1cc0ae4cbb
Add documentation to more `From::from` implementations. For users looking at documentation through IDE popups, this gives them relevant information rather than the generic trait documentation wording “Performs the conversion”. For users reading the documentation for a specific type for any reason, this informs them when the conversion may allocate or copy significant memory versus when it is always a move or cheap copy. Notes on specific cases: * The new documentation for `From<T> for T` explains that it is not a conversion at all. * Also documented `impl<T, U> Into<U> for T where U: From<T>`, the other central blanket implementation of conversion. * The new documentation for construction of maps and sets from arrays of keys mentions the handling of duplicates. Future work could be to do this for *all* code paths that convert an iterable to a map or set. * I did not add documentation to conversions of a specific error type to a more general error type. * I did not add documentation to unstable code. This change was prepared by searching for the text "From<... for" and so may have missed some cases that for whatever reason did not match. I also looked for `Into` impls but did not find any worth documenting by the above criteria.
390 lines
11 KiB
Rust
390 lines
11 KiB
Rust
//! Lazy values and one-time initialization of static data.
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use crate::cell::{Cell, UnsafeCell};
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use crate::fmt;
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use crate::mem;
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use crate::ops::Deref;
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/// A cell which can be written to only once.
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///
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/// Unlike `RefCell`, a `OnceCell` only provides shared `&T` references to its value.
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/// Unlike `Cell`, a `OnceCell` doesn't require copying or replacing the value to access it.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(once_cell)]
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///
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/// use std::lazy::OnceCell;
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///
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/// let cell = OnceCell::new();
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/// assert!(cell.get().is_none());
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///
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/// let value: &String = cell.get_or_init(|| {
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/// "Hello, World!".to_string()
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/// });
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/// assert_eq!(value, "Hello, World!");
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/// assert!(cell.get().is_some());
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/// ```
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#[unstable(feature = "once_cell", issue = "74465")]
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pub struct OnceCell<T> {
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// Invariant: written to at most once.
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inner: UnsafeCell<Option<T>>,
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}
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T> Default for OnceCell<T> {
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fn default() -> Self {
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Self::new()
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}
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}
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T: fmt::Debug> fmt::Debug for OnceCell<T> {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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match self.get() {
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Some(v) => f.debug_tuple("OnceCell").field(v).finish(),
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None => f.write_str("OnceCell(Uninit)"),
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}
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}
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}
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T: Clone> Clone for OnceCell<T> {
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fn clone(&self) -> OnceCell<T> {
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let res = OnceCell::new();
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if let Some(value) = self.get() {
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match res.set(value.clone()) {
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Ok(()) => (),
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Err(_) => unreachable!(),
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}
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}
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res
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}
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}
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T: PartialEq> PartialEq for OnceCell<T> {
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fn eq(&self, other: &Self) -> bool {
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self.get() == other.get()
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}
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}
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T: Eq> Eq for OnceCell<T> {}
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T> const From<T> for OnceCell<T> {
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/// Creates a new `OnceCell<T>` which already contains the given `value`.
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fn from(value: T) -> Self {
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OnceCell { inner: UnsafeCell::new(Some(value)) }
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}
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}
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impl<T> OnceCell<T> {
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/// Creates a new empty cell.
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#[unstable(feature = "once_cell", issue = "74465")]
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#[must_use]
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pub const fn new() -> OnceCell<T> {
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OnceCell { inner: UnsafeCell::new(None) }
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}
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/// Gets the reference to the underlying value.
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///
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/// Returns `None` if the cell is empty.
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#[unstable(feature = "once_cell", issue = "74465")]
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pub fn get(&self) -> Option<&T> {
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// SAFETY: Safe due to `inner`'s invariant
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unsafe { &*self.inner.get() }.as_ref()
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}
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/// Gets the mutable reference to the underlying value.
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///
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/// Returns `None` if the cell is empty.
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#[unstable(feature = "once_cell", issue = "74465")]
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pub fn get_mut(&mut self) -> Option<&mut T> {
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self.inner.get_mut().as_mut()
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}
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/// Sets the contents of the cell to `value`.
