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
//! Lazy values and one-time initialization of static data.

use crate::cell::{Cell, UnsafeCell};
use crate::fmt;
use crate::mem;
use crate::ops::Deref;

/// A cell which can be written to only once.
///
/// Unlike `RefCell`, a `OnceCell` only provides shared `&T` references to its value.
/// Unlike `Cell`, a `OnceCell` doesn't require copying or replacing the value to access it.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::lazy::OnceCell;
///
/// let cell = OnceCell::new();
/// assert!(cell.get().is_none());
///
/// let value: &String = cell.get_or_init(|| {
///     "Hello, World!".to_string()
/// });
/// assert_eq!(value, "Hello, World!");
/// assert!(cell.get().is_some());
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub struct OnceCell<T> {
    // Invariant: written to at most once.
    inner: UnsafeCell<Option<T>>,
}

#[unstable(feature = "once_cell", issue = "74465")]
impl<T> Default for OnceCell<T> {
    fn default() -> Self {
        Self::new()
    }
}

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

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

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

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

#[unstable(feature = "once_cell", issue = "74465")]
impl<T> const From<T> for OnceCell<T> {
    /// Creates a new `OnceCell<T>` which already contains the given `value`.
    fn from(value: T) -> Self {
        OnceCell { inner: UnsafeCell::new(Some(value)) }
    }
}

impl<T> OnceCell<T> {
    /// Creates a new empty cell.
    #[unstable(feature = "once_cell", issue = "74465")]
    #[must_use]
    pub const fn new() -> OnceCell<T> {
        OnceCell { inner: UnsafeCell::new(None) }
    }

    /// Gets the reference to the underlying value.
    ///
    /// Returns `None` if the cell is empty.
    #[unstable(feature = "once_cell", issue = "74465")]
    pub fn get(&self) -> Option<&T> {
        // SAFETY: Safe due to `inner`'s invariant
        unsafe { &*self.inner.get() }.as_ref()
    }

    /// Gets the mutable reference to the underlying value.
    ///
    /// Returns `None` if the cell is empty.
    #[unstable(feature = "once_cell", issue = "74465")]
    pub fn get_mut(&mut self) -> Option<&mut T> {
        self.inner.get_mut().as_mut()
    }

    /// Sets the contents of the cell to `value`.
    ///
    /// # Errors
    ///
    /// This method returns `Ok(())` if the cell was empty and `Err(value)` if
    /// it was full.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::lazy::OnceCell;
    ///
    /// let cell = OnceCell::new();
    /// assert!(cell.get().is_none());
    ///
    /// assert_eq!(cell.set(92), Ok(()));
    /// assert_eq!(cell.set(62), Err(62));
    ///
    /// assert!(cell.get().is_some());
    /// ```
    #[unstable(feature = "once_cell", issue = "74465")]
    pub fn set(&self, value: T) -> Result<(), T> {
        // SAFETY: Safe because we cannot have overlapping mutable borrows
        let slot = unsafe { &*self.inner.get() };
        if slot.is_some() {
            return Err(value);
        }

        // SAFETY: This is the only place where we set the slot, no races
        // due to reentrancy/concurrency are possible, and we've
        // checked that slot is currently `None`, so this write
        // maintains the `inner`'s invariant.
        let slot = unsafe { &mut *self.inner.get() };
        *slot = Some(value);
        Ok(())
    }

    /// Gets the contents of the cell, initializing it with `f`
    /// if the cell was empty.
    ///
    /// # 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`. Doing
    /// so results in a panic.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::lazy::OnceCell;
    ///
    /// let cell = OnceCell::new();
    /// let value = cell.get_or_init(|| 92);
    /// assert_eq!(value, &92);
    /// let value = cell.get_or_init(|| unreachable!());
    /// assert_eq!(value, &92);
    /// ```
    #[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`. Doing
    /// so results in a panic.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::lazy::OnceCell;
    ///
    /// let cell = OnceCell::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))
    /// ```
    #[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>,
    {
        if let Some(val) = self.get() {
            return Ok(val);
        }
        /// Avoid inlining the initialization closure into the common path that fetches
        /// the already initialized value
        #[cold]
        fn outlined_call<F, T, E>(f: F) -> Result<T, E>
        where
            F: FnOnce() -> Result<T, E>,
        {
            f()
        }
        let val = outlined_call(f)?;
        // Note that *some* forms of reentrant initialization might lead to
        // UB (see `reentrant_init` test). I believe that just removing this
        // `assert`, while keeping `set/get` would be sound, but it seems
        // better to panic, rather than to silently use an old value.
        assert!(self.set(val).is_ok(), "reentrant init");
        Ok(self.get().unwrap())
    }

