std/sync/
lazy_lock.rs

1use super::once::OnceExclusiveState;
2use crate::cell::UnsafeCell;
3use crate::mem::ManuallyDrop;
4use crate::ops::{Deref, DerefMut};
5use crate::panic::{RefUnwindSafe, UnwindSafe};
6use crate::sync::Once;
7use crate::{fmt, ptr};
8
9// We use the state of a Once as discriminant value. Upon creation, the state is
10// "incomplete" and `f` contains the initialization closure. In the first call to
11// `call_once`, `f` is taken and run. If it succeeds, `value` is set and the state
12// is changed to "complete". If it panics, the Once is poisoned, so none of the
13// two fields is initialized.
14union Data<T, F> {
15    value: ManuallyDrop<T>,
16    f: ManuallyDrop<F>,
17}
18
19/// A value which is initialized on the first access.
20///
21/// This type is a thread-safe [`LazyCell`], and can be used in statics.
22/// Since initialization may be called from multiple threads, any
23/// dereferencing call will block the calling thread if another
24/// initialization routine is currently running.
25///
26/// [`LazyCell`]: crate::cell::LazyCell
27///
28/// # Poisoning
29///
30/// If the initialization closure passed to [`LazyLock::new`] panics, the lock will be poisoned.
31/// Once the lock is poisoned, any threads that attempt to access this lock (via a dereference
32/// or via an explicit call to [`force()`]) will panic.
33///
34/// This concept is similar to that of poisoning in the [`std::sync::poison`] module. A key
35/// difference, however, is that poisoning in `LazyLock` is _unrecoverable_. All future accesses of
36/// the lock from other threads will panic, whereas a type in [`std::sync::poison`] like
37/// [`std::sync::poison::Mutex`] allows recovery via [`PoisonError::into_inner()`].
38///
39/// [`force()`]: LazyLock::force
40/// [`std::sync::poison`]: crate::sync::poison
41/// [`std::sync::poison::Mutex`]: crate::sync::poison::Mutex
42/// [`PoisonError::into_inner()`]: crate::sync::poison::PoisonError::into_inner
43///
44/// # Examples
45///
46/// Initialize static variables with `LazyLock`.
47/// ```
48/// use std::sync::LazyLock;
49///
50/// // Note: static items do not call [`Drop`] on program termination, so this won't be deallocated.
51/// // this is fine, as the OS can deallocate the terminated program faster than we can free memory
52/// // but tools like valgrind might report "memory leaks" as it isn't obvious this is intentional.
53/// static DEEP_THOUGHT: LazyLock<String> = LazyLock::new(|| {
54/// # mod another_crate {
55/// #     pub fn great_question() -> String { "42".to_string() }
56/// # }
57///     // M3 Ultra takes about 16 million years in --release config
58///     another_crate::great_question()
59/// });
60///
61/// // The `String` is built, stored in the `LazyLock`, and returned as `&String`.
62/// let _ = &*DEEP_THOUGHT;
63/// ```
64///
65/// Initialize fields with `LazyLock`.
66/// ```
67/// use std::sync::LazyLock;
68///
69/// #[derive(Debug)]
70/// struct UseCellLock {
71///     number: LazyLock<u32>,
72/// }
73/// fn main() {
74///     let lock: LazyLock<u32> = LazyLock::new(|| 0u32);
75///
76///     let data = UseCellLock { number: lock };
77///     println!("{}", *data.number);
78/// }
79/// ```
80#[stable(feature = "lazy_cell", since = "1.80.0")]
81pub struct LazyLock<T, F = fn() -> T> {
82    // FIXME(nonpoison_once): if possible, switch to nonpoison version once it is available
83    once: Once,
84    data: UnsafeCell<Data<T, F>>,
85}
86
87impl<T, F: FnOnce() -> T> LazyLock<T, F> {
88    /// Creates a new lazy value with the given initializing function.
89    ///
90    /// # Examples
91    ///
92    /// ```
93    /// use std::sync::LazyLock;
94    ///
95    /// let hello = "Hello, World!".to_string();
96    ///
97    /// let lazy = LazyLock::new(|| hello.to_uppercase());
98    ///
99    /// assert_eq!(&*lazy, "HELLO, WORLD!");
100    /// ```
101    #[inline]
102    #[stable(feature = "lazy_cell", since = "1.80.0")]
103    #[rustc_const_stable(feature = "lazy_cell", since = "1.80.0")]
104    pub const fn new(f: F) -> LazyLock<T, F> {
105        LazyLock { once: Once::new(), data: UnsafeCell::new(Data { f: ManuallyDrop::new(f) }) }
106    }
107
108    /// Creates a new lazy value that is already initialized.
