const fn new() -> OnceLock<T> {
        OnceLock {
            once: Once::new(),
            value: UnsafeCell::new(MaybeUninit::uninit()),
            _marker: PhantomData,
        }
    }

    /// Gets the reference to the underlying value.
    ///
    /// Returns `None` if the cell is empty, or being initialized. This
    /// method never blocks.
    #[inline]
    #[unstable(feature = "once_cell", issue = "74465")]
    pub fn get(&self) -> Option<&T> {
        if self.is_initialized() {
            // Safe b/c checked is_initialized
            Some(unsafe { self.get_unchecked() })
        } else {
            None
        }
    }

    /// Gets the mutable reference to the underlying value.
    ///
    /// Returns `None` if the cell is empty. This method never blocks.
    #[inline]
    #[unstable(feature = "once_cell", issue = "74465")]
    pub fn get_mut(&mut self) -> Option<&mut T> {
        if self.is_initialized() {
            // Safe b/c checked is_initialized and we have a unique access
            Some(unsafe { self.get_unchecked_mut() })
        } else {
            None
        }
    }

    /// Sets the contents of this cell to `value`.
    ///
    /// May block if another thread is currently attempting to initialize the cell. The cell is
    /// guaranteed to contain a value when set returns, though not necessarily the one provided.
    ///
    /// Returns `Ok(())` if the cell's value was set by this call.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::sync::OnceLock;
    ///
    /// static CELL: OnceLock<i32> = OnceLock::new();
    ///
    /// fn main() {
    ///     assert!(CELL.get().is_none());
    ///
    ///     std::thread::spawn(|| {
    ///         assert_eq!(CELL.set(92), Ok(()));
    ///     }).join().unwrap();
    ///
    ///     assert_eq!(CELL.set(62), Err(62));
    ///     assert_eq!(CELL.get(), Some(&92));
    /// }
    /// ```
    #[inline]
    #[unstable(feature = "once_cell", issue = "74465")]
    pub fn set(&self, value: T) -> Result<(), T> {
        let mut value = Some(value);
        self.get_or_init(|| value.take().unwrap());
        match value {
            None => Ok(()),
            Some(value) => Err(value),
        }
    }

    /// Gets the contents of the cell, initializing it with `f` if the cell
    /// was empty.
    ///
    /// Many threads may call `get_or_init` concurrently with different
    /// initializing functions, but it is guaranteed that only one function
    /// will be executed.
    ///
    /// # Panics
    ///
    /// If `f` panics, the panic is propagated to the caller, and the cell
    /// remains uninitialized.
    ///
    /// It is an error to reentrantly initialize the cell from `f`. The
    /// exact outcome is unspecified. Current implementation deadlocks, but
    /// this may be changed to a panic in the future.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::sync::OnceLock;
    ///
    /// let cell = OnceLock::new();
    /// let value = cell.get_or_init(|| 92);
    /// assert_eq!(value, &92);
    /// let value = cell.get_or_init(|| unreachable!());
    /// assert_eq!(value, &92);
    /// ```
    #[inline]
    #[unstable(feature = "once_cell", issue = "74465")]
    pub fn get_or_init<F>(&self, f: F) -> &T
    where
        F: FnOnce() -> T,
    {
        match self.get_or_try_init(|| Ok::<T, !>(f())) {
            Ok(val) => val,
        }
    }

    /// Gets the contents of the cell, initializing it with `f` if
    /// the cell was empty. If the cell was empty and `f` failed, an
    /// error is returned.
    ///
    /// # Panics
    ///
    /// If `f` panics, the panic is propagated to the caller, and
    /// the cell remains uninitialized.
    ///
    /// It is an error to reentrantly initialize the cell from `f`.
    /// The exact outcome is unspecified. Current implementation
    /// deadlocks, but this may be changed to a panic in the future.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::sync::OnceLock;
    ///
    /// let cell = OnceLock::new();
    /// assert_eq!(cell.get_or_try_init(|| Err(())), Err(()));
    /// assert!(cell.get().is_none());
    /// let value = cell.get_or_try_init(|| -> Result<i32, ()> {
    ///     Ok(92)
    /// });
    /// assert_eq!(value, Ok(&92));
    /// assert_eq!(cell.get(), Some(&92))
    /// ```
    #[inline]
    #[unstable(feature = "once_cell", issue = "74465")]
    pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
    where
        F: FnOnce() -> Result<T, E>,
    {
        // Fast path check
        // NOTE: We need to perform an acquire on the state in this method
        // in order to correctly synchronize `LazyLock::force`. This is
        // currently done by calling `self.get()`, which in turn calls
        // `self.is_initialized()`, which in turn performs the acquire.
        if let Some(value) = self.get() {
            return Ok(value);
        }
        self.initialize(f)?;

        debug_assert!(self.is_initialized());

        // SAFETY: The inner value has been initialized
        Ok(unsafe { self.get_unchecked() })
    }

    /// Consumes the `OnceLock`, returning the wrapped value. Returns
    /// `None` if the cell was empty.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::sync::OnceLock;
    ///
    /// let cell: OnceLock<String> = OnceLock::new();
    /// assert_eq!(cell.into_inner(), None);
    ///
    /// let cell = OnceLock::new();
    /// cell.set("hello".to_string()).unwrap();
    /// assert_eq!(cell.into_inner(), Some("hello".to_string()));
    /// ```
    #[inline]
    #[unstable(feature = "once_cell", issue = "74465")]
    pub fn into_inner(mut self) -> Option<T> {
        self.take()
    }

