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Diffstat (limited to 'library/core/src/lazy.rs')
| -rw-r--r-- | library/core/src/lazy.rs | 379 |
1 files changed, 379 insertions, 0 deletions
diff --git a/library/core/src/lazy.rs b/library/core/src/lazy.rs new file mode 100644 index 00000000000..5cf7217ef11 --- /dev/null +++ b/library/core/src/lazy.rs @@ -0,0 +1,379 @@ +//! Lazy values and one-time initialization of static data. + +use crate::cell::{Cell, UnsafeCell}; +use crate::fmt; +use crate::mem; +use crate::ops::Deref; + +/// A cell which can be written to only once. +/// +/// Unlike `RefCell`, a `OnceCell` only provides shared `&T` references to its value. +/// Unlike `Cell`, a `OnceCell` doesn't require copying or replacing the value to access it. +/// +/// # Examples +/// +/// ``` +/// #![feature(once_cell)] +/// +/// use std::lazy::OnceCell; +/// +/// let cell = OnceCell::new(); +/// assert!(cell.get().is_none()); +/// +/// let value: &String = cell.get_or_init(|| { +/// "Hello, World!".to_string() +/// }); +/// assert_eq!(value, "Hello, World!"); +/// assert!(cell.get().is_some()); +/// ``` +#[unstable(feature = "once_cell", issue = "74465")] +pub struct OnceCell<T> { + // Invariant: written to at most once. + inner: UnsafeCell<Option<T>>, +} + +#[unstable(feature = "once_cell", issue = "74465")] +impl<T> Default for OnceCell<T> { + fn default() -> Self { + Self::new() + } +} + +#[unstable(feature = "once_cell", issue = "74465")] +impl<T: fmt::Debug> fmt::Debug for OnceCell<T> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + match self.get() { + Some(v) => f.debug_tuple("OnceCell").field(v).finish(), + None => f.write_str("OnceCell(Uninit)"), + } + } +} + +#[unstable(feature = "once_cell", issue = "74465")] +impl<T: Clone> Clone for OnceCell<T> { + fn clone(&self) -> OnceCell<T> { + let res = OnceCell::new(); + if let Some(value) = self.get() { + match res.set(value.clone()) { + Ok(()) => (), + Err(_) => unreachable!(), + } + } + res + } +} + +#[unstable(feature = "once_cell", issue = "74465")] +impl<T: PartialEq> PartialEq for OnceCell<T> { + fn eq(&self, other: &Self) -> bool { + self.get() == other.get() + } +} + +#[unstable(feature = "once_cell", issue = "74465")] +impl<T: Eq> Eq for OnceCell<T> {} + +#[unstable(feature = "once_cell", issue = "74465")] +impl<T> From<T> for OnceCell<T> { + fn from(value: T) -> Self { + OnceCell { inner: UnsafeCell::new(Some(value)) } + } +} + +impl<T> OnceCell<T> { + /// Creates a new empty cell. + #[unstable(feature = "once_cell", issue = "74465")] + pub const fn new() -> OnceCell<T> { + OnceCell { inner: UnsafeCell::new(None) } + } + + /// Gets the reference to the underlying value. + /// + /// Returns `None` if the cell is empty. + #[unstable(feature = "once_cell", issue = "74465")] + pub fn get(&self) -> Option<&T> { + // Safety: Safe due to `inner`'s invariant + unsafe { &*self.inner.get() }.as_ref() + } + + /// Gets the mutable reference to the underlying value. + /// + /// Returns `None` if the cell is empty. + #[unstable(feature = "once_cell", issue = "74465")] + pub fn get_mut(&mut self) -> Option<&mut T> { + // Safety: Safe because we have unique access + unsafe { &mut *self.inner.get() }.as_mut() + } + + /// Sets the contents of the cell to `value`. + /// + /// # Errors + /// + /// This method returns `Ok(())` if the cell was empty and `Err(value)` if + /// it was full. + /// + /// # Examples + /// + /// ``` + /// #![feature(once_cell)] + /// + /// use std::lazy::OnceCell; + /// + /// let cell = OnceCell::new(); + /// assert!(cell.get().is_none()); + /// + /// assert_eq!(cell.set(92), Ok(())); + /// assert_eq!(cell.set(62), Err(62)); + /// + /// assert!