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Signed-off-by: Valentin Popov <valentin@popov.link>
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Valentin Popov
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# Changelog
## Unreleased
-
## 1.19.0
- Use `portable-atomic` instead of `atomic-polyfill`, [#251](https://github.com/matklad/once_cell/pull/251).
## 1.18.0
- `MSRV` is updated to 1.60.0 to take advantage of `dep:` syntax for cargo features,
removing "implementation details" from publicly visible surface.
## 1.17.2
- Avoid unnecessary synchronization in `Lazy::{force,deref}_mut()`, [#231](https://github.com/matklad/once_cell/pull/231).
## 1.17.1
- Make `OnceRef` implementation compliant with [strict provenance](https://github.com/rust-lang/rust/issues/95228).
## 1.17.0
- Add `race::OnceRef` for storing a `&'a T`.
## 1.16.0
- Add `no_std` implementation based on `critical-section`,
[#195](https://github.com/matklad/once_cell/pull/195).
- Deprecate `atomic-polyfill` feature (use the new `critical-section` instead)
## 1.15.0
- Increase minimal supported Rust version to 1.56.0.
- Implement `UnwindSafe` even if the `std` feature is disabled.
## 1.14.0
- Add extension to `unsync` and `sync` `Lazy` mut API:
- `force_mut`
- `get_mut`
## 1.13.1
- Make implementation compliant with [strict provenance](https://github.com/rust-lang/rust/issues/95228).
- Upgrade `atomic-polyfill` to `1.0`
## 1.13.0
- Add `Lazy::get`, similar to `OnceCell::get`.
## 1.12.1
- Remove incorrect `debug_assert`.
## 1.12.0
- Add `OnceCell::wait`, a blocking variant of `get`.
## 1.11.0
- Add `OnceCell::with_value` to create initialized `OnceCell` in `const` context.
- Improve `Clone` implementation for `OnceCell`.
- Rewrite `parking_lot` version on top of `parking_lot_core`, for even smaller cells!
## 1.10.0
- upgrade `parking_lot` to `0.12.0` (note that this bumps MSRV with `parking_lot` feature enabled to `1.49.0`).
## 1.9.0
- Added an `atomic-polyfill` optional dependency to compile `race` on platforms without atomics
## 1.8.0
- Add `try_insert` API -- a version of `set` that returns a reference.
## 1.7.2
- Improve code size when using parking_lot feature.
## 1.7.1
- Fix `race::OnceBox<T>` to also impl `Default` even if `T` doesn't impl `Default`.
## 1.7.0
- Hide the `race` module behind (default) `race` feature.
Turns out that adding `race` by default was a breaking change on some platforms without atomics.
In this release, we make the module opt-out.
Technically, this is a breaking change for those who use `race` with `no_default_features`.
Given that the `race` module itself only several days old, the breakage is deemed acceptable.
## 1.6.0
- Add `Lazy::into_value`
- Stabilize `once_cell::race` module for "first one wins" no_std-compatible initialization flavor.
- Migrate from deprecated `compare_and_swap` to `compare_exchange`.
## 1.5.2
- `OnceBox` API uses `Box<T>`.
This a breaking change to unstable API.
## 1.5.1
- MSRV is increased to `1.36.0`.
- document `once_cell::race` module.
- introduce `alloc` feature for `OnceBox`.
- fix `OnceBox::set`.
## 1.5.0
- add new `once_cell::race` module for "first one wins" no_std-compatible initialization flavor.
The API is provisional, subject to change and is gated by the `unstable` cargo feature.
## 1.4.1
- upgrade `parking_lot` to `0.11.0`
- make `sync::OnceCell<T>` pass https://doc.rust-lang.org/nomicon/dropck.html#an-escape-hatch[dropck] with `parking_lot` feature enabled.
This fixes a (minor) semver-incompatible changed introduced in `1.4.0`
## 1.4.0
- upgrade `parking_lot` to `0.10` (note that this bumps MSRV with `parking_lot` feature enabled to `1.36.0`).
- add `OnceCell::take`.
- upgrade crossbeam utils (private dependency) to `0.7`.
## 1.3.1
- remove unnecessary `F: fmt::Debug` bound from `impl fmt::Debug for Lazy<T, F>`.
## 1.3.0
- `Lazy<T>` now implements `DerefMut`.
- update implementation according to the latest changes in `std`.
## 1.2.0
- add `sync::OnceCell::get_unchecked`.
## 1.1.0
- implement `Default` for `Lazy`: it creates an empty `Lazy<T>` which is initialized with `T::default` on first access.
- add `OnceCell::get_mut`.
## 1.0.2
- actually add `#![no_std]` attribute if std feature is not enabled.
## 1.0.1
- fix unsoundness in `Lazy<T>` if the initializing function panics. Thanks [@xfix](https://github.com/xfix)!
- implement `RefUnwindSafe` for `Lazy`.
- share more code between `std` and `parking_lot` implementations.
- add F.A.Q section to the docs.
## 1.0.0
- remove `parking_lot` from the list of default features.
- add `std` default feature. Without `std`, only `unsync` module is supported.
- implement `Eq` for `OnceCell`.
- fix wrong `Sync` bound on `sync::Lazy`.
- run the whole test suite with miri.
## 0.2.7
- New implementation of `sync::OnceCell` if `parking_lot` feature is disabled.
It now employs a hand-rolled variant of `std::sync::Once`.
- `sync::OnceCell::get_or_try_init` works without `parking_lot` as well!
- document the effects of `parking_lot` feature: same performance but smaller types.
## 0.2.6
- Updated `Lazy`'s `Deref` impl to requires only `FnOnce` instead of `Fn`
## 0.2.5
- `Lazy` requires only `FnOnce` instead of `Fn`
## 0.2.4
- nicer `fmt::Debug` implementation
## 0.2.3
- update `parking_lot` to `0.9.0`
- fix stacked borrows violation in `unsync::OnceCell::get`
- implement `Clone` for `sync::OnceCell<T> where T: Clone`
## 0.2.2
- add `OnceCell::into_inner` which consumes a cell and returns an option
## 0.2.1
- implement `sync::OnceCell::get_or_try_init` if `parking_lot` feature is enabled
- switch internal `unsafe` implementation of `sync::OnceCell` from `Once` to `Mutex`
- `sync::OnceCell::get_or_init` is twice as fast if cell is already initialized
- implement `std::panic::RefUnwindSafe` and `std::panic::UnwindSafe` for `OnceCell`
- better document behavior around panics
## 0.2.0
- MSRV is now 1.31.1
- `Lazy::new` and `OnceCell::new` are now const-fns
- `unsync_lazy` and `sync_lazy` macros are removed
## 0.1.8
- update crossbeam-utils to 0.6
- enable bors-ng
## 0.1.7
- cells implement `PartialEq` and `From`
- MSRV is down to 1.24.1
- update `parking_lot` to `0.7.1`
## 0.1.6
- `unsync::OnceCell<T>` is `Clone` if `T` is `Clone`.
## 0.1.5
- No changelog until this point :(

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Permission is hereby granted, free of charge, to any
person obtaining a copy of this software and associated
documentation files (the "Software"), to deal in the
Software without restriction, including without
limitation the rights to use, copy, modify, merge,
publish, distribute, sublicense, and/or sell copies of
the Software, and to permit persons to whom the Software
is furnished to do so, subject to the following
conditions:
The above copyright notice and this permission notice
shall be included in all copies or substantial portions
of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
DEALINGS IN THE SOFTWARE.

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<p align="center"><img src="design/logo.png" alt="once_cell"></p>
[![Build Status](https://github.com/matklad/once_cell/actions/workflows/ci.yaml/badge.svg)](https://github.com/matklad/once_cell/actions)
[![Crates.io](https://img.shields.io/crates/v/once_cell.svg)](https://crates.io/crates/once_cell)
[![API reference](https://docs.rs/once_cell/badge.svg)](https://docs.rs/once_cell/)
# Overview
`once_cell` provides two new cell-like types, `unsync::OnceCell` and `sync::OnceCell`. `OnceCell`
might store arbitrary non-`Copy` types, can be assigned to at most once and provide direct access
to the stored contents. In a nutshell, API looks *roughly* like this:
```rust
impl OnceCell<T> {
fn new() -> OnceCell<T> { ... }
fn set(&self, value: T) -> Result<(), T> { ... }
fn get(&self) -> Option<&T> { ... }
}
```
Note that, like with `RefCell` and `Mutex`, the `set` method requires only a shared reference.
Because of the single assignment restriction `get` can return an `&T` instead of `Ref<T>`
or `MutexGuard<T>`.
`once_cell` also has a `Lazy<T>` type, build on top of `OnceCell` which provides the same API as
the `lazy_static!` macro, but without using any macros:
```rust
use std::{sync::Mutex, collections::HashMap};
use once_cell::sync::Lazy;
static GLOBAL_DATA: Lazy<Mutex<HashMap<i32, String>>> = Lazy::new(|| {
let mut m = HashMap::new();
m.insert(13, "Spica".to_string());
m.insert(74, "Hoyten".to_string());
Mutex::new(m)
});
fn main() {
println!("{:?}", GLOBAL_DATA.lock().unwrap());
}
```
More patterns and use-cases are in the [docs](https://docs.rs/once_cell/)!
# Related crates
* [double-checked-cell](https://github.com/niklasf/double-checked-cell)
* [lazy-init](https://crates.io/crates/lazy-init)
* [lazycell](https://crates.io/crates/lazycell)
* [mitochondria](https://crates.io/crates/mitochondria)
* [lazy_static](https://crates.io/crates/lazy_static)
* [async_once_cell](https://crates.io/crates/async_once_cell)
* [generic_once_cell](https://crates.io/crates/generic_once_cell) (bring your own mutex)
Parts of `once_cell` API are included into `std` [as of Rust 1.70.0](https://github.com/rust-lang/rust/pull/105587).