109    #[inline]
110    #[cfg(test)]
111    pub(crate) fn preinit(value: T) -> LazyLock<T, F> {
112        let once = Once::new();
113        once.call_once(|| {});
114        LazyLock { once, data: UnsafeCell::new(Data { value: ManuallyDrop::new(value) }) }
115    }
116
117    /// Consumes this `LazyLock` returning the stored value.
118    ///
119    /// Returns `Ok(value)` if `Lazy` is initialized and `Err(f)` otherwise.
120    ///
121    /// # Panics
122    ///
123    /// Panics if the lock is poisoned.
124    ///
125    /// # Examples
126    ///
127    /// ```
128    /// #![feature(lazy_cell_into_inner)]
129    ///
130    /// use std::sync::LazyLock;
131    ///
132    /// let hello = "Hello, World!".to_string();
133    ///
134    /// let lazy = LazyLock::new(|| hello.to_uppercase());
135    ///
136    /// assert_eq!(&*lazy, "HELLO, WORLD!");
137    /// assert_eq!(LazyLock::into_inner(lazy).ok(), Some("HELLO, WORLD!".to_string()));
138    /// ```
139    #[unstable(feature = "lazy_cell_into_inner", issue = "125623")]
140    pub fn into_inner(mut this: Self) -> Result<T, F> {
141        let state = this.once.state();
142        match state {
143            OnceExclusiveState::Poisoned => panic_poisoned(),
144            state => {
145                let this = ManuallyDrop::new(this);
146                let data = unsafe { ptr::read(&this.data) }.into_inner();
147                match state {
148                    OnceExclusiveState::Incomplete => {
149                        Err(ManuallyDrop::into_inner(unsafe { data.f }))
150                    }
151                    OnceExclusiveState::Complete => {
152                        Ok(ManuallyDrop::into_inner(unsafe { data.value }))
153                    }
154                    OnceExclusiveState::Poisoned => unreachable!(),
155                }
156            }
157        }
158    }
159
160    /// Forces the evaluation of this lazy value and returns a mutable reference to
161    /// the result.
162    ///
163    /// # Panics
164    ///
165    /// If the initialization closure panics (the one that is passed to the [`new()`] method), the
166    /// panic is propagated to the caller, and the lock becomes poisoned. This will cause all future
167    /// accesses of the lock (via [`force()`] or a dereference) to panic.
168    ///
169    /// [`new()`]: LazyLock::new
170    /// [`force()`]: LazyLock::force
171    ///
172    /// # Examples
173    ///
174    /// ```
175    /// use std::sync::LazyLock;
176    ///
177    /// let mut lazy = LazyLock::new(|| 92);
178    ///
179    /// let p = LazyLock::force_mut(&mut lazy);
180    /// assert_eq!(*p, 92);
181    /// *p = 44;
182    /// assert_eq!(*lazy, 44);
183    /// ```
184    #[inline]
185    #[stable(feature = "lazy_get", since = "CURRENT_RUSTC_VERSION")]
186    pub fn force_mut(this: &mut LazyLock<T, F>) -> &mut T {
187        #[cold]
188        /// # Safety
189        /// May only be called when the state is `Incomplete`.
190        unsafe fn really_init_mut<T, F: FnOnce() -> T>(this: &mut LazyLock<T, F>) -> &mut T {
191            struct PoisonOnPanic<'a, T, F>(&'a mut LazyLock<T, F>);
192            impl<T, F> Drop for PoisonOnPanic<'_, T, F> {
193                #[inline]
194                fn drop(&mut self) {
195                    self.0.once.set_state(OnceExclusiveState::Poisoned);
196                }
197            }
198
199            // SAFETY: We always poison if the initializer panics (then we never check the data),
200            // or set the data on success.
201            let f = unsafe { ManuallyDrop::take(&mut this.data.get_mut().f) };
202            // INVARIANT: Initiated from mutable reference, don't drop because we read it.
