1 files changed, 103 insertions, 0 deletions
diff --git a/src/sync/once_lock.rs b/src/sync/once_lock.rs
new file mode 100644
index 0000000..c1fefc9
--- /dev/null
+++ b/src/sync/once_lock.rs
@@ -0,0 +1,103 @@
+// 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 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() };
+        }
+    }
+}