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status = [ "Rust" ]
delete_merged_branches = true

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use std::mem::size_of;
use once_cell::sync::OnceCell;
const N_THREADS: usize = 32;
const N_ROUNDS: usize = 100_000_000;
static CELL: OnceCell<usize> = OnceCell::new();
fn main() {
let start = std::time::Instant::now();
let threads =
(0..N_THREADS).map(|i| std::thread::spawn(move || thread_main(i))).collect::<Vec<_>>();
for thread in threads {
thread.join().unwrap();
}
println!("{:?}", start.elapsed());
println!("size_of::<OnceCell<()>>() = {:?}", size_of::<OnceCell<()>>());
println!("size_of::<OnceCell<bool>>() = {:?}", size_of::<OnceCell<bool>>());
println!("size_of::<OnceCell<u32>>() = {:?}", size_of::<OnceCell<u32>>());
}
fn thread_main(i: usize) {
for _ in 0..N_ROUNDS {
let &value = CELL.get_or_init(|| i);
assert!(value < N_THREADS)
}
}

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//! Benchmark the overhead that the synchronization of `OnceCell::get` causes.
//! We do some other operations that write to memory to get an imprecise but somewhat realistic
//! measurement.
use once_cell::sync::OnceCell;
use std::sync::atomic::{AtomicUsize, Ordering};
const N_THREADS: usize = 16;
const N_ROUNDS: usize = 1_000_000;
static CELL: OnceCell<usize> = OnceCell::new();
static OTHER: AtomicUsize = AtomicUsize::new(0);
fn main() {
let start = std::time::Instant::now();
let threads =
(0..N_THREADS).map(|i| std::thread::spawn(move || thread_main(i))).collect::<Vec<_>>();
for thread in threads {
thread.join().unwrap();
}
println!("{:?}", start.elapsed());
println!("{:?}", OTHER.load(Ordering::Relaxed));
}
#[inline(never)]
fn thread_main(i: usize) {
// The operations we do here don't really matter, as long as we do multiple writes, and
// everything is messy enough to prevent the compiler from optimizing the loop away.
let mut data = [i; 128];
let mut accum = 0usize;
for _ in 0..N_ROUNDS {
let _value = CELL.get_or_init(|| i + 1);
let k = OTHER.fetch_add(data[accum & 0x7F] as usize, Ordering::Relaxed);
for j in data.iter_mut() {
*j = (*j).wrapping_add(accum);
accum = accum.wrapping_add(k);
}
}
}

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extern crate once_cell;
use once_cell::sync::{Lazy, OnceCell};
use std::collections::HashMap;
static HASHMAP: Lazy<HashMap<u32, &'static str>> = Lazy::new(|| {
let mut m = HashMap::new();
m.insert(0, "foo");
m.insert(1, "bar");
m.insert(2, "baz");
m
});
// Same, but completely without macros
fn hashmap() -> &'static HashMap<u32, &'static str> {
static INSTANCE: OnceCell<HashMap<u32, &'static str>> = OnceCell::new();
INSTANCE.get_or_init(|| {
let mut m = HashMap::new();
m.insert(0, "foo");
m.insert(1, "bar");
m.insert(2, "baz");
m
})
}
fn main() {
// First access to `HASHMAP` initializes it
println!("The entry for `0` is \"{}\".", HASHMAP.get(&0).unwrap());
// Any further access to `HASHMAP` just returns the computed value
println!("The entry for `1` is \"{}\".", HASHMAP.get(&1).unwrap());
// The same works for function-style:
assert_eq!(hashmap().get(&0), Some(&"foo"));
assert_eq!(hashmap().get(&1), Some(&"bar"));
}

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fn main() {
let cell = once_cell::sync::OnceCell::<u32>::new();
cell.get_or_init(|| {
cell.get_or_init(|| 1);
2
});
}
/// Dummy test to make it seem hang when compiled as `--test`
/// See https://github.com/matklad/once_cell/issues/79
#[test]
fn dummy_test() {
std::thread::sleep(std::time::Duration::from_secs(4));
}

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use std::{str::FromStr, time::Instant};
use regex::Regex;
macro_rules! regex {
($re:literal $(,)?) => {{
static RE: once_cell::sync::OnceCell<regex::Regex> = once_cell::sync::OnceCell::new();
RE.get_or_init(|| regex::Regex::new($re).unwrap())
}};
}
fn slow() {
let s = r##"13.28.24.13 - - [10/Mar/2016:19:29:25 +0100] "GET /etc/lib/pChart2/examples/index.php?Action=View&Script=../../../../cnf/db.php HTTP/1.1" 404 151 "-" "HTTP_Request2/2.2.1 (http://pear.php.net/package/http_request2) PHP/5.3.16""##;
let mut total = 0;
for _ in 0..1000 {
let re = Regex::new(
r##"^(\S+) (\S+) (\S+) \[([^]]+)\] "([^"]*)" (\d+) (\d+) "([^"]*)" "([^"]*)"$"##,
)
.unwrap();
let size = usize::from_str(re.captures(s).unwrap().get(7).unwrap().as_str()).unwrap();
total += size;
}
println!("{}", total);
}
fn fast() {
let s = r##"13.28.24.13 - - [10/Mar/2016:19:29:25 +0100] "GET /etc/lib/pChart2/examples/index.php?Action=View&Script=../../../../cnf/db.php HTTP/1.1" 404 151 "-" "HTTP_Request2/2.2.1 (http://pear.php.net/package/http_request2) PHP/5.3.16""##;
let mut total = 0;
for _ in 0..1000 {
let re: &Regex = regex!(
r##"^(\S+) (\S+) (\S+) \[([^]]+)\] "([^"]*)" (\d+) (\d+) "([^"]*)" "([^"]*)"$"##,
);
let size = usize::from_str(re.captures(s).unwrap().get(7).unwrap().as_str()).unwrap();
total += size;
}
println!("{}", total);
}
fn main() {
let t = Instant::now();
slow();
println!("slow: {:?}", t.elapsed());
let t = Instant::now();
fast();
println!("fast: {:?}", t.elapsed());
}

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//! Test if the OnceCell properly synchronizes.
//! Needs to be run in release mode.
//!
//! We create a `Vec` with `N_ROUNDS` of `OnceCell`s. All threads will walk the `Vec`, and race to
//! be the first one to initialize a cell.
//! Every thread adds the results of the cells it sees to an accumulator, which is compared at the
//! end.
//! All threads should end up with the same result.
use once_cell::sync::OnceCell;
const N_THREADS: usize = 32;
const N_ROUNDS: usize = 1_000_000;
static CELLS: OnceCell<Vec<OnceCell<usize>>> = OnceCell::new();
static RESULT: OnceCell<usize> = OnceCell::new();
fn main() {
let start = std::time::Instant::now();
CELLS.get_or_init(|| vec![OnceCell::new(); N_ROUNDS]);
let threads =
(0..N_THREADS).map(|i| std::thread::spawn(move || thread_main(i))).collect::<Vec<_>>();
for thread in threads {
thread.join().unwrap();
}
println!("{:?}", start.elapsed());
println!("No races detected");
}
fn thread_main(i: usize) {
let cells = CELLS.get().unwrap();
let mut accum = 0;
for cell in cells.iter() {
let &value = cell.get_or_init(|| i);
accum += value;
}
assert_eq!(RESULT.get_or_init(|| accum), &accum);
}

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use core::panic::{RefUnwindSafe, UnwindSafe};
use portable_atomic::{AtomicBool, Ordering};
use critical_section::{CriticalSection, Mutex};
use crate::unsync;
pub(crate) struct OnceCell<T> {
initialized: AtomicBool,
// Use `unsync::OnceCell` internally since `Mutex` does not provide
// interior mutability and to be able to re-use `get_or_try_init`.
value: Mutex<unsync::OnceCell<T>>,
}
// Why do we need `T: Send`?
// Thread A creates a `OnceCell` and shares it with
// scoped thread B, which fills the cell, which is
// then destroyed by A. That is, destructor observes
// a sent value.
unsafe impl<T: Sync + Send> Sync for OnceCell<T> {}
unsafe impl<T: Send> Send for OnceCell<T> {}
impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for OnceCell<T> {}
impl<T: UnwindSafe> UnwindSafe for OnceCell<T> {}
impl<T> OnceCell<T> {
pub(crate) const fn new() -> OnceCell<T> {
OnceCell { initialized: AtomicBool::new(false), value: Mutex::new(unsync::OnceCell::new()) }
}
pub(crate) const fn with_value(value: T) -> OnceCell<T> {
OnceCell {
initialized: AtomicBool::new(true),
value: Mutex::new(unsync::OnceCell::with_value(value)),
}
}
#[inline]
pub(crate) fn is_initialized(&self) -> bool {
self.initialized.load(Ordering::Acquire)
}
#[cold]
pub(crate) fn initialize<F, E>(&self, f: F) -> Result<(), E>
where
F: FnOnce() -> Result<T, E>,
{
critical_section::with(|cs| {
let cell = self.value.borrow(cs);
cell.get_or_try_init(f).map(|_| {
self.initialized.store(true, Ordering::Release);
})
})
}
/// Get the reference to the underlying value, without checking if the cell
/// is initialized.
///
/// # Safety
///
/// Caller must ensure that the cell is in initialized state, and that
/// the contents are acquired by (synchronized to) this thread.
pub(crate) unsafe fn get_unchecked(&self) -> &T {
debug_assert!(self.is_initialized());
// SAFETY: The caller ensures that the value is initialized and access synchronized.
self.value.borrow(CriticalSection::new()).get().unwrap_unchecked()
}
#[inline]
pub(crate) fn get_mut(&mut self) -> Option<&mut T> {
self.value.get_mut().get_mut()
}
#[inline]
pub(crate) fn into_inner(self) -> Option<T> {
self.value.into_inner().into_inner()
}
}