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///
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/// # Errors
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///
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/// This method returns `Ok(())` if the cell was empty and `Err(value)` if
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/// it was full.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(once_cell)]
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///
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/// use std::lazy::OnceCell;
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///
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/// let cell = OnceCell::new();
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/// assert!(cell.get().is_none());
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///
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/// assert_eq!(cell.set(92), Ok(()));
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/// assert_eq!(cell.set(62), Err(62));
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///
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/// assert!(cell.get().is_some());
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/// ```
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#[unstable(feature = "once_cell", issue = "74465")]
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pub fn set(&self, value: T) -> Result<(), T> {
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// SAFETY: Safe because we cannot have overlapping mutable borrows
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let slot = unsafe { &*self.inner.get() };
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if slot.is_some() {
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return Err(value);
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}
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// SAFETY: This is the only place where we set the slot, no races
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// due to reentrancy/concurrency are possible, and we've
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// checked that slot is currently `None`, so this write
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// maintains the `inner`'s invariant.
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let slot = unsafe { &mut *self.inner.get() };
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*slot = Some(value);
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Ok(())
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}
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/// Gets the contents of the cell, initializing it with `f`
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/// if the cell was empty.
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///
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/// # Panics
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///
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/// If `f` panics, the panic is propagated to the caller, and the cell
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/// remains uninitialized.
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///
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/// It is an error to reentrantly initialize the cell from `f`. Doing
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/// so results in a panic.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(once_cell)]
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///
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/// use std::lazy::OnceCell;
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///
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/// let cell = OnceCell::new();
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/// let value = cell.get_or_init(|| 92);
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/// assert_eq!(value, &92);
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/// let value = cell.get_or_init(|| unreachable!());
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/// assert_eq!(value, &92);
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/// ```
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#[unstable(feature = "once_cell", issue = "74465")]
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pub fn get_or_init<F>(&self, f: F) -> &T
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where
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F: FnOnce() -> T,
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{
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match self.get_or_try_init(|| Ok::<T, !>(f())) {
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Ok(val) => val,
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}
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}
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/// Gets the contents of the cell, initializing it with `f` if
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/// the cell was empty. If the cell was empty and `f` failed, an
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/// error is returned.
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///
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/// # Panics
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///
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/// If `f` panics, the panic is propagated to the caller, and the cell
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/// remains uninitialized.
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///
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/// It is an error to reentrantly initialize the cell from `f`. Doing
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/// so results in a panic.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(once_cell)]
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///
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/// use std::lazy::OnceCell;
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///
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/// let cell = OnceCell::new();
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/// assert_eq!(cell.get_or_try_init(|| Err(())), Err(()));
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/// assert!(cell.get().is_none());
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/// let value = cell.get_or_try_init(|| -> Result<i32, ()> {
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/// Ok(92)
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/// });
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/// assert_eq!(value, Ok(&92));
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/// assert_eq!(cell.get(), Some(&92))
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/// ```
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#[unstable(feature = "once_cell", issue = "74465")]
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pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
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where
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F: FnOnce() -> Result<T, E>,
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{
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if let Some(val) = self.get() {
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return Ok(val);
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}
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/// Avoid inlining the initialization closure into the common path that fetches
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/// the already initialized value
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#[cold]
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fn outlined_call<F, T, E>(f: F) -> Result<T, E>
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where
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F: FnOnce() -> Result<T, E>,
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{
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f()
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}
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let val = outlined_call(f)?;
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// Note that *some* forms of reentrant initialization might lead to
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// UB (see `reentrant_init` test). I believe that just removing this
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// `assert`, while keeping `set/get` would be sound, but it seems
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// better to panic, rather than to silently use an old value.
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assert!(self.set(val).is_ok(), "reentrant init");
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Ok(self.get().unwrap())
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}
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/// Consumes the cell, returning the wrapped value.
