1 files changed, 121 insertions, 0 deletions
diff --git a/library/std/src/sync/lazy_lock.rs b/library/std/src/sync/lazy_lock.rs
new file mode 100644
index 000000000..535cc1c42
--- /dev/null
+++ b/library/std/src/sync/lazy_lock.rs
@@ -0,0 +1,121 @@
+use crate::cell::Cell;
+use crate::fmt;
+use crate::ops::Deref;
+use crate::panic::{RefUnwindSafe, UnwindSafe};
+use crate::sync::OnceLock;
+
+/// A value which is initialized on the first access.
+///
+/// This type is a thread-safe `Lazy`, and can be used in statics.
+///
+/// # Examples
+///
+/// ```
+/// #![feature(once_cell)]
+///
+/// use std::collections::HashMap;
+///
+/// use std::sync::LazyLock;
+///
+/// static HASHMAP: LazyLock<HashMap<i32, String>> = LazyLock::new(|| {
+///     println!("initializing");
+///     let mut m = HashMap::new();
+///     m.insert(13, "Spica".to_string());
+///     m.insert(74, "Hoyten".to_string());
+///     m
+/// });
+///
+/// fn main() {
+///     println!("ready");
+///     std::thread::spawn(|| {
+///         println!("{:?}", HASHMAP.get(&13));
+///     }).join().unwrap();
+///     println!("{:?}", HASHMAP.get(&74));
+///
+///     // Prints:
+///     //   ready
+///     //   initializing
+///     //   Some("Spica")
+///     //   Some("Hoyten")
+/// }
+/// ```
+#[unstable(feature = "once_cell", issue = "74465")]
+pub struct LazyLock<T, F = fn() -> T> {
+    cell: OnceLock<T>,
+    init: Cell<Option<F>>,
+}
+
+impl<T, F> LazyLock<T, F> {
+    /// Creates a new lazy value with the given initializing
+    /// function.
+    #[unstable(feature = "once_cell", issue = "74465")]
+    pub const fn new(f: F) -> LazyLock<T, F> {
+        LazyLock { cell: OnceLock::new(), init: Cell::new(Some(f)) }
+    }
+}
+
+impl<T, F: FnOnce() -> T> LazyLock<T, F> {
+    /// Forces the evaluation of this lazy value and
+    /// returns a reference to result. This is equivalent
+    /// to the `Deref` impl, but is explicit.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(once_cell)]
+    ///
+    /// use std::sync::LazyLock;
+    ///
+    /// let lazy = LazyLock::new(|| 92);
+    ///
+    /// assert_eq!(LazyLock::force(&lazy), &92);
+    /// assert_eq!(&*lazy, &92);
+    /// ```
+    #[unstable(feature = "once_cell", issue = "74465")]
+    pub fn force(this: &LazyLock<T, F>) -> &T {
+        this.cell.get_or_init(|| match this.init.take() {
+            Some(f) => f(),
+            None => panic!("Lazy instance has previously been poisoned"),
+        })
+    }
+}
+
+#[unstable(feature = "once_cell", issue = "74465")]
+impl<T, F: FnOnce() -> T> Deref for LazyLock<T, F> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        LazyLock::force(self)
+    }
+}
+
+#[unstable(feature = "once_cell", issue = "74465")]
+impl<T: Default> Default for LazyLock<T> {
+    /// Creates a new lazy value using `Default` as the initializing function.
+    fn default() -> LazyLock<T> {
+        LazyLock::new(T::default)
+    }
+}
+
+#[unstable(feature = "once_cell", issue = "74465")]
+impl<T: fmt::Debug, F> fmt::Debug for LazyLock<T, F> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Lazy").field("cell", &self.cell).finish_non_exhaustive()
+    }
+}
+
+// We never create a `&F` from a `&LazyLock<T, F>` so it is fine
+// to not impl `Sync` for `F`
+// we do create a `&mut Option<F>` in `force`, but this is
+// properly synchronized, so it only happens once
+// so it also does not contribute to this impl.
+#[unstable(feature = "once_cell", issue = "74465")]
+unsafe impl<T, F: Send> Sync for LazyLock<T, F> where OnceLock<T>: Sync {}
+// auto-derived `Send` impl is OK.
+
+#[unstable(feature = "once_cell", issue = "74465")]
+impl<T, F: UnwindSafe> RefUnwindSafe for LazyLock<T, F> where OnceLock<T>: RefUnwindSafe {}
+#[unstable(feature = "once_cell", issue = "74465")]
+impl<T, F: UnwindSafe> UnwindSafe for LazyLock<T, F> where OnceLock<T>: UnwindSafe {}
+
+#[cfg(test)]
+mod tests;