library/std/src/lazy.rs

   1 //! Lazy values and one-time initialization of static data.
   2
   3 #[cfg(test)]
   4 mod tests;
   5
   6 use crate::{
   7     cell::{Cell, UnsafeCell},
   8     fmt,
   9     marker::PhantomData,
  10     mem::MaybeUninit,
  11     ops::{Deref, Drop},
  12     panic::{RefUnwindSafe, UnwindSafe},
  13     pin::Pin,
  14     sync::Once,
  15 };
  16
  17 #[doc(inline)]
  18 #[unstable(feature = "once_cell", issue = "74465")]
  19 pub use core::lazy::*;
  20
  21 /// A synchronization primitive which can be written to only once.
  22 ///
  23 /// This type is a thread-safe `OnceCell`.
  24 ///
  25 /// # Examples
  26 ///
  27 /// ```
  28 /// #![feature(once_cell)]
  29 ///
  30 /// use std::lazy::SyncOnceCell;
  31 ///
  32 /// static CELL: SyncOnceCell<String> = SyncOnceCell::new();
  33 /// assert!(CELL.get().is_none());
  34 ///
  35 /// std::thread::spawn(|| {
  36 ///     let value: &String = CELL.get_or_init(|| {
  37 ///         "Hello, World!".to_string()
  38 ///     });
  39 ///     assert_eq!(value, "Hello, World!");
  40 /// }).join().unwrap();
  41 ///
  42 /// let value: Option<&String> = CELL.get();
  43 /// assert!(value.is_some());
  44 /// assert_eq!(value.unwrap().as_str(), "Hello, World!");
  45 /// ```
  46 #[unstable(feature = "once_cell", issue = "74465")]
  47 pub struct SyncOnceCell<T> {
  48     once: Once,
  49     // Whether or not the value is initialized is tracked by `state_and_queue`.
  50     value: UnsafeCell<MaybeUninit<T>>,
  51     /// `PhantomData` to make sure dropck understands we're dropping T in our Drop impl.
  52     ///
  53     /// ```compile_fail,E0597
  54     /// #![feature(once_cell)]
  55     ///
  56     /// use std::lazy::SyncOnceCell;
  57     ///
  58     /// struct A<'a>(&'a str);
  59     ///
  60     /// impl<'a> Drop for A<'a> {
  61     ///     fn drop(&mut self) {}
  62     /// }
  63     ///
  64     /// let cell = SyncOnceCell::new();
  65     /// {
  66     ///     let s = String::new();
  67     ///     let _ = cell.set(A(&s));
  68     /// }
  69     /// ```
  70     _marker: PhantomData<T>,
  71 }
  72
  73 // Why do we need `T: Send`?
  74 // Thread A creates a `SyncOnceCell` and shares it with
  75 // scoped thread B, which fills the cell, which is
  76 // then destroyed by A. That is, destructor observes
  77 // a sent value.
  78 #[unstable(feature = "once_cell", issue = "74465")]
  79 unsafe impl<T: Sync + Send> Sync for SyncOnceCell<T> {}
  80 #[unstable(feature = "once_cell", issue = "74465")]
  81 unsafe impl<T: Send> Send for SyncOnceCell<T> {}
  82
  83 #[unstable(feature = "once_cell", issue = "74465")]
  84 impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for SyncOnceCell<T> {}
  85 #[unstable(feature = "once_cell", issue = "74465")]
  86 impl<T: UnwindSafe> UnwindSafe for SyncOnceCell<T> {}
  87
  88 #[unstable(feature = "once_cell", issue = "74465")]
  89 impl<T> Default for SyncOnceCell<T> {
  90     fn default() -> SyncOnceCell<T> {
  91         SyncOnceCell::new()
  92     }
  93 }
  94
  95 #[unstable(feature = "once_cell", issue = "74465")]
  96 impl<T: fmt::Debug> fmt::Debug for SyncOnceCell<T> {
  97     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  98         match self.get() {
  99             Some(v) => f.debug_tuple("Once").field(v).finish(),
 100             None => f.write_str("Once(Uninit)"),
 101         }
 102     }
 103 }
 104
 105 #[unstable(feature = "once_cell", issue = "74465")]
 106 impl<T: Clone> Clone for SyncOnceCell<T> {
 107     fn clone(&self) -> SyncOnceCell<T> {
 108         let cell = Self::new();
 109         if let Some(value) = self.get() {
 110             match cell.set(value.clone()) {
 111                 Ok(()) => (),
 112                 Err(_) => unreachable!(),
 113             }
 114         }
 115         cell
 116     }
 117 }
 118
 119 #[unstable(feature = "once_cell", issue = "74465")]
 120 impl<T> From<T> for SyncOnceCell<T> {
 121     fn from(value: T) -> Self {
 122         let cell = Self::new();
 123         match cell.set(value) {
 124             Ok(()) => cell,
 125             Err(_) => unreachable!(),
 126         }
 127     }
 128 }
 129
 130 #[unstable(feature = "once_cell", issue = "74465")]
 131 impl<T: PartialEq> PartialEq for SyncOnceCell<T> {
 132     fn eq(&self, other: &SyncOnceCell<T>) -> bool {
 133         self.get() == other.get()
 134     }
 135 }
 136
 137 #[unstable(feature = "once_cell", issue = "74465")]
 138 impl<T: Eq> Eq for SyncOnceCell<T> {}
 139
 140 impl<T> SyncOnceCell<T> {
 141     /// Creates a new empty cell.