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use std::{
cell::UnsafeCell,
panic::{RefUnwindSafe, UnwindSafe},
sync::atomic::{AtomicU8, Ordering},
};
pub(crate) struct OnceCell<T> {
state: AtomicU8,
value: UnsafeCell<Option<T>>,
}
const INCOMPLETE: u8 = 0x0;
const RUNNING: u8 = 0x1;
const COMPLETE: u8 = 0x2;
// Why do we need `T: Send`?
// Thread A creates a `OnceCell` and shares it with
// scoped thread B, which fills the cell, which is
// then destroyed by A. That is, destructor observes
// a sent value.
unsafe impl<T: Sync + Send> Sync for OnceCell<T> {}
unsafe impl<T: Send> Send for OnceCell<T> {}
impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for OnceCell<T> {}
impl<T: UnwindSafe> UnwindSafe for OnceCell<T> {}
impl<T> OnceCell<T> {
pub(crate) const fn new() -> OnceCell<T> {
OnceCell { state: AtomicU8::new(INCOMPLETE), value: UnsafeCell::new(None) }
}
pub(crate) const fn with_value(value: T) -> OnceCell<T> {
OnceCell { state: AtomicU8::new(COMPLETE), value: UnsafeCell::new(Some(value)) }
}
/// Safety: synchronizes with store to value via Release/Acquire.
#[inline]
pub(crate) fn is_initialized(&self) -> bool {
self.state.load(Ordering::Acquire) == COMPLETE
}
/// Safety: synchronizes with store to value via `is_initialized` or mutex
/// lock/unlock, writes value only once because of the mutex.
#[cold]
pub(crate) fn initialize<F, E>(&self, f: F) -> Result<(), E>
where
F: FnOnce() -> Result<T, E>,
{
let mut f = Some(f);
let mut res: Result<(), E> = Ok(());
let slot: *mut Option<T> = self.value.get();
initialize_inner(&self.state, &mut || {
// We are calling user-supplied function and need to be careful.
// - if it returns Err, we unlock mutex and return without touching anything
// - if it panics, we unlock mutex and propagate panic without touching anything
// - if it calls `set` or `get_or_try_init` re-entrantly, we get a deadlock on
// mutex, which is important for safety. We *could* detect this and panic,
// but that is more complicated
// - finally, if it returns Ok, we store the value and store the flag with
// `Release`, which synchronizes with `Acquire`s.
let f = unsafe { f.take().unwrap_unchecked() };
match f() {
Ok(value) => unsafe {
// Safe b/c we have a unique access and no panic may happen
// until the cell is marked as initialized.
debug_assert!((*slot).is_none());
*slot = Some(value);
true
},
Err(err) => {
res = Err(err);
false
}
}
});
res
}
#[cold]
pub(crate) fn wait(&self) {
let key = &self.state as *const _ as usize;
unsafe {
parking_lot_core::park(
key,
|| self.state.load(Ordering::Acquire) != COMPLETE,
|| (),
|_, _| (),
parking_lot_core::DEFAULT_PARK_TOKEN,
None,
);
}
}
/// Get the reference to the underlying value, without checking if the cell
/// is initialized.
///
/// # Safety
///
/// Caller must ensure that the cell is in initialized state, and that
/// the contents are acquired by (synchronized to) this thread.
pub(crate) unsafe fn get_unchecked(&self) -> &T {
debug_assert!(self.is_initialized());
let slot = &*self.value.get();
slot.as_ref().unwrap_unchecked()
}
/// Gets the mutable reference to the underlying value.
/// Returns `None` if the cell is empty.
pub(crate) fn get_mut(&mut self) -> Option<&mut T> {
// Safe b/c we have an exclusive access
let slot: &mut Option<T> = unsafe { &mut *self.value.get() };
slot.as_mut()
}
/// Consumes this `OnceCell`, returning the wrapped value.
/// Returns `None` if the cell was empty.
pub(crate) fn into_inner(self) -> Option<T> {
self.value.into_inner()
}
}
struct Guard<'a> {
state: &'a AtomicU8,
new_state: u8,
}
impl<'a> Drop for Guard<'a> {
fn drop(&mut self) {
self.state.store(self.new_state, Ordering::Release);
unsafe {
let key = self.state as *const AtomicU8 as usize;
parking_lot_core::unpark_all(key, parking_lot_core::DEFAULT_UNPARK_TOKEN);
}
}
}
// Note: this is intentionally monomorphic
#[inline(never)]
fn initialize_inner(state: &AtomicU8, init: &mut dyn FnMut() -> bool) {
loop {
let exchange =
state.compare_exchange_weak(INCOMPLETE, RUNNING, Ordering::Acquire, Ordering::Acquire);
match exchange {
Ok(_) => {
let mut guard = Guard { state, new_state: INCOMPLETE };
if init() {
guard.new_state = COMPLETE;
}
return;
}
Err(COMPLETE) => return,
Err(RUNNING) => unsafe {
let key = state as *const AtomicU8 as usize;
parking_lot_core::park(
key,
|| state.load(Ordering::Relaxed) == RUNNING,
|| (),
|_, _| (),
parking_lot_core::DEFAULT_PARK_TOKEN,
None,
);
},
Err(INCOMPLETE) => (),
Err(_) => debug_assert!(false),
}
}
}
#[test]
fn test_size() {
use std::mem::size_of;
assert_eq!(size_of::<OnceCell<bool>>(), 1 * size_of::<bool>() + size_of::<u8>());
}