    /// Consumes the cell, returning the wrapped value.
    ///
    /// Returns `None` if the cell was empty.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::lazy::OnceCell;
    ///
    /// let cell: OnceCell<String> = OnceCell::new();
    /// assert_eq!(cell.into_inner(), None);
    ///
    /// let cell = OnceCell::new();
    /// cell.set("hello".to_string()).unwrap();
    /// assert_eq!(cell.into_inner(), Some("hello".to_string()));
    /// ```
    #[unstable(feature = "once_cell", issue = "74465")]
    pub fn into_inner(self) -> Option<T> {
        // Because `into_inner` takes `self` by value, the compiler statically verifies
        // that it is not currently borrowed. So it is safe to move out `Option<T>`.
        self.inner.into_inner()
    }

    /// Takes the value out of this `OnceCell`, moving it back to an uninitialized state.
    ///
    /// Has no effect and returns `None` if the `OnceCell` hasn't been initialized.
    ///
    /// Safety is guaranteed by requiring a mutable reference.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::lazy::OnceCell;
    ///
    /// let mut cell: OnceCell<String> = OnceCell::new();
    /// assert_eq!(cell.take(), None);
    ///
    /// let mut cell = OnceCell::new();
    /// cell.set("hello".to_string()).unwrap();
    /// assert_eq!(cell.take(), Some("hello".to_string()));
    /// assert_eq!(cell.get(), None);
    /// ```
    #[unstable(feature = "once_cell", issue = "74465")]
    pub fn take(&mut self) -> Option<T> {
        mem::take(self).into_inner()
    }
}

/// A value which is initialized on the first access.
///
/// # Examples
///
/// ```
/// #![feature(once_cell)]
///
/// use std::lazy::Lazy;
///
/// let lazy: Lazy<i32> = Lazy::new(|| {
///     println!("initializing");
///     92
/// });
/// println!("ready");
/// println!("{}", *lazy);
/// println!("{}", *lazy);
///
/// // Prints:
/// //   ready
/// //   initializing
/// //   92
/// //   92
/// ```
#[unstable(feature = "once_cell", issue = "74465")]
pub struct Lazy<T, F = fn() -> T> {
    cell: OnceCell<T>,
    init: Cell<Option<F>>,
}

#[unstable(feature = "once_cell", issue = "74465")]
impl<T: fmt::Debug, F> fmt::Debug for Lazy<T, F> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Lazy").field("cell", &self.cell).field("init", &"..").finish()
    }
}

impl<T, F> Lazy<T, F> {
    /// Creates a new lazy value with the given initializing function.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// # fn main() {
    /// use std::lazy::Lazy;
    ///
    /// let hello = "Hello, World!".to_string();
    ///
    /// let lazy = Lazy::new(|| hello.to_uppercase());
    ///
    /// assert_eq!(&*lazy, "HELLO, WORLD!");
    /// # }
    /// ```
    #[unstable(feature = "once_cell", issue = "74465")]
    pub const fn new(init: F) -> Lazy<T, F> {
        Lazy { cell: OnceCell::new(), init: Cell::new(Some(init)) }
    }
}

impl<T, F: FnOnce() -> T> Lazy<T, F> {
    /// Forces the evaluation of this lazy value and returns a reference to
    /// the result.
    ///
    /// This is equivalent to the `Deref` impl, but is explicit.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(once_cell)]
    ///
    /// use std::lazy::Lazy;
    ///
    /// let lazy = Lazy::new(|| 92);
    ///
    /// assert_eq!(Lazy::force(&lazy), &92);
    /// assert_eq!(&*lazy, &92);
    /// ```
    #[unstable(feature = "once_cell", issue = "74465")]
    pub fn force(this: &Lazy<T, F>) -> &T {
        this.cell.get_or_init(|| match this.init.take() {
            Some(f) => f(),
            None => panic!("`Lazy` instance has previously been poisoned"),
        })
    }
}

#[unstable(feature = "once_cell", issue = "74465")]
impl<T, F: FnOnce() -> T> Deref for Lazy<T, F> {
    type Target = T;
    fn deref(&self) -> &T {
        Lazy::force(self)
    }
}

#[unstable(feature = "once_cell", issue = "74465")]
impl<T: Default> Default for Lazy<T> {
    /// Creates a new lazy value using `Default` as the initializing function.
    fn default() -> Lazy<T> {
        Lazy::new(T::default)
    }
}