203            let guard = PoisonOnPanic(this);
204            let data = f();
205            guard.0.data.get_mut().value = ManuallyDrop::new(data);
206            guard.0.once.set_state(OnceExclusiveState::Complete);
207            core::mem::forget(guard);
208            // SAFETY: We put the value there above.
209            unsafe { &mut this.data.get_mut().value }
210        }
211
212        let state = this.once.state();
213        match state {
214            OnceExclusiveState::Poisoned => panic_poisoned(),
215            // SAFETY: The `Once` states we completed the initialization.
216            OnceExclusiveState::Complete => unsafe { &mut this.data.get_mut().value },
217            // SAFETY: The state is `Incomplete`.
218            OnceExclusiveState::Incomplete => unsafe { really_init_mut(this) },
219        }
220    }
221
222    /// Forces the evaluation of this lazy value and returns a reference to
223    /// result. This is equivalent to the `Deref` impl, but is explicit.
224    ///
225    /// This method will block the calling thread if another initialization
226    /// routine is currently running.
227    ///
228    /// # Panics
229    ///
230    /// If the initialization closure panics (the one that is passed to the [`new()`] method), the
231    /// panic is propagated to the caller, and the lock becomes poisoned. This will cause all future
232    /// accesses of the lock (via [`force()`] or a dereference) to panic.
233    ///
234    /// [`new()`]: LazyLock::new
235    /// [`force()`]: LazyLock::force
236    ///
237    /// # Examples
238    ///
239    /// ```
240    /// use std::sync::LazyLock;
241    ///
242    /// let lazy = LazyLock::new(|| 92);
243    ///
244    /// assert_eq!(LazyLock::force(&lazy), &92);
245    /// assert_eq!(&*lazy, &92);
246    /// ```
247    #[inline]
248    #[stable(feature = "lazy_cell", since = "1.80.0")]
249    #[rustc_should_not_be_called_on_const_items]
250    pub fn force(this: &LazyLock<T, F>) -> &T {
251        this.once.call_once_force(|state| {
252            if state.is_poisoned() {
253                panic_poisoned();
254            }
255
256            // SAFETY: `call_once` only runs this closure once, ever.
257            let data = unsafe { &mut *this.data.get() };
258            let f = unsafe { ManuallyDrop::take(&mut data.f) };
259            let value = f();
260            data.value = ManuallyDrop::new(value);
261        });
262
263        // SAFETY:
264        // There are four possible scenarios:
265        // * the closure was called and initialized `value`.
266        // * the closure was called and panicked, so this point is never reached.
267        // * the closure was not called, but a previous call initialized `value`.
268        // * the closure was not called because the Once is poisoned, which we handled above.
269        // So `value` has definitely been initialized and will not be modified again.
270        unsafe { &*(*this.data.get()).value }
271    }
272}
273
274impl<T, F> LazyLock<T, F> {
275    /// Returns a mutable reference to the value if initialized. Otherwise (if uninitialized or
276    /// poisoned), returns `None`.
277    ///
278    /// # Examples
279    ///
280    /// ```
281    /// use std::sync::LazyLock;
282    ///
283    /// let mut lazy = LazyLock::new(|| 92);
284    ///
285    /// assert_eq!(LazyLock::get_mut(&mut lazy), None);
286    /// let _ = LazyLock::force(&lazy);
287    /// *LazyLock::get_mut(&mut lazy).unwrap() = 44;
288    /// assert_eq!(*lazy, 44);
289    /// ```
290    #[inline]
291    #[stable(feature = "lazy_get", since = "CURRENT_RUSTC_VERSION")]
292    pub fn get_mut(this: &mut LazyLock<T, F>) -> Option<&mut T> {
293        // `state()` does not perform an atomic load, so prefer it over `is_complete()`.
294        let state = this.once.state();
295        match state {
296            // SAFETY:
297            // The closure has been run successfully, so `value` has been initialized.
298            OnceExclusiveState::Complete => Some(unsafe { &mut this.data.get_mut().value }),
299            _ => None,
300        }
301    }
302
303    /// Returns a reference to the value if initialized. Otherwise (if uninitialized or poisoned),
304    /// returns `None`.
