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Diffstat (limited to 'src/libstd/lazy.rs')
| -rw-r--r-- | src/libstd/lazy.rs | 844 |
1 files changed, 844 insertions, 0 deletions
diff --git a/src/libstd/lazy.rs b/src/libstd/lazy.rs new file mode 100644 index 00000000000..86e1cfae582 --- /dev/null +++ b/src/libstd/lazy.rs @@ -0,0 +1,844 @@ +//! Lazy values and one-time initialization of static data. + +use crate::{ + cell::{Cell, UnsafeCell}, + fmt, + mem::{self, MaybeUninit}, + ops::{Deref, Drop}, + panic::{RefUnwindSafe, UnwindSafe}, + sync::Once, +}; + +#[doc(inline)] +#[unstable(feature = "once_cell", issue = "74465")] +pub use core::lazy::*; + +/// A synchronization primitive which can be written to only once. +/// +/// This type is a thread-safe `OnceCell`. +/// +/// # Examples +/// +/// ``` +/// #![feature(once_cell)] +/// +/// use std::lazy::SyncOnceCell; +/// +/// static CELL: SyncOnceCell<String> = SyncOnceCell::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 SyncOnceCell<T> { + once: Once, + // Whether or not the value is initialized is tracked by `state_and_queue`. + value: UnsafeCell<MaybeUninit<T>>, +} + +// Why do we need `T: Send`? +// Thread A creates a `SyncOnceCell` 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 SyncOnceCell<T> {} +#[unstable(feature = "once_cell", issue = "74465")] +unsafe impl<T: Send> Send for SyncOnceCell<T> {} + +#[unstable(feature = "once_cell", issue = "74465")] +impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for SyncOnceCell<T> {} +#[unstable(feature = "once_cell", issue = "74465")] +impl<T: UnwindSafe> UnwindSafe for SyncOnceCell<T> {} + +#[unstable(feature = "once_cell", issue = "74465")] +impl<T> Default for SyncOnceCell<T> { + fn default() -> SyncOnceCell<T> { + SyncOnceCell::new() + } +} + +#[unstable(feature = "once_cell", issue = "74465")] +impl<T: fmt::Debug> fmt::Debug for SyncOnceCell<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 SyncOnceCell<T> { + fn clone(&self) -> SyncOnceCell<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 SyncOnceCell<T> { + 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 SyncOnceCell<T> { + fn eq(&self, other: &SyncOnceCell<T>) -> bool { + self.get() == other.get() + } +} + +#[unstable(feature = "once_cell", issue = "74465")] +impl<T: Eq> Eq for SyncOnceCell<T> {} + +impl<T> SyncOnceCell<T> { + /// Creates a new empty cell. + #[unstable(feature = "once_cell", issue = "74465")] + pub const fn new() -> SyncOnceCell<T> { + SyncOnceCell { once: Once::new(), value: UnsafeCell::new(MaybeUninit::uninit()) } + } + + /// 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`. + /// + /// Returns `Ok(())` if the cell's value was updated. + /// + /// # Examples + /// + /// ``` + /// #![feature(once_cell)] + /// + /// use std::lazy::SyncOnceCell; + /// + /// static CELL: SyncOnceCell<i32> = SyncOnceCell::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::lazy::SyncOnceCell; + /// + /// let cell = SyncOnceCell::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::lazy::SyncOnceCell; + /// + /// let cell = SyncOnceCell::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 `SyncLazy::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() }) + } + + /// Consumes the `SyncOnceCell`, returning the wrapped value. Returns + /// `None` if the cell was empty. + /// + /// # Examples + /// + /// ``` + /// #![feature(once_cell)] + /// + /// use std::lazy::SyncOnceCell; + /// + /// let cell: SyncOnceCell<String> = SyncOnceCell::new(); + /// assert_eq!