 142     #[unstable(feature = "once_cell", issue = "74465")]
 143     pub const fn new() -> SyncOnceCell<T> {
 144         SyncOnceCell {
 145             once: Once::new(),
 146             value: UnsafeCell::new(MaybeUninit::uninit()),
 147             _marker: PhantomData,
 148         }
 149     }
 150
 151     /// Gets the reference to the underlying value.
 152     ///
 153     /// Returns `None` if the cell is empty, or being initialized. This
 154     /// method never blocks.
 155     #[unstable(feature = "once_cell", issue = "74465")]
 156     pub fn get(&self) -> Option<&T> {
 157         if self.is_initialized() {
 158             // Safe b/c checked is_initialized
 159             Some(unsafe { self.get_unchecked() })
 160         } else {
 161             None
 162         }
 163     }
 164
 165     /// Gets the mutable reference to the underlying value.
 166     ///
 167     /// Returns `None` if the cell is empty. This method never blocks.
 168     #[unstable(feature = "once_cell", issue = "74465")]
 169     pub fn get_mut(&mut self) -> Option<&mut T> {
 170         if self.is_initialized() {
 171             // Safe b/c checked is_initialized and we have a unique access
 172             Some(unsafe { self.get_unchecked_mut() })
 173         } else {
 174             None
 175         }
 176     }
 177
 178     /// Sets the contents of this cell to `value`.
 179     ///
 180     /// Returns `Ok(())` if the cell's value was updated.
 181     ///
 182     /// # Examples
 183     ///
 184     /// ```
 185     /// #![feature(once_cell)]
 186     ///
 187     /// use std::lazy::SyncOnceCell;
 188     ///
 189     /// static CELL: SyncOnceCell<i32> = SyncOnceCell::new();
 190     ///
 191     /// fn main() {
 192     ///     assert!(CELL.get().is_none());
 193     ///
 194     ///     std::thread::spawn(|| {
 195     ///         assert_eq!(CELL.set(92), Ok(()));
 196     ///     }).join().unwrap();
 197     ///
 198     ///     assert_eq!(CELL.set(62), Err(62));
 199     ///     assert_eq!(CELL.get(), Some(&92));
 200     /// }
 201     /// ```
 202     #[unstable(feature = "once_cell", issue = "74465")]
 203     pub fn set(&self, value: T) -> Result<(), T> {
 204         let mut value = Some(value);
 205         self.get_or_init(|| value.take().unwrap());
 206         match value {
 207             None => Ok(()),
 208             Some(value) => Err(value),
 209         }
 210     }
 211
 212     /// Gets the contents of the cell, initializing it with `f` if the cell
 213     /// was empty.
 214     ///
 215     /// Many threads may call `get_or_init` concurrently with different
 216     /// initializing functions, but it is guaranteed that only one function
 217     /// will be executed.
 218     ///
 219     /// # Panics
 220     ///
 221     /// If `f` panics, the panic is propagated to the caller, and the cell
 222     /// remains uninitialized.
 223     ///
 224     /// It is an error to reentrantly initialize the cell from `f`. The
 225     /// exact outcome is unspecified. Current implementation deadlocks, but
 226     /// this may be changed to a panic in the future.
