// Based on unstable std::sync::OnceLock.
//
// Source:
https://github.com/rust-lang/rust/blob/8e9c93df464b7ada3fc7a1c8ccddd9dcb24ee0a0/library/std/src/sync/once_lock.rs
use core::cell::UnsafeCell;
use core::mem::MaybeUninit;
use std::sync::Once;
pub(crate) struct OnceLock<T> {
once: Once,
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(),
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.once.is_completed() {
// SAFETY: The inner value has been initialized
return unsafe { self.get_unchecked() };
}
self.initialize(f);
// SAFETY: The inner value has been initialized
unsafe { self.get_unchecked() }
}
#[cold]
fn initialize<F>(&self, f: F)
where
F: FnOnce() -> T,
{
let slot = self.value.get();
self.once.call_once(|| {
let value = f();
unsafe { slot.write(MaybeUninit::new(value)) }
});
}
/// # Safety
///
/// The value must be initialized
unsafe fn get_unchecked(&self) -> &T {
debug_assert!(self.once.is_completed());
&*self.value.get().cast::<T>()
}
}
impl<T> Drop for OnceLock<T> {
fn drop(&mut self) {
if self.once.is_completed() {
// SAFETY: The inner value has been initialized
unsafe { (*self.value.get()).assume_init_drop() };
}
}
}
