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// Based on unstable std::sync::OnceLock.
//
use core::cell::UnsafeCell;
use core::mem::MaybeUninit;
use core::sync::atomic::{AtomicBool, Ordering};
use std::sync::Once;
pub(crate) struct OnceLock<T> {
once: Once,
// Once::is_completed requires Rust 1.43, so use this to track of whether they have been initialized.
is_initialized: AtomicBool,
value: UnsafeCell<MaybeUninit<T>>,
// Unlike std::sync::OnceLock, we don't need PhantomData here because
// we don't use #[may_dangle].
}
unsafe impl<T: Sync + Send> Sync for OnceLock<T> {}
unsafe impl<T: Send> Send for OnceLock<T> {}
impl<T> OnceLock<T> {
/// Creates a new empty cell.
#[must_use]
pub(crate) const fn new() -> Self {
Self {
once: Once::new(),
is_initialized: AtomicBool::new(false),
value: UnsafeCell::new(MaybeUninit::uninit()),
}
}
/// 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.
pub(crate) fn get_or_init<F>(&self, f: F) -> &T
where
F: FnOnce() -> T,
{
// Fast path check
if self.is_initialized() {
// SAFETY: The inner value has been initialized
return unsafe { self.get_unchecked() };
}
self.initialize(f);
debug_assert!(self.is_initialized());
// SAFETY: The inner value has been initialized
unsafe { self.get_unchecked() }
}
#[inline]
fn is_initialized(&self) -> bool {
self.is_initialized.load(Ordering::Acquire)
}
#[cold]
fn initialize<F>(&self, f: F)
where
F: FnOnce() -> T,
{
let slot = self.value.get().cast::<T>();
let is_initialized = &self.is_initialized;
self.once.call_once(|| {
let value = f();
unsafe {
slot.write(value);
}
is_initialized.store(true, Ordering::Release);
});
}
/// # Safety
///
/// The value must be initialized
unsafe fn get_unchecked(&self) -> &T {
debug_assert!(self.is_initialized());
&*self.value.get().cast::<T>()
}
}
impl<T> Drop for OnceLock<T> {
fn drop(&mut self) {
if self.is_initialized() {
// SAFETY: The inner value has been initialized
unsafe { self.value.get().cast::<T>().drop_in_place() };
}
}
}