 227     ///
 228     /// # Examples
 229     ///
 230     /// ```
 231     /// #![feature(once_cell)]
 232     ///
 233     /// use std::lazy::SyncOnceCell;
 234     ///
 235     /// let cell = SyncOnceCell::new();
 236     /// let value = cell.get_or_init(|| 92);
 237     /// assert_eq!(value, &92);
 238     /// let value = cell.get_or_init(|| unreachable!());
 239     /// assert_eq!(value, &92);
 240     /// ```
 241     #[unstable(feature = "once_cell", issue = "74465")]
 242     pub fn get_or_init<F>(&self, f: F) -> &T
 243     where
 244         F: FnOnce() -> T,
 245     {
 246         match self.get_or_try_init(|| Ok::<T, !>(f())) {
 247             Ok(val) => val,
 248         }
 249     }
 250
 251     /// Gets the contents of the cell, initializing it with `f` if
 252     /// the cell was empty. If the cell was empty and `f` failed, an
 253     /// error is returned.
 254     ///
 255     /// # Panics
 256     ///
 257     /// If `f` panics, the panic is propagated to the caller, and
 258     /// the cell remains uninitialized.
 259     ///
 260     /// It is an error to reentrantly initialize the cell from `f`.
 261     /// The exact outcome is unspecified. Current implementation
 262     /// deadlocks, but this may be changed to a panic in the future.
 263     ///
 264     /// # Examples
 265     ///
 266     /// ```
 267     /// #![feature(once_cell)]
 268     ///
 269     /// use std::lazy::SyncOnceCell;
 270     ///
 271     /// let cell = SyncOnceCell::new();
 272     /// assert_eq!(cell.get_or_try_init(|| Err(())), Err(()));
 273     /// assert!(cell.get().is_none());
 274     /// let value = cell.get_or_try_init(|| -> Result<i32, ()> {
 275     ///     Ok(92)
 276     /// });
 277     /// assert_eq!(value, Ok(&92));
 278     /// assert_eq!(cell.get(), Some(&92))
 279     /// ```
 280     #[unstable(feature = "once_cell", issue = "74465")]
 281     pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
 282     where
 283         F: FnOnce() -> Result<T, E>,
 284     {
 285         // Fast path check
 286         // NOTE: We need to perform an acquire on the state in this method
 287         // in order to correctly synchronize `SyncLazy::force`. This is
 288         // currently done by calling `self.get()`, which in turn calls
 289         // `self.is_initialized()`, which in turn performs the acquire.
 290         if let Some(value) = self.get() {
 291             return Ok(value);
 292         }
 293         self.initialize(f)?;
 294
 295         debug_assert!(self.is_initialized());
 296
 297         // SAFETY: The inner value has been initialized
 298         Ok(unsafe { self.get_unchecked() })
 299     }
 300
 301     /// Internal-only API that gets the contents of the cell, initializing it
 302     /// in two steps with `f` and `g` if the cell was empty.
 303     ///
 304     /// `f` is called to construct the value, which is then moved into the cell
 305     /// and given as a (pinned) mutable reference to `g` to finish
 306     /// initialization.
 307     ///
 308     /// This allows `g` to inspect an manipulate the value after it has been
 309     /// moved into its final place in the cell, but before the cell is
 310     /// considered initialized.
 311     ///
 312     /// # Panics
 313     ///
 314     /// If `f` or `g` panics, the panic is propagated to the caller, and the
 315     /// cell remains uninitialized.
 316     ///
 317     /// With the current implementation, if `g` panics, the value from `f` will
 318     /// not be dropped. This should probably be fixed if this is ever used for
 319     /// a type where this matters.
 320     ///
 321     /// It is an error to reentrantly initialize the cell from `f`. The exact
 322     /// outcome is unspecified. Current implementation deadlocks, but this may
 323     /// be changed to a panic in the future.
 324     pub(crate) fn get_or_init_pin<F, G>(self: Pin<&Self>, f: F, g: G) -> Pin<&T>
 325     where
 326         F: FnOnce() -> T,
 327         G: FnOnce(Pin<&mut T>),
 328     {
 329         if let Some(value) = self.get_ref().get() {
 330             // SAFETY: The inner value was already initialized, and will not be
 331             // moved anymore.
 332             return unsafe { Pin::new_unchecked(value) };
 333         }
 334
 335         let slot = &self.value;
 336
 337         // Ignore poisoning from other threads
 338         // If another thread panics, then we'll be able to run our closure
 339         self.once.call_once_force(|_| {
 340             let value = f();
 341             // SAFETY: We use the Once (self.once) to guarantee unique access
 342             // to the UnsafeCell (slot).
 343             let value: &mut T = unsafe { (&mut *slot.get()).write(value) };
 344             // SAFETY: The value has been written to its final place in
 345             // self.value. We do not to move it anymore, which we promise here
 346             // with a Pin<&mut T>.
 347             g(unsafe { Pin::new_unchecked(value) });
 348         });
 349
 350         // SAFETY: The inner value has been initialized, and will not be moved
 351         // anymore.