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// There's a lot of scary concurrent code in this module, but it is copied from
// `std::sync::Once` with two changes:
// * no poisoning
// * init function can fail
use std::{
cell::{Cell, UnsafeCell},
panic::{RefUnwindSafe, UnwindSafe},
sync::atomic::{AtomicBool, AtomicPtr, Ordering},
thread::{self, Thread},
};
#[derive(Debug)]
pub(crate) struct OnceCell<T> {
// This `queue` field is the core of the implementation. It encodes two
// pieces of information:
//
// * The current state of the cell (`INCOMPLETE`, `RUNNING`, `COMPLETE`)
// * Linked list of threads waiting for the current cell.
//
// State is encoded in two low bits. Only `INCOMPLETE` and `RUNNING` states
// allow waiters.
queue: AtomicPtr<Waiter>,
value: UnsafeCell<Option<T>>,
}
// Why do we need `T: Send`?
// Thread A creates a `OnceCell` and shares it with
// scoped thread B, which fills the cell, which is
// then destroyed by A. That is, destructor observes
// a sent value.
unsafe impl<T: Sync + Send> Sync for OnceCell<T> {}
unsafe impl<T: Send> Send for OnceCell<T> {}
impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for OnceCell<T> {}
impl<T: UnwindSafe> UnwindSafe for OnceCell<T> {}
impl<T> OnceCell<T> {
pub(crate) const fn new() -> OnceCell<T> {
OnceCell { queue: AtomicPtr::new(INCOMPLETE_PTR), value: UnsafeCell::new(None) }
}
pub(crate) const fn with_value(value: T) -> OnceCell<T> {
OnceCell { queue: AtomicPtr::new(COMPLETE_PTR), value: UnsafeCell::new(Some(value)) }
}
/// Safety: synchronizes with store to value via Release/(Acquire|SeqCst).
#[inline]
pub(crate) fn is_initialized(&self) -> bool {
// An `Acquire` load is enough because that makes all the initialization
// operations visible to us, and, this being a fast path, weaker
// ordering helps with performance. This `Acquire` synchronizes with
// `SeqCst` operations on the slow path.
self.queue.load(Ordering::Acquire) == COMPLETE_PTR
}
/// Safety: synchronizes with store to value via SeqCst read from state,
/// writes value only once because we never get to INCOMPLETE state after a
/// successful write.
#[cold]
pub(crate) fn initialize<F, E>(&self, f: F) -> Result<(), E>
where
F: FnOnce() -> Result<T, E>,
{
let mut f = Some(f);
let mut res: Result<(), E> = Ok(());
let slot: *mut Option<T> = self.value.get();
initialize_or_wait(
&self.queue,
Some(&mut || {
let f = unsafe { f.take().unwrap_unchecked() };
match f() {
Ok(value) => {
unsafe { *slot = Some(value) };
true
}
Err(err) => {
res = Err(err);
false
}
}
}),
);
res
}
#[cold]
pub(crate) fn wait(&self) {
initialize_or_wait(&self.queue, None);
}
/// Get the reference to the underlying value, without checking if the cell
/// is initialized.
///
/// # Safety
///
/// Caller must ensure that the cell is in initialized state, and that
/// the contents are acquired by (synchronized to) this thread.
pub(crate) unsafe fn get_unchecked(&self) -> &T {
debug_assert!(self.is_initialized());
let slot = &*self.value.get();
slot.as_ref().unwrap_unchecked()
}
/// Gets the mutable reference to the underlying value.
/// Returns `None` if the cell is empty.
pub(crate) fn get_mut(&mut self) -> Option<&mut T> {
// Safe b/c we have a unique access.
unsafe { &mut *self.value.get() }.as_mut()
}
/// Consumes this `OnceCell`, returning the wrapped value.
/// Returns `None` if the cell was empty.
#[inline]
pub(crate) 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.value.into_inner()
}
}
// Three states that a OnceCell can be in, encoded into the lower bits of `queue` in
// the OnceCell structure.
const INCOMPLETE: usize = 0x0;
const RUNNING: usize = 0x1;
const COMPLETE: usize = 0x2;
const INCOMPLETE_PTR: *mut Waiter = INCOMPLETE as *mut Waiter;
const COMPLETE_PTR: *mut Waiter = COMPLETE as *mut Waiter;
// Mask to learn about the state. All other bits are the queue of waiters if
// this is in the RUNNING state.
const STATE_MASK: usize = 0x3;
/// Representation of a node in the linked list of waiters in the RUNNING state.
/// A waiters is stored on the stack of the waiting threads.
#[repr(align(4))] // Ensure the two lower bits are free to use as state bits.
struct Waiter {
thread: Cell<Option<Thread>>,
signaled: AtomicBool,
next: *mut Waiter,
}
/// Drains and notifies the queue of waiters on drop.
struct Guard<'a> {
queue: &'a AtomicPtr<Waiter>,
new_queue: *mut Waiter,
}
impl Drop for Guard<'_> {
fn drop(&mut self) {
let queue = self.queue.swap(self.new_queue, Ordering::AcqRel);
let state = strict::addr(queue) & STATE_MASK;
assert_eq!(state, RUNNING);
unsafe {
let mut waiter = strict::map_addr(queue, |q| q & !STATE_MASK);
while !waiter.is_null() {
let next = (*waiter).next;
let thread = (*waiter).thread.take().unwrap();
(*waiter).signaled.store(true, Ordering::Release);
waiter = next;
thread.unpark();
}
}
}
}
// Corresponds to `std::sync::Once::call_inner`.
//
// Originally copied from std, but since modified to remove poisoning and to
// support wait.
//
// Note: this is intentionally monomorphic
#[inline(never)]
fn initialize_or_wait(queue: &AtomicPtr<Waiter>, mut init: Option<&mut dyn FnMut() -> bool>) {
let mut curr_queue = queue.load(Ordering::Acquire);
loop {
let curr_state = strict::addr(curr_queue) & STATE_MASK;
match (curr_state, &mut init) {
(COMPLETE, _) => return,
(INCOMPLETE, Some(init)) => {
let exchange = queue.compare_exchange(
curr_queue,
strict::map_addr(curr_queue, |q| (q & !STATE_MASK) | RUNNING),
Ordering::Acquire,
Ordering::Acquire,
);
if let Err(new_queue) = exchange {
curr_queue = new_queue;
continue;
}
let mut guard = Guard { queue, new_queue: INCOMPLETE_PTR };
if init() {
guard.new_queue = COMPLETE_PTR;
}
return;
}
(INCOMPLETE, None) | (RUNNING, _) => {
wait(queue, curr_queue);
curr_queue = queue.load(Ordering::Acquire);
}
_ => debug_assert!(false),
}
}
}
fn wait(queue: &AtomicPtr<Waiter>, mut curr_queue: *mut Waiter) {
let curr_state = strict::addr(curr_queue) & STATE_MASK;
loop {
let node = Waiter {
thread: Cell::new(Some(thread::current())),
signaled: AtomicBool::new(false),
next: strict::map_addr(curr_queue, |q| q & !STATE_MASK),
};
let me = &node as *const Waiter as *mut Waiter;
let exchange = queue.compare_exchange(
curr_queue,
strict::map_addr(me, |q| q | curr_state),
Ordering::Release,
Ordering::Relaxed,
);
if let Err(new_queue) = exchange {
if strict::addr(new_queue) & STATE_MASK != curr_state {
return;
}
curr_queue = new_queue;
continue;
}
while !node.signaled.load(Ordering::Acquire) {
thread::park();
}
break;
}
}
// Polyfill of strict provenance from https://crates.io/crates/sptr.
//
// Use free-standing function rather than a trait to keep things simple and
// avoid any potential conflicts with future stabile std API.
mod strict {
#[must_use]
#[inline]
pub(crate) fn addr<T>(ptr: *mut T) -> usize
where
T: Sized,
{
// FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
// SAFETY: Pointer-to-integer transmutes are valid (if you are okay with losing the
// provenance).
unsafe { core::mem::transmute(ptr) }
}
#[must_use]
#[inline]
pub(crate) fn with_addr<T>(ptr: *mut T, addr: usize) -> *mut T
where
T: Sized,
{
// FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
//
// In the mean-time, this operation is defined to be "as if" it was
// a wrapping_offset, so we can emulate it as such. This should properly
// restore pointer provenance even under today's compiler.
let self_addr = self::addr(ptr) as isize;
let dest_addr = addr as isize;
let offset = dest_addr.wrapping_sub(self_addr);
// This is the canonical desugarring of this operation,
// but `pointer::cast` was only stabilized in 1.38.
// self.cast::<u8>().wrapping_offset(offset).cast::<T>()
(ptr as *mut u8).wrapping_offset(offset) as *mut T
}
#[must_use]
#[inline]
pub(crate) fn map_addr<T>(ptr: *mut T, f: impl FnOnce(usize) -> usize) -> *mut T
where
T: Sized,
{
self::with_addr(ptr, f(addr(ptr)))
}
}
// These test are snatched from std as well.
#[cfg(test)]
mod tests {
use std::panic;
use std::{sync::mpsc::channel, thread};
use super::OnceCell;
impl<T> OnceCell<T> {
fn init(&self, f: impl FnOnce() -> T) {
enum Void {}
let _ = self.initialize(|| Ok::<T, Void>(f()));
}
}
#[test]
fn smoke_once() {
static O: OnceCell<()> = OnceCell::new();
let mut a = 0;
O.init(|| a += 1);
assert_eq!(a, 1);
O.init(|| a += 1);
assert_eq!(a, 1);
}
#[test]
fn stampede_once() {
static O: OnceCell<()> = OnceCell::new();
static mut RUN: bool = false;
let (tx, rx) = channel();
for _ in 0..10 {
let tx = tx.clone();
thread::spawn(move || {
for _ in 0..4 {
thread::yield_now()
}
unsafe {
O.init(|| {
assert!(!RUN);
RUN = true;
});
assert!(RUN);
}
tx.send(()).unwrap();
});
}
unsafe {
O.init(|| {
assert!(!RUN);
RUN = true;
});
assert!(RUN);
}
for _ in 0..10 {
rx.recv().unwrap();
}
}
#[test]
fn poison_bad() {
static O: OnceCell<()> = OnceCell::new();
// poison the once
let t = panic::catch_unwind(|| {
O.init(|| panic!());
});
assert!(t.is_err());
// we can subvert poisoning, however
let mut called = false;
O.init(|| {
called = true;
});
assert!(called);
// once any success happens, we stop propagating the poison
O.init(|| {});
}
#[test]
fn wait_for_force_to_finish() {
static O: OnceCell<()> = OnceCell::new();
// poison the once
let t = panic::catch_unwind(|| {
O.init(|| panic!());
});
assert!(t.is_err());
// make sure someone's waiting inside the once via a force
let (tx1, rx1) = channel();
let (tx2, rx2) = channel();
let t1 = thread::spawn(move || {
O.init(|| {
tx1.send(()).unwrap();
rx2.recv().unwrap();
});
});
rx1.recv().unwrap();
// put another waiter on the once
let t2 = thread::spawn(|| {
let mut called = false;
O.init(|| {
called = true;
});
assert!(!called);
});
tx2.send(()).unwrap();
assert!(t1.join().is_ok());
assert!(t2.join().is_ok());
}
#[test]
#[cfg(target_pointer_width = "64")]
fn test_size() {
use std::mem::size_of;
assert_eq!(size_of::<OnceCell<u32>>(), 4 * size_of::<u32>());
}
}

