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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496
use crate::cell::UnsafeCell;
use crate::fmt;
use crate::marker::PhantomData;
use crate::mem::MaybeUninit;
use crate::panic::{RefUnwindSafe, UnwindSafe};
use crate::pin::Pin;
use crate::sync::Once;
/// A synchronization primitive which can be written to only once.
///
/// This type is a thread-safe `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 `state_and_queue`.
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.
#[unstable(feature = "once_cell", issue = "74465")]
#[must_use]
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.
#[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.
#[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));
/// }
/// ```
#[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);
/// ```
#[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))
/// ```
#[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() })
}
/// Internal-only API that gets the contents of the cell, initializing it
/// in two steps with `f` and `g` if the cell was empty.
///
/// `f` is called to construct the value, which is then moved into the cell
/// and given as a (pinned) mutable reference to `g` to finish
/// initialization.
///
/// This allows `g` to inspect an manipulate the value after it has been
/// moved into its final place in the cell, but before the cell is
/// considered initialized.
///
/// # Panics
///
/// If `f` or `g` panics, the panic is propagated to the caller, and the
/// cell remains uninitialized.
///
/// With the current implementation, if `g` panics, the value from `f` will
/// not be dropped. This should probably be fixed if this is ever used for
/// a type where this matters.
///
/// 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.
pub(crate) fn get_or_init_pin<F, G>(self: Pin<&Self>, f: F, g: G) -> Pin<&T>
where
F: FnOnce() -> T,
G: FnOnce(Pin<&mut T>),
{
if let Some(value) = self.get_ref().get() {
// SAFETY: The inner value was already initialized, and will not be
// moved anymore.
return unsafe { Pin::new_unchecked(value) };
}
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(|_| {
let value = f();
// SAFETY: We use the Once (self.once) to guarantee unique access
// to the UnsafeCell (slot).
let value: &mut T = unsafe { (&mut *slot.get()).write(value) };
// SAFETY: The value has been written to its final place in
// self.value. We do not to move it anymore, which we promise here
// with a Pin<&mut T>.
g(unsafe { Pin::new_unchecked(value) });
});
// SAFETY: The inner value has been initialized, and will not be moved
// anymore.
unsafe { Pin::new_unchecked(self.get_ref().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()));
/// ```
#[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);
/// ```
#[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
unsafe fn get_unchecked(&self) -> &T {
debug_assert!(self.is_initialized());
(&*self.value.get()).assume_init_ref()
}
/// # Safety
///
/// The value must be initialized
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());
/// }
/// ```
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> {
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(())
/// # }
/// ```
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> {
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> {
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;