 352         unsafe { Pin::new_unchecked(self.get_ref().get_unchecked()) }
 353     }
 354
 355     /// Consumes the `SyncOnceCell`, returning the wrapped value. Returns
 356     /// `None` if the cell was empty.
 357     ///
 358     /// # Examples
 359     ///
 360     /// ```
 361     /// #![feature(once_cell)]
 362     ///
 363     /// use std::lazy::SyncOnceCell;
 364     ///
 365     /// let cell: SyncOnceCell<String> = SyncOnceCell::new();
 366     /// assert_eq!(cell.into_inner(), None);
 367     ///
 368     /// let cell = SyncOnceCell::new();
 369     /// cell.set("hello".to_string()).unwrap();
 370     /// assert_eq!(cell.into_inner(), Some("hello".to_string()));
 371     /// ```
 372     #[unstable(feature = "once_cell", issue = "74465")]
 373     pub fn into_inner(mut self) -> Option<T> {
 374         self.take()
 375     }
 376
 377     /// Takes the value out of this `SyncOnceCell`, moving it back to an uninitialized state.
 378     ///
 379     /// Has no effect and returns `None` if the `SyncOnceCell` hasn't been initialized.
 380     ///
 381     /// Safety is guaranteed by requiring a mutable reference.
 382     ///
 383     /// # Examples
 384     ///
 385     /// ```
 386     /// #![feature(once_cell)]
 387     ///
 388     /// use std::lazy::SyncOnceCell;
 389     ///
 390     /// let mut cell: SyncOnceCell<String> = SyncOnceCell::new();
 391     /// assert_eq!(cell.take(), None);
 392     ///
 393     /// let mut cell = SyncOnceCell::new();
 394     /// cell.set("hello".to_string()).unwrap();
 395     /// assert_eq!(cell.take(), Some("hello".to_string()));
 396     /// assert_eq!(cell.get(), None);
 397     /// ```
 398     #[unstable(feature = "once_cell", issue = "74465")]
 399     pub fn take(&mut self) -> Option<T> {
 400         if self.is_initialized() {
 401             self.once = Once::new();
 402             // SAFETY: `self.value` is initialized and contains a valid `T`.
 403             // `self.once` is reset, so `is_initialized()` will be false again
 404             // which prevents the value from being read twice.
 405             unsafe { Some((&mut *self.value.get()).assume_init_read()) }
 406         } else {
 407             None
 408         }
 409     }
 410
 411     #[inline]
 412     fn is_initialized(&self) -> bool {
 413         self.once.is_completed()
 414     }
 415
 416     #[cold]
 417     fn initialize<F, E>(&self, f: F) -> Result<(), E>
 418     where
 419         F: FnOnce() -> Result<T, E>,
 420     {
 421         let mut res: Result<(), E> = Ok(());
 422         let slot = &self.value;
 423
 424         // Ignore poisoning from other threads
 425         // If another thread panics, then we'll be able to run our closure
 426         self.once.call_once_force(|p| {
 427             match f() {
 428                 Ok(value) => {
 429                     unsafe { (&mut *slot.get()).write(value) };
 430                 }
 431                 Err(e) => {
 432                     res = Err(e);
 433
 434                     // Treat the underlying `Once` as poisoned since we
 435                     // failed to initialize our value. Calls
 436                     p.poison();
 437                 }
 438             }
 439         });
 440         res
 441     }
 442
 443     /// Safety: The value must be initialized
 444     unsafe fn get_unchecked(&self) -> &T {
 445         debug_assert!(self.is_initialized());
 446         (&*self.value.get()).assume_init_ref()
 447     }
 448
 449     /// Safety: The value must be initialized
 450     unsafe fn get_unchecked_mut(&mut self) -> &mut T {
 451         debug_assert!(self.is_initialized());
 452         (&mut *self.value.get()).assume_init_mut()
 453     }
 454 }
 455
 456 unsafe impl<#[may_dangle] T> Drop for SyncOnceCell<T> {
 457     fn drop(&mut self) {
 458         if self.is_initialized() {
 459             // Safety: The cell is initialized and being dropped, so it can't
 460             // be accessed again. We also don't touch the `T` other than
 461             // dropping it, which validates our usage of #[may_dangle].
 462             unsafe { (&mut *self.value.get()).assume_init_drop() };
 463         }
 464     }
 465 }
 466
 467 /// A value which is initialized on the first access.
 468 ///
 469 /// This type is a thread-safe `Lazy`, and can be used in statics.