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//! Thread-safe, non-blocking, "first one wins" flavor of `OnceCell`.
//!
//! If two threads race to initialize a type from the `race` module, they
//! don't block, execute initialization function together, but only one of
//! them stores the result.
//!
//! This module does not require `std` feature.
//!
//! # Atomic orderings
//!
//! All types in this module use `Acquire` and `Release`
//! [atomic orderings](Ordering) for all their operations. While this is not
//! strictly necessary for types other than `OnceBox`, it is useful for users as
//! it allows them to be certain that after `get` or `get_or_init` returns on
//! one thread, any side-effects caused by the setter thread prior to them
//! calling `set` or `get_or_init` will be made visible to that thread; without
//! it, it's possible for it to appear as if they haven't happened yet from the
//! getter thread's perspective. This is an acceptable tradeoff to make since
//! `Acquire` and `Release` have very little performance overhead on most
//! architectures versus `Relaxed`.
#[cfg(feature = "critical-section")]
use portable_atomic as atomic;
#[cfg(not(feature = "critical-section"))]
use core::sync::atomic;
use atomic::{AtomicPtr, AtomicUsize, Ordering};
use core::cell::UnsafeCell;
use core::marker::PhantomData;
use core::num::NonZeroUsize;
use core::ptr;
/// A thread-safe cell which can be written to only once.
#[derive(Default, Debug)]
pub struct OnceNonZeroUsize {
inner: AtomicUsize,
}
impl OnceNonZeroUsize {
/// Creates a new empty cell.
#[inline]
pub const fn new() -> OnceNonZeroUsize {
OnceNonZeroUsize { inner: AtomicUsize::new(0) }
}
/// Gets the underlying value.
#[inline]
pub fn get(&self) -> Option<NonZeroUsize> {
let val = self.inner.load(Ordering::Acquire);
NonZeroUsize::new(val)
}
/// Sets the contents of this cell to `value`.
///
/// Returns `Ok(())` if the cell was empty and `Err(())` if it was
/// full.
#[inline]
pub fn set(&self, value: NonZeroUsize) -> Result<(), ()> {
let exchange =
self.inner.compare_exchange(0, value.get(), Ordering::AcqRel, Ordering::Acquire);
match exchange {
Ok(_) => Ok(()),
Err(_) => Err(()),
}
}
/// Gets the contents of the cell, initializing it with `f` if the cell was
/// empty.
///
/// If several threads concurrently run `get_or_init`, more than one `f` can
/// be called. However, all threads will return the same value, produced by
/// some `f`.
pub fn get_or_init<F>(&self, f: F) -> NonZeroUsize
where
F: FnOnce() -> NonZeroUsize,
{
enum Void {}
match self.get_or_try_init(|| Ok::<NonZeroUsize, Void>(f())) {
Ok(val) => val,
Err(void) => match void {},
}
}
/// 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.
///
/// If several threads concurrently run `get_or_init`, more than one `f` can
/// be called. However, all threads will return the same value, produced by
/// some `f`.
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<NonZeroUsize, E>
where
F: FnOnce() -> Result<NonZeroUsize, E>,
{
let val = self.inner.load(Ordering::Acquire);
let res = match NonZeroUsize::new(val) {
Some(it) => it,
None => {
let mut val = f()?.get();
let exchange =
self.inner.compare_exchange(0, val, Ordering::AcqRel, Ordering::Acquire);
if let Err(old) = exchange {
val = old;
}
unsafe { NonZeroUsize::new_unchecked(val) }
}
};
Ok(res)
}
}
/// A thread-safe cell which can be written to only once.
#[derive(Default, Debug)]
pub struct OnceBool {
inner: OnceNonZeroUsize,
}
impl OnceBool {
/// Creates a new empty cell.
#[inline]
pub const fn new() -> OnceBool {
OnceBool { inner: OnceNonZeroUsize::new() }
}
/// Gets the underlying value.
#[inline]
pub fn get(&self) -> Option<bool> {
self.inner.get().map(OnceBool::from_usize)
}
/// Sets the contents of this cell to `value`.
///
/// Returns `Ok(())` if the cell was empty and `Err(())` if it was
/// full.
#[inline]
pub fn set(&self, value: bool) -> Result<(), ()> {
self.inner.set(OnceBool::to_usize(value))
}
/// Gets the contents of the cell, initializing it with `f` if the cell was
/// empty.
///
/// If several threads concurrently run `get_or_init`, more than one `f` can
/// be called. However, all threads will return the same value, produced by
/// some `f`.
pub fn get_or_init<F>(&self, f: F) -> bool
where
F: FnOnce() -> bool,
{
OnceBool::from_usize(self.inner.get_or_init(|| OnceBool::to_usize(f())))
}
/// 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.
///
/// If several threads concurrently run `get_or_init`, more than one `f` can
/// be called. However, all threads will return the same value, produced by
/// some `f`.
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<bool, E>
where
F: FnOnce() -> Result<bool, E>,
{
self.inner.get_or_try_init(|| f().map(OnceBool::to_usize)).map(OnceBool::from_usize)
}
#[inline]
fn from_usize(value: NonZeroUsize) -> bool {
value.get() == 1
}
#[inline]
fn to_usize(value: bool) -> NonZeroUsize {
unsafe { NonZeroUsize::new_unchecked(if value { 1 } else { 2 }) }
}
}
/// A thread-safe cell which can be written to only once.
pub struct OnceRef<'a, T> {
inner: AtomicPtr<T>,
ghost: PhantomData<UnsafeCell<&'a T>>,
}
// TODO: Replace UnsafeCell with SyncUnsafeCell once stabilized
unsafe impl<'a, T: Sync> Sync for OnceRef<'a, T> {}
impl<'a, T> core::fmt::Debug for OnceRef<'a, T> {
fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
write!(f, "OnceRef({:?})", self.inner)
}
}
impl<'a, T> Default for OnceRef<'a, T> {
fn default() -> Self {
Self::new()
}
}
impl<'a, T> OnceRef<'a, T> {
/// Creates a new empty cell.
pub const fn new() -> OnceRef<'a, T> {
OnceRef { inner: AtomicPtr::new(ptr::null_mut()), ghost: PhantomData }
}
/// Gets a reference to the underlying value.
pub fn get(&self) -> Option<&'a T> {
let ptr = self.inner.load(Ordering::Acquire);
unsafe { ptr.as_ref() }
}
/// Sets the contents of this cell to `value`.
///
/// Returns `Ok(())` if the cell was empty and `Err(value)` if it was
/// full.
pub fn set(&self, value: &'a T) -> Result<(), ()> {
let ptr = value as *const T as *mut T;
let exchange =
self.inner.compare_exchange(ptr::null_mut(), ptr, Ordering::AcqRel, Ordering::Acquire);
match exchange {
Ok(_) => Ok(()),
Err(_) => Err(()),
}
}
/// Gets the contents of the cell, initializing it with `f` if the cell was
/// empty.
///
/// If several threads concurrently run `get_or_init`, more than one `f` can
/// be called. However, all threads will return the same value, produced by
/// some `f`.
pub fn get_or_init<F>(&self, f: F) -> &'a T
where
F: FnOnce() -> &'a T,
{
enum Void {}
match self.get_or_try_init(|| Ok::<&'a T, Void>(f())) {
Ok(val) => val,
Err(void) => match void {},
}
}
/// 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.
///
/// If several threads concurrently run `get_or_init`, more than one `f` can
/// be called. However, all threads will return the same value, produced by
/// some `f`.
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&'a T, E>
where
F: FnOnce() -> Result<&'a T, E>,
{
let mut ptr = self.inner.load(Ordering::Acquire);
if ptr.is_null() {
// TODO replace with `cast_mut` when MSRV reaches 1.65.0 (also in `set`)
ptr = f()? as *const T as *mut T;
let exchange = self.inner.compare_exchange(
ptr::null_mut(),
ptr,
Ordering::AcqRel,
Ordering::Acquire,
);
if let Err(old) = exchange {
ptr = old;
}
}
Ok(unsafe { &*ptr })
}
/// ```compile_fail
/// use once_cell::race::OnceRef;
///
/// let mut l = OnceRef::new();
///
/// {
/// let y = 2;
/// let mut r = OnceRef::new();
/// r.set(&y).unwrap();
/// core::mem::swap(&mut l, &mut r);
/// }
///
/// // l now contains a dangling reference to y
/// eprintln!("uaf: {}", l.get().unwrap());
/// ```
fn _dummy() {}
}
#[cfg(feature = "alloc")]
pub use self::once_box::OnceBox;
#[cfg(feature = "alloc")]
mod once_box {
use super::atomic::{AtomicPtr, Ordering};
use core::{marker::PhantomData, ptr};
use alloc::boxed::Box;
/// A thread-safe cell which can be written to only once.
pub struct OnceBox<T> {
inner: AtomicPtr<T>,
ghost: PhantomData<Option<Box<T>>>,
}
impl<T> core::fmt::Debug for OnceBox<T> {
fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
write!(f, "OnceBox({:?})", self.inner.load(Ordering::Relaxed))
}
}
impl<T> Default for OnceBox<T> {
fn default() -> Self {
Self::new()
}
}
impl<T> Drop for OnceBox<T> {
fn drop(&mut self) {
let ptr = *self.inner.get_mut();
if !ptr.is_null() {
drop(unsafe { Box::from_raw(ptr) })
}
}
}
impl<T> OnceBox<T> {
/// Creates a new empty cell.
pub const fn new() -> OnceBox<T> {
OnceBox { inner: AtomicPtr::new(ptr::null_mut()), ghost: PhantomData }
}
/// Gets a reference to the underlying value.
pub fn get(&self) -> Option<&T> {
let ptr = self.inner.load(Ordering::Acquire);
if ptr.is_null() {
return None;
}
Some(unsafe { &*ptr })
}
/// Sets the contents of this cell to `value`.
///
/// Returns `Ok(())` if the cell was empty and `Err(value)` if it was
/// full.
pub fn set(&self, value: Box<T>) -> Result<(), Box<T>> {
let ptr = Box::into_raw(value);
let exchange = self.inner.compare_exchange(
ptr::null_mut(),
ptr,
Ordering::AcqRel,
Ordering::Acquire,
);
if exchange.is_err() {
let value = unsafe { Box::from_raw(ptr) };
return Err(value);
}
Ok(())
}
/// Gets the contents of the cell, initializing it with `f` if the cell was
/// empty.
///
/// If several threads concurrently run `get_or_init`, more than one `f` can
/// be called. However, all threads will return the same value, produced by
/// some `f`.
pub fn get_or_init<F>(&self, f: F) -> &T
where
F: FnOnce() -> Box<T>,
{
enum Void {}
match self.get_or_try_init(|| Ok::<Box<T>, Void>(f())) {
Ok(val) => val,
Err(void) => match void {},
}
}
/// 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.
///
/// If several threads concurrently run `get_or_init`, more than one `f` can
/// be called. However, all threads will return the same value, produced by
/// some `f`.
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
where
F: FnOnce() -> Result<Box<T>, E>,
{
let mut ptr = self.inner.load(Ordering::Acquire);
if ptr.is_null() {
let val = f()?;
ptr = Box::into_raw(val);
let exchange = self.inner.compare_exchange(
ptr::null_mut(),
ptr,
Ordering::AcqRel,
Ordering::Acquire,
);
if let Err(old) = exchange {
drop(unsafe { Box::from_raw(ptr) });
ptr = old;
}
};
Ok(unsafe { &*ptr })
}
}
unsafe impl<T: Sync + Send> Sync for OnceBox<T> {}
/// ```compile_fail
/// struct S(*mut ());
/// unsafe impl Sync for S {}
///
/// fn share<T: Sync>(_: &T) {}
/// share(&once_cell::race::OnceBox::<S>::new());
/// ```
fn _dummy() {}
}