(cell.into_inner(), None); + /// + /// let cell = SyncOnceCell::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> { + // Safety: Safe because we immediately free `self` without dropping + let inner = unsafe { self.take_inner() }; + + // Don't drop this `SyncOnceCell`. We just moved out one of the fields, but didn't set + // the state to uninitialized. + mem::ManuallyDrop::new(self); + inner + } + + /// Takes the value out of this `SyncOnceCell`, moving it back to an uninitialized state. + /// + /// Has no effect and returns `None` if the `SyncOnceCell` hasn't been initialized. + /// + /// Safety is guaranteed by requiring a mutable reference. + /// + /// # Examples + /// + /// ``` + /// #![feature(once_cell)] + /// + /// use std::lazy::SyncOnceCell; + /// + /// let mut cell: SyncOnceCell<String> = SyncOnceCell::new(); + /// assert_eq!(cell.take(), None); + /// + /// let mut cell = SyncOnceCell::new(); + /// cell.set("hello".to_string()).unwrap(); + /// assert_eq!(cell.take(), Some("hello".to_string())); + /// assert_eq!(cell.get(), None); + /// ``` + #[unstable(feature = "once_cell", issue = "74465")] + pub fn take(&mut self) -> Option<T> { + mem::take(self).into_inner() + } + + /// Takes the wrapped value out of a `SyncOnceCell`. + /// Afterwards the cell is no longer initialized. + /// + /// Safety: The cell must now be free'd WITHOUT dropping. No other usages of the cell + /// are valid. Only used by `into_inner` and `drop`. + unsafe fn take_inner(&mut self) -> Option<T> { + // The mutable reference guarantees there are no other threads that can observe us + // taking out the wrapped value. + // Right after this function `self` is supposed to be freed, so it makes little sense + // to atomically set the state to uninitialized. + if self.is_initialized() { + let value = mem::replace(&mut self.value, UnsafeCell::new(MaybeUninit::uninit())); + Some(value.into_inner().assume_init()) + } 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()).get_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()).get_mut() + } +} + +impl<T> Drop for SyncOnceCell<T> { + fn drop(&mut self) { + // Safety: The cell is being dropped, so it can't be accessed again + unsafe { self.take_inner() }; + } +} + +/// A value which is initialized on the first access. +/// +/// This type is a thread-safe `Lazy`, and can be used in statics. +/// +/// # Examples +/// +/// ``` +/// #![feature(once_cell)] +/// +/// use std::collections::HashMap; +/// +/// use std::lazy::SyncLazy; +/// +/// static HASHMAP: SyncLazy<HashMap<i32, String>> = SyncLazy::new(|| { +/// println!("initializing"); +/// let mut m = HashMap::new(); +/// m.insert(13, "Spica".to_string()); +/// m.insert(74, "Hoyten".to_string()); +/// m +/// }); +/// +/// fn main() { +/// println!("ready"); +/// std::thread::spawn(|| { +/// println!("{:?}", HASHMAP.get(&13)); +/// }).join().unwrap(); +/// println!("{:?}", HASHMAP.get(&74)); +/// +/// // Prints: +/// // ready +/// // initializing +/// // Some("Spica") +/// // Some("Hoyten") +/// } +/// ``` +#[unstable(feature = "once_cell", issue = "74465")] +pub struct SyncLazy<T, F = fn() -> T> { + cell: SyncOnceCell<T>, + init: Cell<Option<F>>, +} + +#[unstable(feature = "once_cell", issue = "74465")] +impl<T: fmt::Debug, F> fmt::Debug for SyncLazy<T, F> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + f.debug_struct("Lazy").field("cell", &self.cell).field("init", &"..").finish() + } +} + +// We never create a `&F` from a `&SyncLazy<T, F>` so it is fine +// to not impl `Sync` for `F` +// we do create a `&mut Option<F>` in `force`, but this is +// properly synchronized, so it only happens once +// so it