mirror of https://github.com/rust-lang/rust
199 lines
6.3 KiB
Rust
199 lines
6.3 KiB
Rust
use crate::ops::Deref;
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use crate::{fmt, mem};
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use super::UnsafeCell;
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enum State<T, F> {
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Uninit(F),
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Init(T),
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Poisoned,
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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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/// For a thread-safe version of this struct, see [`std::sync::LazyLock`].
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///
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/// [`std::sync::LazyLock`]: ../../std/sync/struct.LazyLock.html
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///
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/// # Examples
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///
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/// ```
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/// use std::cell::LazyCell;
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///
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/// let lazy: LazyCell<i32> = LazyCell::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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#[stable(feature = "lazy_cell", since = "CURRENT_RUSTC_VERSION")]
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pub struct LazyCell<T, F = fn() -> T> {
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state: UnsafeCell<State<T, F>>,
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}
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impl<T, F: FnOnce() -> T> LazyCell<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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/// use std::cell::LazyCell;
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///
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/// let hello = "Hello, World!".to_string();
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///
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/// let lazy = LazyCell::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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#[inline]
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#[stable(feature = "lazy_cell", since = "CURRENT_RUSTC_VERSION")]
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#[rustc_const_stable(feature = "lazy_cell", since = "CURRENT_RUSTC_VERSION")]
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pub const fn new(f: F) -> LazyCell<T, F> {
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LazyCell { state: UnsafeCell::new(State::Uninit(f)) }
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}
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/// Consumes this `LazyCell` returning the stored value.
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///
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/// Returns `Ok(value)` if `Lazy` is initialized and `Err(f)` otherwise.
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///
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/// # Examples
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///
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/// ```
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/// #![feature(lazy_cell_consume)]
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///
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/// use std::cell::LazyCell;
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///
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/// let hello = "Hello, World!".to_string();
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///
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/// let lazy = LazyCell::new(|| hello.to_uppercase());
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///
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/// assert_eq!(&*lazy, "HELLO, WORLD!");
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/// assert_eq!(LazyCell::into_inner(lazy).ok(), Some("HELLO, WORLD!".to_string()));
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/// ```
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#[unstable(feature = "lazy_cell_consume", issue = "125623")]
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pub fn into_inner(this: Self) -> Result<T, F> {
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match this.state.into_inner() {
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State::Init(data) => Ok(data),
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State::Uninit(f) => Err(f),
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State::Poisoned => panic!("LazyCell instance has previously been poisoned"),
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}
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}
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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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/// use std::cell::LazyCell;
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///
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/// let lazy = LazyCell::new(|| 92);
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///
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/// assert_eq!(LazyCell::force(&lazy), &92);
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/// assert_eq!(&*lazy, &92);
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/// ```
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#[inline]
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#[stable(feature = "lazy_cell", since = "CURRENT_RUSTC_VERSION")]
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pub fn force(this: &LazyCell<T, F>) -> &T {
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// SAFETY:
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// This invalidates any mutable references to the data. The resulting
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// reference lives either until the end of the borrow of `this` (in the
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// initialized case) or is invalidated in `really_init` (in the
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// uninitialized case; `really_init` will create and return a fresh reference).
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let state = unsafe { &*this.state.get() };
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match state {
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State::Init(data) => data,
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// SAFETY: The state is uninitialized.
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State::Uninit(_) => unsafe { LazyCell::really_init(this) },
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State::Poisoned => panic!("LazyCell has previously been poisoned"),
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}
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}
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/// # Safety
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/// May only be called when the state is `Uninit`.
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#[cold]
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unsafe fn really_init(this: &LazyCell<T, F>) -> &T {
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// SAFETY:
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// This function is only called when the state is uninitialized,
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// so no references to `state` can exist except for the reference
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// in `force`, which is invalidated here and not accessed again.
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let state = unsafe { &mut *this.state.get() };
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// Temporarily mark the state as poisoned. This prevents reentrant
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// accesses and correctly poisons the cell if the closure panicked.
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let State::Uninit(f) = mem::replace(state, State::Poisoned) else { unreachable!() };
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let data = f();
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// SAFETY:
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// If the closure accessed the cell through something like a reentrant
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// mutex, but caught the panic resulting from the state being poisoned,
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// the mutable borrow for `state` will be invalidated, so we need to
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// go through the `UnsafeCell` pointer here. The state can only be
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// poisoned at this point, so using `write` to skip the destructor
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// of `State` should help the optimizer.
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unsafe { this.state.get().write(State::Init(data)) };
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// SAFETY:
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// The previous references were invalidated by the `write` call above,
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// so do a new shared borrow of the state instead.
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let state = unsafe { &*this.state.get() };
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let State::Init(data) = state else { unreachable!() };
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data
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}
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}
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impl<T, F> LazyCell<T, F> {
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#[inline]
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fn get(&self) -> Option<&T> {
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// SAFETY:
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// This is sound for the same reason as in `force`: once the state is
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// initialized, it will not be mutably accessed again, so this reference
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// will stay valid for the duration of the borrow to `self`.
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let state = unsafe { &*self.state.get() };
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match state {
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State::Init(data) => Some(data),
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_ => None,
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}
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}
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}
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#[stable(feature = "lazy_cell", since = "CURRENT_RUSTC_VERSION")]
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impl<T, F: FnOnce() -> T> Deref for LazyCell<T, F> {
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type Target = T;
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#[inline]
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fn deref(&self) -> &T {
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LazyCell::force(self)
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}
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}
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#[stable(feature = "lazy_cell", since = "CURRENT_RUSTC_VERSION")]
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impl<T: Default> Default for LazyCell<T> {
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/// Creates a new lazy value using `Default` as the initializing function.
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#[inline]
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fn default() -> LazyCell<T> {
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LazyCell::new(T::default)
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}
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}
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#[stable(feature = "lazy_cell", since = "CURRENT_RUSTC_VERSION")]
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impl<T: fmt::Debug, F> fmt::Debug for LazyCell<T, F> {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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let mut d = f.debug_tuple("LazyCell");
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match self.get() {
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Some(data) => d.field(data),
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None => d.field(&format_args!("<uninit>")),
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};
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d.finish()
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}
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}
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