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mod unsync_once_cell;
#[cfg(any(feature = "std", feature = "critical-section"))]
mod sync_once_cell;
mod unsync_lazy;
#[cfg(any(feature = "std", feature = "critical-section"))]
mod sync_lazy;
#[cfg(feature = "race")]
mod race;
#[cfg(all(feature = "race", feature = "alloc"))]
mod race_once_box;

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#[cfg(feature = "std")]
use std::sync::Barrier;
use std::{
num::NonZeroUsize,
sync::atomic::{AtomicUsize, Ordering::SeqCst},
thread::scope,
};
use once_cell::race::{OnceBool, OnceNonZeroUsize};
#[test]
fn once_non_zero_usize_smoke_test() {
let cnt = AtomicUsize::new(0);
let cell = OnceNonZeroUsize::new();
let val = NonZeroUsize::new(92).unwrap();
scope(|s| {
s.spawn(|| {
assert_eq!(
cell.get_or_init(|| {
cnt.fetch_add(1, SeqCst);
val
}),
val
);
assert_eq!(cnt.load(SeqCst), 1);
assert_eq!(
cell.get_or_init(|| {
cnt.fetch_add(1, SeqCst);
val
}),
val
);
assert_eq!(cnt.load(SeqCst), 1);
});
});
assert_eq!(cell.get(), Some(val));
assert_eq!(cnt.load(SeqCst), 1);
}
#[test]
fn once_non_zero_usize_set() {
let val1 = NonZeroUsize::new(92).unwrap();
let val2 = NonZeroUsize::new(62).unwrap();
let cell = OnceNonZeroUsize::new();
assert!(cell.set(val1).is_ok());
assert_eq!(cell.get(), Some(val1));
assert!(cell.set(val2).is_err());
assert_eq!(cell.get(), Some(val1));
}
#[cfg(feature = "std")]
#[test]
fn once_non_zero_usize_first_wins() {
let val1 = NonZeroUsize::new(92).unwrap();
let val2 = NonZeroUsize::new(62).unwrap();
let cell = OnceNonZeroUsize::new();
let b1 = Barrier::new(2);
let b2 = Barrier::new(2);
let b3 = Barrier::new(2);
scope(|s| {
s.spawn(|| {
let r1 = cell.get_or_init(|| {
b1.wait();
b2.wait();
val1
});
assert_eq!(r1, val1);
b3.wait();
});
b1.wait();
s.spawn(|| {
let r2 = cell.get_or_init(|| {
b2.wait();
b3.wait();
val2
});
assert_eq!(r2, val1);
});
});
assert_eq!(cell.get(), Some(val1));
}
#[test]
fn once_bool_smoke_test() {
let cnt = AtomicUsize::new(0);
let cell = OnceBool::new();
scope(|s| {
s.spawn(|| {
assert_eq!(
cell.get_or_init(|| {
cnt.fetch_add(1, SeqCst);
false
}),
false
);
assert_eq!(cnt.load(SeqCst), 1);
assert_eq!(
cell.get_or_init(|| {
cnt.fetch_add(1, SeqCst);
false
}),
false
);
assert_eq!(cnt.load(SeqCst), 1);
});
});
assert_eq!(cell.get(), Some(false));
assert_eq!(cnt.load(SeqCst), 1);
}
#[test]
fn once_bool_set() {
let cell = OnceBool::new();
assert!(cell.set(false).is_ok());
assert_eq!(cell.get(), Some(false));
assert!(cell.set(true).is_err());
assert_eq!(cell.get(), Some(false));
}

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#[cfg(feature = "std")]
use std::sync::Barrier;
use std::sync::{
atomic::{AtomicUsize, Ordering::SeqCst},
Arc,
};
use once_cell::race::OnceBox;
#[derive(Default)]
struct Heap {
total: Arc<AtomicUsize>,
}
#[derive(Debug)]
struct Pebble<T> {
val: T,
total: Arc<AtomicUsize>,
}
impl<T> Drop for Pebble<T> {
fn drop(&mut self) {
self.total.fetch_sub(1, SeqCst);
}
}
impl Heap {
fn total(&self) -> usize {
self.total.load(SeqCst)
}
fn new_pebble<T>(&self, val: T) -> Pebble<T> {
self.total.fetch_add(1, SeqCst);
Pebble { val, total: Arc::clone(&self.total) }
}
}
#[cfg(feature = "std")]
#[test]
fn once_box_smoke_test() {
use std::thread::scope;
let heap = Heap::default();
let global_cnt = AtomicUsize::new(0);
let cell = OnceBox::new();
let b = Barrier::new(128);
scope(|s| {
for _ in 0..128 {
s.spawn(|| {
let local_cnt = AtomicUsize::new(0);
cell.get_or_init(|| {
global_cnt.fetch_add(1, SeqCst);
local_cnt.fetch_add(1, SeqCst);
b.wait();
Box::new(heap.new_pebble(()))
});
assert_eq!(local_cnt.load(SeqCst), 1);
cell.get_or_init(|| {
global_cnt.fetch_add(1, SeqCst);
local_cnt.fetch_add(1, SeqCst);
Box::new(heap.new_pebble(()))
});
assert_eq!(local_cnt.load(SeqCst), 1);
});
}
});
assert!(cell.get().is_some());
assert!(global_cnt.load(SeqCst) > 10);
assert_eq!(heap.total(), 1);
drop(cell);
assert_eq!(heap.total(), 0);
}
#[test]
fn once_box_set() {
let heap = Heap::default();
let cell = OnceBox::new();
assert!(cell.get().is_none());
assert!(cell.set(Box::new(heap.new_pebble("hello"))).is_ok());
assert_eq!(cell.get().unwrap().val, "hello");
assert_eq!(heap.total(), 1);
assert!(cell.set(Box::new(heap.new_pebble("world"))).is_err());
assert_eq!(cell.get().unwrap().val, "hello");
assert_eq!(heap.total(), 1);
drop(cell);
assert_eq!(heap.total(), 0);
}
#[cfg(feature = "std")]
#[test]
fn once_box_first_wins() {
use std::thread::scope;
let cell = OnceBox::new();
let val1 = 92;
let val2 = 62;
let b1 = Barrier::new(2);
let b2 = Barrier::new(2);
let b3 = Barrier::new(2);
scope(|s| {
s.spawn(|| {
let r1 = cell.get_or_init(|| {
b1.wait();
b2.wait();
Box::new(val1)
});
assert_eq!(*r1, val1);
b3.wait();
});
b1.wait();
s.spawn(|| {
let r2 = cell.get_or_init(|| {
b2.wait();
b3.wait();
Box::new(val2)
});
assert_eq!(*r2, val1);
});
});
assert_eq!(cell.get(), Some(&val1));
}
#[test]
fn once_box_reentrant() {
let cell = OnceBox::new();
let res = cell.get_or_init(|| {
cell.get_or_init(|| Box::new("hello".to_string()));
Box::new("world".to_string())
});
assert_eq!(res, "hello");
}
#[test]
fn once_box_default() {
struct Foo;
let cell: OnceBox<Foo> = Default::default();
assert!(cell.get().is_none());
}