    /// Takes the value out of this `OnceLock`, moving it back to an uninitialized state.
    ///
    /// Has no effect and returns `None` if the `OnceLock` hasn't been initialized.
    ///
    /// Safety is guaranteed by requiring a mutable reference.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::sync::OnceLock;
    ///
    /// let mut cell: OnceLock<String> = OnceLock::new();
    /// assert_eq!(cell.take(), None);
    ///
    /// let mut cell = OnceLock::new();
    /// cell.set("hello".to_string()).unwrap();
    /// assert_eq!(cell.take(), Some("hello".to_string()));
    /// assert_eq!(cell.get(), None);
    /// ```
    #[inline]
    #[unstable(feature = "once_cell", issue = "74465")]
    pub fn take(&mut self) -> Option<T> {
        if self.is_initialized() {
            self.once = Once::new();
            // SAFETY: `self.value` is initialized and contains a valid `T`.
            // `self.once` is reset, so `is_initialized()` will be false again
            // which prevents the value from being read twice.
            unsafe { Some((&mut *self.value.get()).assume_init_read()) }
        } else {
            None
        }
    }

    #[inline]
    fn is_initialized(&self) -> bool {
        self.once.is_completed()
    }

    #[cold]
    fn initialize<F, E>(&self, f: F) -> Result<(), E>
    where
        F: FnOnce() -> Result<T, E>,
    {
        let mut res: Result<(), E> = Ok(());
        let slot = &self.value;

        // Ignore poisoning from other threads
        // If another thread panics, then we'll be able to run our closure
        self.once.call_once_force(|p| {
            match f() {
                Ok(value) => {
                    unsafe { (&mut *slot.get()).write(value) };
                }
                Err(e) => {
                    res = Err(e);

                    // Treat the underlying `Once` as poisoned since we
                    // failed to initialize our value. Calls
                    p.poison();
                }
            }
        });
        res
    }

    /// # Safety
    ///
    /// The value must be initialized
    #[inline]
    unsafe fn get_unchecked(&self) -> &T {
        debug_assert!(self.is_initialized());
        (&*self.value.get()).assume_init_ref()
    }

    /// # Safety
    ///
    /// The value must be initialized
    #[inline]
    unsafe fn get_unchecked_mut(&mut self) -> &mut T {
        debug_assert!(self.is_initialized());
        (&mut *self.value.get()).assume_init_mut()
    }
}

// Why do we need `T: Send`?
// Thread A creates a `OnceLock` and shares it with
// scoped thread B, which fills the cell, which is
// then destroyed by A. That is, destructor observes
// a sent value.
#[unstable(feature = "once_cell", issue = "74465")]
unsafe impl<T: Sync + Send> Sync for OnceLock<T> {}
#[unstable(feature = "once_cell", issue = "74465")]
unsafe impl<T: Send> Send for OnceLock<T> {}

#[unstable(feature = "once_cell", issue = "74465")]
impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for OnceLock<T> {}
#[unstable(feature = "once_cell", issue = "74465")]
impl<T: UnwindSafe> UnwindSafe for OnceLock<T> {}

#[unstable(feature = "once_cell", issue = "74465")]
#[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
impl<T> const Default for OnceLock<T> {
    /// Creates a new empty cell.
    ///
    /// # Example
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::sync::OnceLock;
    ///
    /// fn main() {
    ///     assert_eq!(OnceLock::<()>::new(), OnceLock::default());
    /// }
    /// ```
    #[inline]
    fn default() -> OnceLock<T> {
        OnceLock::new()
    }
}

#[unstable(feature = "once_cell", issue = "74465")]
impl<T: fmt::Debug> fmt::Debug for OnceLock<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self.get() {
            Some(v) => f.debug_tuple("Once").field(v).finish(),
            None => f.write_str("Once(Uninit)"),
        }
    }
}

#[unstable(feature = "once_cell", issue = "74465")]
impl<T: Clone> Clone for OnceLock<T> {
    #[inline]
    fn clone(&self) -> OnceLock<T> {
        let cell = Self::new();
        if let Some(value) = self.get() {
            match cell.set(value.clone()) {
                Ok(()) => (),
                Err(_) => unreachable!(),
            }
        }
        cell
    }
}

#[unstable(feature = "once_cell", issue = "74465")]
impl<T> From<T> for OnceLock<T> {
    /// Create a new cell with its contents set to `value`.
    ///
    /// # Example
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::sync::OnceLock;
    ///
    /// # fn main() -> Result<(), i32> {
    /// let a = OnceLock::from(3);
    /// let b = OnceLock::new();
    /// b.set(3)?;
    /// assert_eq!(a, b);
    /// Ok(())
    /// # }
    /// ```
    #[inline]
    fn from(value: T) -> Self {
        let cell = Self::new();
        match cell.set(value) {
            Ok(()) => cell,
            Err(_) => unreachable!(),
        }
    }
}

#[unstable(feature = "once_cell", issue = "74465")]
impl<T: PartialEq> PartialEq for OnceLock<T> {
    #[inline]
    fn eq(&self, other: &OnceLock<T>) -> bool {
        self.get() == other.get()
    }
}

#[unstable(feature = "once_cell", issue = "74465")]
impl<T: Eq> Eq for OnceLock<T> {}

#[unstable(feature = "once_cell", issue = "74465")]
unsafe impl<#[may_dangle] T> Drop for OnceLock<T> {
    #[inline]
    fn drop(&mut self) {
        if self.is_initialized() {
            // SAFETY: The cell is initialized and being dropped, so it can't
            // be accessed again. We also don't touch the `T` other than
            // dropping it, which validates our usage of #[may_dangle].
            unsafe { (&mut *self.value.get()).assume_init_drop() };
        }
    }
}

#[cfg(test)]
mod tests;