also does not contribute to this impl. +#[unstable(feature = "once_cell", issue = "74465")] +unsafe impl<T, F: Send> Sync for SyncLazy<T, F> where SyncOnceCell<T>: Sync {} +// auto-derived `Send` impl is OK. + +#[unstable(feature = "once_cell", issue = "74465")] +impl<T, F: RefUnwindSafe> RefUnwindSafe for SyncLazy<T, F> where SyncOnceCell<T>: RefUnwindSafe {} + +impl<T, F> SyncLazy<T, F> { + /// Creates a new lazy value with the given initializing + /// function. + #[unstable(feature = "once_cell", issue = "74465")] + pub const fn new(f: F) -> SyncLazy<T, F> { + SyncLazy { cell: SyncOnceCell::new(), init: Cell::new(Some(f)) } + } +} + +impl<T, F: FnOnce() -> T> SyncLazy<T, F> { + /// Forces the evaluation of this lazy value and + /// returns a reference to result. This is equivalent + /// to the `Deref` impl, but is explicit. + /// + /// # Examples + /// + /// ``` + /// #![feature(once_cell)] + /// + /// use std::lazy::SyncLazy; + /// + /// let lazy = SyncLazy::new(|| 92); + /// + /// assert_eq!(SyncLazy::force(&lazy), &92); + /// assert_eq!(&*lazy, &92); + /// ``` + #[unstable(feature = "once_cell", issue = "74465")] + pub fn force(this: &SyncLazy<T, F>) -> &T { + this.cell.get_or_init(|| match this.init.take() { + Some(f) => f(), + None => panic!("Lazy instance has previously been poisoned"), + }) + } +} + +#[unstable(feature = "once_cell", issue = "74465")] +impl<T, F: FnOnce() -> T> Deref for SyncLazy<T, F> { + type Target = T; + fn deref(&self) -> &T { + SyncLazy::force(self) + } +} + +#[unstable(feature = "once_cell", issue = "74465")] +impl<T: Default> Default for SyncLazy<T> { + /// Creates a new lazy value using `Default` as the initializing function. + fn default() -> SyncLazy<T> { + SyncLazy::new(T::default) + } +} + +#[cfg(test)] +mod tests { + use crate::{ + lazy::{Lazy, SyncLazy, SyncOnceCell}, + panic, + sync::{ + atomic::{AtomicUsize, Ordering::SeqCst}, + mpsc::channel, + Mutex, + }, + }; + + #[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<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_poisoning() { + let x: Lazy<String> = Lazy::new(|| panic!("kaboom")); + for _ in 0..2 { + let res = panic::catch_unwind(panic::AssertUnwindSafe(|| x.len())); + assert!(res.is_err()); + } + } + + // miri doesn't support threads + #[cfg(not(miri))] + fn spawn_and_wait<R: Send + 'static>(f: impl FnOnce() -> R + Send + 'static) -> R { + crate::thread::spawn(f).join().unwrap() + } + + #[cfg(not(miri))] + fn spawn(f: impl FnOnce() + Send + 'static) { + let _ = crate::thread::spawn(f); + } + + // "stub threads" for Miri + #[cfg(miri)] + fn spawn_and_wait<R: Send + 'static>(f: impl FnOnce() -> R + Send + 'static) -> R { + f(()) + } + + #[cfg(miri)] + fn spawn(f: impl FnOnce() + Send + 'static) { + f(()) + } + + #[test] + fn sync_once_cell() { + static ONCE_CELL: SyncOnceCell<i32> = SyncOnceCell::new(); + + assert!(ONCE_CELL.get().is_none()); + + spawn_and_wait(|| { + ONCE_CELL.get_or_init(|| 92); + assert_eq!(ONCE_CELL.get(), Some(&92)); + }); + + ONCE_CELL.get_or_init(|| panic!("Kabom!")); + assert_eq!(ONCE_CELL.get(), Some(&92)); + } + + #[test] + fn sync_once_cell_get_mut() { + let mut c = SyncOnceCell::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 sync_once_cell_get_unchecked() { + let c = SyncOnceCell::new(); + c.set(92).unwrap(); + unsafe { + assert_eq!(c.get_unchecked(), &92); + } + } + + #[test] + fn sync_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 = SyncOnceCell::new(); + spawn_and_wait(move || { + x.get_or_init(|| Dropper); + assert_eq!(DROP_CNT.load(SeqCst), 0); + drop(x); + }); + + assert_eq!