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use std::{
cell::Cell,
sync::atomic::{AtomicUsize, Ordering::SeqCst},
thread::scope,
};
use once_cell::sync::{Lazy, OnceCell};
#[test]
fn lazy_new() {
let called = AtomicUsize::new(0);
let x = Lazy::new(|| {
called.fetch_add(1, SeqCst);
92
});
assert_eq!(called.load(SeqCst), 0);
scope(|s| {
s.spawn(|| {
let y = *x - 30;
assert_eq!(y, 62);
assert_eq!(called.load(SeqCst), 1);
});
});
let y = *x - 30;
assert_eq!(y, 62);
assert_eq!(called.load(SeqCst), 1);
}
#[test]
fn lazy_deref_mut() {
let called = AtomicUsize::new(0);
let mut x = Lazy::new(|| {
called.fetch_add(1, SeqCst);
92
});
assert_eq!(called.load(SeqCst), 0);
let y = *x - 30;
assert_eq!(y, 62);
assert_eq!(called.load(SeqCst), 1);
*x /= 2;
assert_eq!(*x, 46);
assert_eq!(called.load(SeqCst), 1);
}
#[test]
fn lazy_force_mut() {
let called = Cell::new(0);
let mut x = Lazy::new(|| {
called.set(called.get() + 1);
92
});
assert_eq!(called.get(), 0);
let v = Lazy::force_mut(&mut x);
assert_eq!(called.get(), 1);
*v /= 2;
assert_eq!(*x, 46);
assert_eq!(called.get(), 1);
}
#[test]
fn lazy_get_mut() {
let called = Cell::new(0);
let mut x: Lazy<u32, _> = Lazy::new(|| {
called.set(called.get() + 1);
92
});
assert_eq!(called.get(), 0);
assert_eq!(*x, 92);
let mut_ref: &mut u32 = Lazy::get_mut(&mut x).unwrap();
assert_eq!(called.get(), 1);
*mut_ref /= 2;
assert_eq!(*x, 46);
assert_eq!(called.get(), 1);
}
#[test]
fn lazy_default() {
static CALLED: AtomicUsize = AtomicUsize::new(0);
struct Foo(u8);
impl Default for Foo {
fn default() -> Self {
CALLED.fetch_add(1, SeqCst);
Foo(42)
}
}
let lazy: Lazy<std::sync::Mutex<Foo>> = <_>::default();
assert_eq!(CALLED.load(SeqCst), 0);
assert_eq!(lazy.lock().unwrap().0, 42);
assert_eq!(CALLED.load(SeqCst), 1);
lazy.lock().unwrap().0 = 21;
assert_eq!(lazy.lock().unwrap().0, 21);
assert_eq!(CALLED.load(SeqCst), 1);
}
#[test]
fn static_lazy() {
static XS: Lazy<Vec<i32>> = Lazy::new(|| {
let mut xs = Vec::new();
xs.push(1);
xs.push(2);
xs.push(3);
xs
});
scope(|s| {
s.spawn(|| {
assert_eq!(&*XS, &vec![1, 2, 3]);
});
});
assert_eq!(&*XS, &vec![1, 2, 3]);
}
#[test]
fn static_lazy_via_fn() {
fn xs() -> &'static Vec<i32> {
static XS: OnceCell<Vec<i32>> = OnceCell::new();
XS.get_or_init(|| {
let mut xs = Vec::new();
xs.push(1);
xs.push(2);
xs.push(3);
xs
})
}
assert_eq!(xs(), &vec![1, 2, 3]);
}
#[test]
fn lazy_into_value() {
let l: Lazy<i32, _> = Lazy::new(|| panic!());
assert!(matches!(Lazy::into_value(l), Err(_)));
let l = Lazy::new(|| -> i32 { 92 });
Lazy::force(&l);
assert!(matches!(Lazy::into_value(l), Ok(92)));
}
#[test]
fn lazy_poisoning() {
let x: Lazy<String> = Lazy::new(|| panic!("kaboom"));
for _ in 0..2 {
let res = std::panic::catch_unwind(|| x.len());
assert!(res.is_err());
}
}
#[test]
// https://github.com/rust-lang/rust/issues/34761#issuecomment-256320669
fn arrrrrrrrrrrrrrrrrrrrrr() {
let lazy: Lazy<&String, _>;
{
let s = String::new();
lazy = Lazy::new(|| &s);
_ = *lazy;
}
}
#[test]
fn lazy_is_sync_send() {
fn assert_traits<T: Send + Sync>() {}
assert_traits::<Lazy<String>>();
}

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use std::{
sync::atomic::{AtomicUsize, Ordering::SeqCst},
thread::scope,
};
#[cfg(feature = "std")]
use std::sync::Barrier;
#[cfg(not(feature = "std"))]
use core::cell::Cell;
use once_cell::sync::{Lazy, OnceCell};
#[test]
fn once_cell() {
let c = OnceCell::new();
assert!(c.get().is_none());
scope(|s| {
s.spawn(|| {
c.get_or_init(|| 92);
assert_eq!(c.get(), Some(&92));
});
});
c.get_or_init(|| panic!("Kabom!"));
assert_eq!(c.get(), Some(&92));
}
#[test]
fn once_cell_with_value() {
static CELL: OnceCell<i32> = OnceCell::with_value(12);
assert_eq!(CELL.get(), Some(&12));
}
#[test]
fn once_cell_get_mut() {
let mut c = OnceCell::new();
assert!(c.get_mut().is_none());
c.set(90).unwrap();
*c.get_mut().unwrap() += 2;
assert_eq!(c.get_mut(), Some(&mut 92));
}
#[test]
fn once_cell_get_unchecked() {
let c = OnceCell::new();
c.set(92).unwrap();
unsafe {
assert_eq!(c.get_unchecked(), &92);
}
}
#[test]
fn once_cell_drop() {
static DROP_CNT: AtomicUsize = AtomicUsize::new(0);
struct Dropper;
impl Drop for Dropper {
fn drop(&mut self) {
DROP_CNT.fetch_add(1, SeqCst);
}
}
let x = OnceCell::new();
scope(|s| {
s.spawn(|| {
x.get_or_init(|| Dropper);
assert_eq!(DROP_CNT.load(SeqCst), 0);
drop(x);
});
});
assert_eq!(DROP_CNT.load(SeqCst), 1);
}
#[test]
fn once_cell_drop_empty() {
let x = OnceCell::<String>::new();
drop(x);
}
#[test]
fn clone() {
let s = OnceCell::new();
let c = s.clone();
assert!(c.get().is_none());
s.set("hello".to_string()).unwrap();
let c = s.clone();
assert_eq!(c.get().map(String::as_str), Some("hello"));
}
#[test]
fn get_or_try_init() {
let cell: OnceCell<String> = OnceCell::new();
assert!(cell.get().is_none());
let res = std::panic::catch_unwind(|| cell.get_or_try_init(|| -> Result<_, ()> { panic!() }));
assert!(res.is_err());
assert!(cell.get().is_none());
assert_eq!(cell.get_or_try_init(|| Err(())), Err(()));
assert_eq!(cell.get_or_try_init(|| Ok::<_, ()>("hello".to_string())), Ok(&"hello".to_string()));
assert_eq!(cell.get(), Some(&"hello".to_string()));
}
#[cfg(feature = "std")]
#[test]
fn wait() {
let cell: OnceCell<String> = OnceCell::new();
scope(|s| {
s.spawn(|| cell.set("hello".to_string()));
let greeting = cell.wait();
assert_eq!(greeting, "hello")
});
}
#[cfg(feature = "std")]
#[test]
fn get_or_init_stress() {
let n_threads = if cfg!(miri) { 30 } else { 1_000 };
let n_cells = if cfg!(miri) { 30 } else { 1_000 };
let cells: Vec<_> = std::iter::repeat_with(|| (Barrier::new(n_threads), OnceCell::new()))
.take(n_cells)
.collect();
scope(|s| {
for t in 0..n_threads {
let cells = &cells;
s.spawn(move || {
for (i, (b, s)) in cells.iter().enumerate() {
b.wait();
let j = if t % 2 == 0 { s.wait() } else { s.get_or_init(|| i) };
assert_eq!(*j, i);
}
});
}
});
}
#[test]
fn from_impl() {
assert_eq!(OnceCell::from("value").get(), Some(&"value"));
assert_ne!(OnceCell::from("foo").get(), Some(&"bar"));
}
#[test]
fn partialeq_impl() {
assert!(OnceCell::from("value") == OnceCell::from("value"));
assert!(OnceCell::from("foo") != OnceCell::from("bar"));
assert!(OnceCell::<String>::new() == OnceCell::new());
assert!(OnceCell::<String>::new() != OnceCell::from("value".to_owned()));
}
#[test]
fn into_inner() {
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()));
}
#[test]
fn debug_impl() {
let cell = OnceCell::new();
assert_eq!(format!("{:#?}", cell), "OnceCell(Uninit)");
cell.set(vec!["hello", "world"]).unwrap();
assert_eq!(
format!("{:#?}", cell),
r#"OnceCell(
[
"hello",
"world",
],
)"#
);
}
#[test]
#[cfg_attr(miri, ignore)] // miri doesn't support processes
#[cfg(feature = "std")]
fn reentrant_init() {
let examples_dir = {
let mut exe = std::env::current_exe().unwrap();
exe.pop();
exe.pop();
exe.push("examples");
exe
};
let bin = examples_dir
.join("reentrant_init_deadlocks")
.with_extension(std::env::consts::EXE_EXTENSION);
let mut guard = Guard { child: std::process::Command::new(bin).spawn().unwrap() };
std::thread::sleep(std::time::Duration::from_secs(2));
let status = guard.child.try_wait().unwrap();
assert!(status.is_none());
struct Guard {
child: std::process::Child,
}
impl Drop for Guard {
fn drop(&mut self) {
let _ = self.child.kill();
}
}
}
#[cfg(not(feature = "std"))]
#[test]
#[should_panic(expected = "reentrant init")]
fn reentrant_init() {
let x: OnceCell<Box<i32>> = OnceCell::new();
let dangling_ref: Cell<Option<&i32>> = Cell::new(None);
x.get_or_init(|| {
let r = x.get_or_init(|| Box::new(92));
dangling_ref.set(Some(r));
Box::new(62)
});
eprintln!("use after free: {:?}", dangling_ref.get().unwrap());
}
#[test]
fn eval_once_macro() {
macro_rules! eval_once {
(|| -> $ty:ty {
$($body:tt)*
}) => {{
static ONCE_CELL: OnceCell<$ty> = OnceCell::new();
fn init() -> $ty {
$($body)*
}
ONCE_CELL.get_or_init(init)
}};
}
let fib: &'static Vec<i32> = eval_once! {
|| -> Vec<i32> {
let mut res = vec![1, 1];
for i in 0..10 {
let next = res[i] + res[i + 1];
res.push(next);
}
res
}
};
assert_eq!(fib[5], 8)
}
#[test]
fn once_cell_does_not_leak_partially_constructed_boxes() {
let n_tries = if cfg!(miri) { 10 } else { 100 };
let n_readers = 10;
let n_writers = 3;
const MSG: &str = "Hello, World";
for _ in 0..n_tries {
let cell: OnceCell<String> = OnceCell::new();
scope(|scope| {
for _ in 0..n_readers {
scope.spawn(|| loop {
if let Some(msg) = cell.get() {
assert_eq!(msg, MSG);
break;
}
});
}
for _ in 0..n_writers {
let _ = scope.spawn(|| cell.set(MSG.to_owned()));
}
});
}
}
#[cfg(feature = "std")]
#[test]
fn get_does_not_block() {
let cell = OnceCell::new();
let barrier = Barrier::new(2);
scope(|scope| {
scope.spawn(|| {
cell.get_or_init(|| {
barrier.wait();
barrier.wait();
"hello".to_string()
});
});
barrier.wait();
assert_eq!(cell.get(), None);
barrier.wait();
});
assert_eq!(cell.get(), Some(&"hello".to_string()));
}
#[test]
// https://github.com/rust-lang/rust/issues/34761#issuecomment-256320669
fn arrrrrrrrrrrrrrrrrrrrrr() {
let cell = OnceCell::new();
{
let s = String::new();
cell.set(&s).unwrap();
}
}
#[test]
fn once_cell_is_sync_send() {
fn assert_traits<T: Send + Sync>() {}
assert_traits::<OnceCell<String>>();
assert_traits::<Lazy<String>>();
}