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///
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/// Returns `None` if the cell was empty.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(once_cell)]
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///
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/// use std::lazy::OnceCell;
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///
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/// let cell: OnceCell<String> = OnceCell::new();
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/// assert_eq!(cell.into_inner(), None);
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///
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/// let cell = OnceCell::new();
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/// cell.set("hello".to_string()).unwrap();
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/// assert_eq!(cell.into_inner(), Some("hello".to_string()));
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/// ```
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#[unstable(feature = "once_cell", issue = "74465")]
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pub fn into_inner(self) -> Option<T> {
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// Because `into_inner` takes `self` by value, the compiler statically verifies
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// that it is not currently borrowed. So it is safe to move out `Option<T>`.
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self.inner.into_inner()
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}
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/// Takes the value out of this `OnceCell`, moving it back to an uninitialized state.
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///
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/// Has no effect and returns `None` if the `OnceCell` hasn't been initialized.
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///
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/// Safety is guaranteed by requiring a mutable reference.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(once_cell)]
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///
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/// use std::lazy::OnceCell;
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///
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/// let mut cell: OnceCell<String> = OnceCell::new();
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/// assert_eq!(cell.take(), None);
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///
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/// let mut cell = OnceCell::new();
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/// cell.set("hello".to_string()).unwrap();
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/// assert_eq!(cell.take(), Some("hello".to_string()));
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/// assert_eq!(cell.get(), None);
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/// ```
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#[unstable(feature = "once_cell", issue = "74465")]
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pub fn take(&mut self) -> Option<T> {
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mem::take(self).into_inner()
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}
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}
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/// A value which is initialized on the first access.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(once_cell)]
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///
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/// use std::lazy::Lazy;
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///
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/// let lazy: Lazy<i32> = Lazy::new(|| {
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/// println!("initializing");
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/// 92
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/// });
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/// println!("ready");
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/// println!("{}", *lazy);
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/// println!("{}", *lazy);
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///
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/// // Prints:
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/// // ready
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/// // initializing
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/// // 92
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/// // 92
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/// ```
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#[unstable(feature = "once_cell", issue = "74465")]
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pub struct Lazy<T, F = fn() -> T> {
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cell: OnceCell<T>,
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init: Cell<Option<F>>,
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}
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T: fmt::Debug, F> fmt::Debug for Lazy<T, F> {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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f.debug_struct("Lazy").field("cell", &self.cell).field("init", &"..").finish()
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}
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}
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impl<T, F> Lazy<T, F> {
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/// Creates a new lazy value with the given initializing function.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(once_cell)]
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///
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/// # fn main() {
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/// use std::lazy::Lazy;
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///
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/// let hello = "Hello, World!".to_string();
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///
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/// let lazy = Lazy::new(|| hello.to_uppercase());
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///
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/// assert_eq!(&*lazy, "HELLO, WORLD!");
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/// # }
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/// ```
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#[unstable(feature = "once_cell", issue = "74465")]
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pub const fn new(init: F) -> Lazy<T, F> {
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Lazy { cell: OnceCell::new(), init: Cell::new(Some(init)) }
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}
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}
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impl<T, F: FnOnce() -> T> Lazy<T, F> {
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/// Forces the evaluation of this lazy value and returns a reference to
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/// the result.
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///
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/// This is equivalent to the `Deref` impl, but is explicit.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(once_cell)]
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///
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/// use std::lazy::Lazy;
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///
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/// let lazy = Lazy::new(|| 92);
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///
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/// assert_eq!(Lazy::force(&lazy), &92);
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/// assert_eq!(&*lazy, &92);
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/// ```
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#[unstable(feature = "once_cell", issue = "74465")]
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pub fn force(this: &Lazy<T, F>) -> &T {
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this.cell.get_or_init(|| match this.init.take() {
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Some(f) => f(),
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None => panic!("`Lazy` instance has previously been poisoned"),
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})
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}
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}
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T, F: FnOnce() -> T> Deref for Lazy<T, F> {
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type Target = T;
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fn deref(&self) -> &T {
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Lazy::force(self)
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}
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}
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#[unstable(feature = "once_cell", issue = "74465")]
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impl<T: Default> Default for Lazy<T> {
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/// Creates a new lazy value using `Default` as the initializing function.
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fn default() -> Lazy<T> {
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Lazy::new(T::default)
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}
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}
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