305    ///
306    /// # Examples
307    ///
308    /// ```
309    /// use std::sync::LazyLock;
310    ///
311    /// let lazy = LazyLock::new(|| 92);
312    ///
313    /// assert_eq!(LazyLock::get(&lazy), None);
314    /// let _ = LazyLock::force(&lazy);
315    /// assert_eq!(LazyLock::get(&lazy), Some(&92));
316    /// ```
317    #[inline]
318    #[stable(feature = "lazy_get", since = "CURRENT_RUSTC_VERSION")]
319    #[rustc_should_not_be_called_on_const_items]
320    pub fn get(this: &LazyLock<T, F>) -> Option<&T> {
321        if this.once.is_completed() {
322            // SAFETY:
323            // The closure has been run successfully, so `value` has been initialized
324            // and will not be modified again.
325            Some(unsafe { &(*this.data.get()).value })
326        } else {
327            None
328        }
329    }
330}
331
332#[stable(feature = "lazy_cell", since = "1.80.0")]
333impl<T, F> Drop for LazyLock<T, F> {
334    fn drop(&mut self) {
335        match self.once.state() {
336            OnceExclusiveState::Incomplete => unsafe {
337                ManuallyDrop::drop(&mut self.data.get_mut().f)
338            },
339            OnceExclusiveState::Complete => unsafe {
340                ManuallyDrop::drop(&mut self.data.get_mut().value)
341            },
342            OnceExclusiveState::Poisoned => {}
343        }
344    }
345}
346
347#[stable(feature = "lazy_cell", since = "1.80.0")]
348impl<T, F: FnOnce() -> T> Deref for LazyLock<T, F> {
349    type Target = T;
350
351    /// Dereferences the value.
352    ///
353    /// This method will block the calling thread if another initialization
354    /// routine is currently running.
355    ///
356    /// # Panics
357    ///
358    /// If the initialization closure panics (the one that is passed to the [`new()`] method), the
359    /// panic is propagated to the caller, and the lock becomes poisoned. This will cause all future
360    /// accesses of the lock (via [`force()`] or a dereference) to panic.
361    ///
362    /// [`new()`]: LazyLock::new
363    /// [`force()`]: LazyLock::force
364    #[inline]
365    fn deref(&self) -> &T {
366        LazyLock::force(self)
367    }
368}
369
370#[stable(feature = "lazy_deref_mut", since = "1.89.0")]
371impl<T, F: FnOnce() -> T> DerefMut for LazyLock<T, F> {
372    /// # Panics
373    ///
374    /// If the initialization closure panics (the one that is passed to the [`new()`] method), the
375    /// panic is propagated to the caller, and the lock becomes poisoned. This will cause all future
376    /// accesses of the lock (via [`force()`] or a dereference) to panic.
377    ///
378    /// [`new()`]: LazyLock::new
379    /// [`force()`]: LazyLock::force
380    #[inline]
381    fn deref_mut(&mut self) -> &mut T {
382        LazyLock::force_mut(self)
383    }
384}
385
386#[stable(feature = "lazy_cell", since = "1.80.0")]
387impl<T: Default> Default for LazyLock<T> {
388    /// Creates a new lazy value using `Default` as the initializing function.
389    #[inline]
390    fn default() -> LazyLock<T> {
391        LazyLock::new(T::default)
392    }
393}
394
395#[stable(feature = "lazy_cell", since = "1.80.0")]
396impl<T: fmt::Debug, F> fmt::Debug for LazyLock<T, F> {
397    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
398        let mut d = f.debug_tuple("LazyLock");
399        match LazyLock::get(self) {
400            Some(v) => d.field(v),
401            None => d.field(&format_args!("<uninit>")),
402        };
403        d.finish()
404    }
405}
406
407#[cold]
408#[inline(never)]
409fn panic_poisoned() -> ! {
410    panic!("LazyLock instance has previously been poisoned")
411}
412
413// We never create a `&F` from a `&LazyLock<T, F>` so it is fine
414// to not impl `Sync` for `F`.
415#[stable(feature = "lazy_cell", since = "1.80.0")]
416unsafe impl<T: Sync + Send, F: Send> Sync for LazyLock<T, F> {}
417// auto-derived `Send` impl is OK.
418
419#[stable(feature = "lazy_cell", since = "1.80.0")]
420impl<T: RefUnwindSafe + UnwindSafe, F: UnwindSafe> RefUnwindSafe for LazyLock<T, F> {}
421#[stable(feature = "lazy_cell", since = "1.80.0")]
422impl<T: UnwindSafe, F: UnwindSafe> UnwindSafe for LazyLock<T, F> {}
std/sync/lazy_lock.rs

std/sync/
lazy_lock.rs