(cell.get().is_some()); + /// ``` + #[unstable(feature = "once_cell", issue = "74465")] + pub fn set(&self, value: T) -> Result<(), T> { + // Safety: Safe because we cannot have overlapping mutable borrows + let slot = unsafe { &*self.inner.get() }; + if slot.is_some() { + return Err(value); + } + + // Safety: This is the only place where we set the slot, no races + // due to reentrancy/concurrency are possible, and we've + // checked that slot is currently `None`, so this write + // maintains the `inner`'s invariant. + let slot = unsafe { &mut *self.inner.get() }; + *slot = Some(value); + Ok(()) + } + + /// Gets the contents of the cell, initializing it with `f` + /// if the cell was empty. + /// + /// # 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`. Doing + /// so results in a panic. + /// + /// # Examples + /// + /// ``` + /// #![feature(once_cell)] + /// + /// use std::lazy::OnceCell; + /// + /// let cell = OnceCell::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`. Doing + /// so results in a panic. + /// + /// # Examples + /// + /// ``` + /// #![feature(once_cell)] + /// + /// use std::lazy::OnceCell; + /// + /// let cell = OnceCell::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>, + { + if let Some(val) = self.get() { + return Ok(val); + } + let val = f()?; + // Note that *some* forms of reentrant initialization might lead to + // UB (see `reentrant_init` test). I believe that just removing this + // `assert`, while keeping `set/get` would be sound, but it seems + // better to panic, rather than to silently use an old value. + assert!(self.set(val).is_ok(), "reentrant init"); + Ok(self.get().unwrap()) + } + + /// Consumes the cell, returning the wrapped value. + /// + /// Returns `None` if the cell was empty. + /// + /// # Examples + /// + /// ``` + /// #![feature(once_cell)] + /// + /// use std::lazy::OnceCell; + /// + /// 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())); + /// ``` + #[unstable(feature = "once_cell", issue = "74465")] + pub 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.inner.into_inner() + } + + /// Takes the value out of this `OnceCell`, moving it back to an uninitialized state. + /// + /// Has no effect and returns `None` if the `OnceCell` hasn't been initialized. + /// + /// Safety is guaranteed by requiring a mutable reference. + /// + /// # Examples + /// + /// ``` + /// #![feature(once_cell)] + /// + /// use std::lazy::OnceCell; + /// + /// let mut cell: OnceCell<String> = OnceCell::new(); + /// assert_eq!(cell.take(), None); + /// + /// let mut cell = OnceCell::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() + } +} + +/// A value which is initialized on the first access. +/// +/// # Examples +/// +/// ``` +/// #![feature(once_cell)] +/// +/// use std::lazy::Lazy; +/// +/// let lazy: Lazy<i32> = Lazy::new(|| { +/// println!("initializing"); +/// 92 +/// }); +/// println!("ready"); +/// println!("{}", *lazy); +/// println!("{}", *lazy); +/// +/// // Prints: +/// // ready +/// // initializing +/// // 92 +/// // 92 +/// ``` +#[unstable(feature = "once_cell", issue = "74465")] +pub struct Lazy<T, F = fn() -> T> { + cell: OnceCell<T>, + init: Cell<Option<F>>, +} + +#[unstable(feature = "once_cell", issue = "74465")] +impl<T: fmt::Debug, F> fmt::Debug for Lazy<T, F> { + fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { + f.debug_struct("Lazy").field("cell", &self.cell).field("init", &"..").finish() + } +} + +impl<T, F> Lazy<T, F> { + /// Creates a new lazy value with the given initializing function. + /// + /// # Examples + /// + /// ``` + /// #![feature(once_cell)] + /// + /// # fn main() { + /// use std::lazy::Lazy; + /// + /// let hello = "Hello, World!".to_string(); + /// + /// let lazy = Lazy::new(|| hello.to_uppercase()); + /// + /// assert_eq!(&*lazy, "HELLO, WORLD!"); + /// # } + /// ``` + #[unstable(feature = "once_cell", issue = "74465")] + pub const fn new(init: F) -> Lazy<T, F> { + Lazy { cell: OnceCell::new(), init: Cell::new(Some(init)) } + } +} + +impl<T, F: FnOnce() -> T> Lazy<T, F> { + /// Forces the evaluation of this lazy value and returns a reference to + /// the result. + /// + /// This is equivalent to the `Deref` impl, but is explicit. + /// + /// # Examples + /// + /// ``` + /// #![feature(once_cell)] + /// + /// use std::lazy::Lazy; + /// + /// let lazy = Lazy::new(|| 92); + /// + /// assert_eq!(Lazy::force(&lazy), &92); + /// assert_eq!(&*lazy, &92); + /// ``` + #[unstable(feature = "once_cell", issue = "74465")] + pub fn force(this: &Lazy<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 Lazy<T, F> { + type Target = T; + fn deref(&self) -> &T { + Lazy::force(self) + } +} + +#[unstable(feature = "once_cell", issue = "74465")] +impl<T: Default> Default for Lazy<T> { + /// Creates a new lazy value using `Default` as the initializing function. + fn default() -> Lazy<T> { + Lazy::new(T::default) + } +} |