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use core::{
cell::Cell,
sync::atomic::{AtomicUsize, Ordering::SeqCst},
};
use once_cell::unsync::Lazy;
#[test]
fn lazy_new() {
let called = Cell::new(0);
let x = Lazy::new(|| {
called.set(called.get() + 1);
92
});
assert_eq!(called.get(), 0);
let y = *x - 30;
assert_eq!(y, 62);
assert_eq!(called.get(), 1);
let y = *x - 30;
assert_eq!(y, 62);
assert_eq!(called.get(), 1);
}
#[test]
fn lazy_deref_mut() {
let called = Cell::new(0);
let mut x = Lazy::new(|| {
called.set(called.get() + 1);
92
});
assert_eq!(called.get(), 0);
let y = *x - 30;
assert_eq!(y, 62);
assert_eq!(called.get(), 1);
*x /= 2;
assert_eq!(*x, 46);
assert_eq!(called.get(), 1);
}
#[test]
fn lazy_force_mut() {
let called = Cell::new(0);
let mut x = Lazy::new(|| {
called.set(called.get() + 1);
92
});
assert_eq!(called.get(), 0);
let v = Lazy::force_mut(&mut x);
assert_eq!(called.get(), 1);
*v /= 2;
assert_eq!(*x, 46);
assert_eq!(called.get(), 1);
}
#[test]
fn lazy_get_mut() {
let called = Cell::new(0);
let mut x: Lazy<u32, _> = Lazy::new(|| {
called.set(called.get() + 1);
92
});
assert_eq!(called.get(), 0);
assert_eq!(*x, 92);
let mut_ref: &mut u32 = Lazy::get_mut(&mut x).unwrap();
assert_eq!(called.get(), 1);
*mut_ref /= 2;
assert_eq!(*x, 46);
assert_eq!(called.get(), 1);
}
#[test]
fn lazy_default() {
static CALLED: AtomicUsize = AtomicUsize::new(0);
struct Foo(u8);
impl Default for Foo {
fn default() -> Self {
CALLED.fetch_add(1, SeqCst);
Foo(42)
}
}
let lazy: Lazy<std::sync::Mutex<Foo>> = <_>::default();
assert_eq!(CALLED.load(SeqCst), 0);
assert_eq!(lazy.lock().unwrap().0, 42);
assert_eq!(CALLED.load(SeqCst), 1);
lazy.lock().unwrap().0 = 21;
assert_eq!(lazy.lock().unwrap().0, 21);
assert_eq!(CALLED.load(SeqCst), 1);
}
#[test]
fn lazy_into_value() {
let l: Lazy<i32, _> = Lazy::new(|| panic!());
assert!(matches!(Lazy::into_value(l), Err(_)));
let l = Lazy::new(|| -> i32 { 92 });
Lazy::force(&l);
assert!(matches!(Lazy::into_value(l), Ok(92)));
}
#[test]
#[cfg(feature = "std")]
fn lazy_poisoning() {
let x: Lazy<String> = Lazy::new(|| panic!("kaboom"));
for _ in 0..2 {
let res = std::panic::catch_unwind(|| x.len());
assert!(res.is_err());
}
}
#[test]
// https://github.com/rust-lang/rust/issues/34761#issuecomment-256320669
fn arrrrrrrrrrrrrrrrrrrrrr() {
let lazy: Lazy<&String, _>;
{
let s = String::new();
lazy = Lazy::new(|| &s);
_ = *lazy;
}
}

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use core::{
cell::Cell,
sync::atomic::{AtomicUsize, Ordering::SeqCst},
};
use once_cell::unsync::OnceCell;
#[test]
fn once_cell() {
let c = OnceCell::new();
assert!(c.get().is_none());
c.get_or_init(|| 92);
assert_eq!(c.get(), Some(&92));
c.get_or_init(|| panic!("Kabom!"));
assert_eq!(c.get(), Some(&92));
}
#[test]
fn once_cell_with_value() {
const CELL: OnceCell<i32> = OnceCell::with_value(12);
let cell = CELL;
assert_eq!(cell.get(), Some(&12));
}
#[test]
fn once_cell_get_mut() {
let mut c = OnceCell::new();
assert!(c.get_mut().is_none());
c.set(90).unwrap();
*c.get_mut().unwrap() += 2;
assert_eq!(c.get_mut(), Some(&mut 92));
}
#[test]
fn once_cell_drop() {
static DROP_CNT: AtomicUsize = AtomicUsize::new(0);
struct Dropper;
impl Drop for Dropper {
fn drop(&mut self) {
DROP_CNT.fetch_add(1, SeqCst);
}
}
let x = OnceCell::new();
x.get_or_init(|| Dropper);
assert_eq!(DROP_CNT.load(SeqCst), 0);
drop(x);
assert_eq!(DROP_CNT.load(SeqCst), 1);
}
#[test]
fn once_cell_drop_empty() {
let x = OnceCell::<String>::new();
drop(x);
}
#[test]
fn clone() {
let s = OnceCell::new();
let c = s.clone();
assert!(c.get().is_none());
s.set("hello".to_string()).unwrap();
let c = s.clone();
assert_eq!(c.get().map(String::as_str), Some("hello"));
}
#[test]
fn get_or_try_init() {
let cell: OnceCell<String> = OnceCell::new();
assert!(cell.get().is_none());
let res = std::panic::catch_unwind(|| cell.get_or_try_init(|| -> Result<_, ()> { panic!() }));
assert!(res.is_err());
assert!(cell.get().is_none());
assert_eq!(cell.get_or_try_init(|| Err(())), Err(()));
assert_eq!(cell.get_or_try_init(|| Ok::<_, ()>("hello".to_string())), Ok(&"hello".to_string()));
assert_eq!(cell.get(), Some(&"hello".to_string()));
}
#[test]
fn from_impl() {
assert_eq!(OnceCell::from("value").get(), Some(&"value"));
assert_ne!(OnceCell::from("foo").get(), Some(&"bar"));
}
#[test]
fn partialeq_impl() {
assert!(OnceCell::from("value") == OnceCell::from("value"));
assert!(OnceCell::from("foo") != OnceCell::from("bar"));
assert!(OnceCell::<String>::new() == OnceCell::new());
assert!(OnceCell::<String>::new() != OnceCell::from("value".to_owned()));
}
#[test]
fn into_inner() {
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()));
}
#[test]
fn debug_impl() {
let cell = OnceCell::new();
assert_eq!(format!("{:#?}", cell), "OnceCell(Uninit)");
cell.set(vec!["hello", "world"]).unwrap();
assert_eq!(
format!("{:#?}", cell),
r#"OnceCell(
[
"hello",
"world",
],
)"#
);
}
#[test]
#[should_panic(expected = "reentrant init")]
fn reentrant_init() {
let x: OnceCell<Box<i32>> = OnceCell::new();
let dangling_ref: Cell<Option<&i32>> = Cell::new(None);
x.get_or_init(|| {
let r = x.get_or_init(|| Box::new(92));
dangling_ref.set(Some(r));
Box::new(62)
});
eprintln!("use after free: {:?}", dangling_ref.get().unwrap());
}
#[test]
fn aliasing_in_get() {
let x = OnceCell::new();
x.set(42).unwrap();
let at_x = x.get().unwrap(); // --- (shared) borrow of inner `Option<T>` --+
let _ = x.set(27); // <-- temporary (unique) borrow of inner `Option<T>` |
println!("{}", at_x); // <------- up until here ---------------------------+
}
#[test]
// https://github.com/rust-lang/rust/issues/34761#issuecomment-256320669
fn arrrrrrrrrrrrrrrrrrrrrr() {
let cell = OnceCell::new();
{
let s = String::new();
cell.set(&s).unwrap();
}
}