 470 ///
 471 /// # Examples
 472 ///
 473 /// ```
 474 /// #![feature(once_cell)]
 475 ///
 476 /// use std::collections::HashMap;
 477 ///
 478 /// use std::lazy::SyncLazy;
 479 ///
 480 /// static HASHMAP: SyncLazy<HashMap<i32, String>> = SyncLazy::new(|| {
 481 ///     println!("initializing");
 482 ///     let mut m = HashMap::new();
 483 ///     m.insert(13, "Spica".to_string());
 484 ///     m.insert(74, "Hoyten".to_string());
 485 ///     m
 486 /// });
 487 ///
 488 /// fn main() {
 489 ///     println!("ready");
 490 ///     std::thread::spawn(|| {
 491 ///         println!("{:?}", HASHMAP.get(&13));
 492 ///     }).join().unwrap();
 493 ///     println!("{:?}", HASHMAP.get(&74));
 494 ///
 495 ///     // Prints:
 496 ///     //   ready
 497 ///     //   initializing
 498 ///     //   Some("Spica")
 499 ///     //   Some("Hoyten")
 500 /// }
 501 /// ```
 502 #[unstable(feature = "once_cell", issue = "74465")]
 503 pub struct SyncLazy<T, F = fn() -> T> {
 504     cell: SyncOnceCell<T>,
 505     init: Cell<Option<F>>,
 506 }
 507
 508 #[unstable(feature = "once_cell", issue = "74465")]
 509 impl<T: fmt::Debug, F> fmt::Debug for SyncLazy<T, F> {
 510     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 511         f.debug_struct("Lazy").field("cell", &self.cell).field("init", &"..").finish()
 512     }
 513 }
 514
 515 // We never create a `&F` from a `&SyncLazy<T, F>` so it is fine
 516 // to not impl `Sync` for `F`
 517 // we do create a `&mut Option<F>` in `force`, but this is
 518 // properly synchronized, so it only happens once
 519 // so it also does not contribute to this impl.
 520 #[unstable(feature = "once_cell", issue = "74465")]
 521 unsafe impl<T, F: Send> Sync for SyncLazy<T, F> where SyncOnceCell<T>: Sync {}
 522 // auto-derived `Send` impl is OK.
 523
 524 #[unstable(feature = "once_cell", issue = "74465")]
 525 impl<T, F: UnwindSafe> RefUnwindSafe for SyncLazy<T, F> where SyncOnceCell<T>: RefUnwindSafe {}
 526 #[unstable(feature = "once_cell", issue = "74465")]
 527 impl<T, F: UnwindSafe> UnwindSafe for SyncLazy<T, F> where SyncOnceCell<T>: UnwindSafe {}
 528
 529 impl<T, F> SyncLazy<T, F> {
 530     /// Creates a new lazy value with the given initializing
 531     /// function.
 532     #[unstable(feature = "once_cell", issue = "74465")]
 533     pub const fn new(f: F) -> SyncLazy<T, F> {
 534         SyncLazy { cell: SyncOnceCell::new(), init: Cell::new(Some(f)) }
 535     }
 536 }
 537
 538 impl<T, F: FnOnce() -> T> SyncLazy<T, F> {
 539     /// Forces the evaluation of this lazy value and
 540     /// returns a reference to result. This is equivalent
 541     /// to the `Deref` impl, but is explicit.
 542     ///
 543     /// # Examples
 544     ///
 545     /// ```
 546     /// #![feature(once_cell)]
 547     ///
 548     /// use std::lazy::SyncLazy;
 549     ///
 550     /// let lazy = SyncLazy::new(|| 92);
 551     ///
 552     /// assert_eq!(SyncLazy::force(&lazy), &92);
 553     /// assert_eq!(&*lazy, &92);
 554     /// ```
 555     #[unstable(feature = "once_cell", issue = "74465")]
 556     pub fn force(this: &SyncLazy<T, F>) -> &T {
 557         this.cell.get_or_init(|| match this.init.take() {
 558             Some(f) => f(),
 559             None => panic!("Lazy instance has previously been poisoned"),
 560         })
 561     }
 562 }
 563
 564 #[unstable(feature = "once_cell", issue = "74465")]
 565 impl<T, F: FnOnce() -> T> Deref for SyncLazy<T, F> {
 566     type Target = T;
 567     fn deref(&self) -> &T {
 568         SyncLazy::force(self)
 569     }
 570 }
 571
 572 #[unstable(feature = "once_cell", issue = "74465")]
 573 impl<T: Default> Default for SyncLazy<T> {
 574     /// Creates a new lazy value using `Default` as the initializing function.
 575     fn default() -> SyncLazy<T> {
 576         SyncLazy::new(T::default)
 577     }
 578 }