(DROP_CNT.load(SeqCst), 1); + } + + #[test] + fn sync_once_cell_drop_empty() { + let x = SyncOnceCell::<String>::new(); + drop(x); + } + + #[test] + fn clone() { + let s = SyncOnceCell::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: SyncOnceCell<String> = SyncOnceCell::new(); + assert!(cell.get().is_none()); + + let res = panic::catch_unwind(|| cell.get_or_try_init(|| -> Result<_, ()> { panic!() })); + assert!(res.is_err()); + assert!(!cell.is_initialized()); + 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!(SyncOnceCell::from("value").get(), Some(&"value")); + assert_ne!(SyncOnceCell::from("foo").get(), Some(&"bar")); + } + + #[test] + fn partialeq_impl() { + assert!(SyncOnceCell::from("value") == SyncOnceCell::from("value")); + assert!(SyncOnceCell::from("foo") != SyncOnceCell::from("bar")); + + assert!(SyncOnceCell::<String>::new() == SyncOnceCell::new()); + assert!(SyncOnceCell::<String>::new() != SyncOnceCell::from("value".to_owned())); + } + + #[test] + fn into_inner() { + let cell: SyncOnceCell<String> = SyncOnceCell::new(); + assert_eq!(cell.into_inner(), None); + let cell = SyncOnceCell::new(); + cell.set("hello".to_string()).unwrap(); + assert_eq!(cell.into_inner(), Some("hello".to_string())); + } + + #[test] + fn sync_lazy_new() { + static CALLED: AtomicUsize = AtomicUsize::new(0); + static SYNC_LAZY: SyncLazy<i32> = SyncLazy::new(|| { + CALLED.fetch_add(1, SeqCst); + 92 + }); + + assert_eq!(CALLED.load(SeqCst), 0); + + spawn_and_wait(|| { + let y = *SYNC_LAZY - 30; + assert_eq!(y, 62); + assert_eq!(CALLED.load(SeqCst), 1); + }); + + let y = *SYNC_LAZY - 30; + assert_eq!(y, 62); + assert_eq!(CALLED.load(SeqCst), 1); + } + + #[test] + fn sync_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: SyncLazy<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] + #[cfg_attr(miri, ignore)] // leaks memory + fn static_sync_lazy() { + static XS: SyncLazy<Vec<i32>> = SyncLazy::new(|| { + let mut xs = Vec::new(); + xs.push(1); + xs.push(2); + xs.push(3); + xs + }); + + spawn_and_wait(|| { + assert_eq!(&*XS, &vec![1, 2, 3]); + }); + + assert_eq!(&*XS, &vec![1, 2, 3]); + } + + #[test] + #[cfg_attr(miri, ignore)] // leaks memory + fn static_sync_lazy_via_fn() { + fn xs() -> &'static Vec<i32> { + static XS: SyncOnceCell<Vec<i32>> = SyncOnceCell::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 sync_lazy_poisoning() { + let x: SyncLazy<String> = SyncLazy::new(|| panic!("kaboom")); + for _ in 0..2 { + let res = panic::catch_unwind(|| x.len()); + assert!(res.is_err()); + } + } + + #[test] + fn is_sync_send() { + fn assert_traits<T: Send + Sync>() {} + assert_traits::<SyncOnceCell<String>>(); + assert_traits::<SyncLazy<String>>(); + } + + #[test] + fn eval_once_macro() { + macro_rules! eval_once { + (|| -> $ty:ty { + $($body:tt)* + }) => {{ + static ONCE_CELL: SyncOnceCell<$ty> = SyncOnceCell::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] + #[cfg_attr(miri, ignore)] // deadlocks without real threads + fn sync_once_cell_does_not_leak_partially_constructed_boxes() { + static ONCE_CELL: SyncOnceCell<String> = SyncOnceCell::new(); + + let n_readers = 10; + let n_writers = 3; + const MSG: &str = "Hello, World"; + + let (tx, rx) = channel(); + + for _ in 0..n_readers { + let tx = tx.clone(); + spawn(move || { + loop { + if let Some(msg) = ONCE_CELL.get() { + tx.send(msg).unwrap(); + break; + } + } + }); + } + for _ in 0..n_writers { + spawn(move || { + let _ = ONCE_CELL.set(MSG.to_owned()); + }); + } + + for _ in 0..n_readers { + let msg = rx.recv().unwrap(); + assert_eq!(msg, MSG); + } + } +} |