1 files changed, 76 insertions, 0 deletions
diff --git a/src/lazy.rs b/src/lazy.rs
new file mode 100644
index 0000000..4f004e6
--- /dev/null
+++ b/src/lazy.rs
@@ -0,0 +1,76 @@
+#[cfg(feature = "std")]
+pub(crate) use once_cell::sync::Lazy;
+
+#[cfg(not(feature = "std"))]
+pub(crate) use self::spin::Lazy;
+
+#[cfg(not(feature = "std"))]
+mod spin {
+    //! This is the `once_cell::sync::Lazy` type, but modified to use our
+    //! `spin::Once` type rather than `OnceCell`. This is used to replace
+    //! `once_cell::sync::Lazy` on `no-std` builds.
+    use crate::spin::Once;
+    use core::{cell::Cell, fmt, ops::Deref};
+
+    /// Re-implementation of `once_cell::sync::Lazy` on top of `spin::Once`
+    /// rather than `OnceCell`.
+    ///
+    /// This is used when the standard library is disabled.
+    pub(crate) struct Lazy<T, F = fn() -> T> {
+        cell: Once<T>,
+        init: Cell<Option<F>>,
+    }
+
+    impl<T: fmt::Debug, F> fmt::Debug for Lazy<T, F> {
+        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+            f.debug_struct("Lazy")
+                .field("cell", &self.cell)
+                .field("init", &"..")
+                .finish()
+        }
+    }
+
+    // We never create a `&F` from a `&Lazy<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.
+    unsafe impl<T, F: Send> Sync for Lazy<T, F> where Once<T>: Sync {}
+    // auto-derived `Send` impl is OK.
+
+    impl<T, F> Lazy<T, F> {
+        /// Creates a new lazy value with the given initializing function.
+        pub(crate) const fn new(init: F) -> Lazy<T, F> {
+            Lazy {
+                cell: Once::new(),
+                init: Cell::new(Some(init)),
+            }
+        }
+    }
+
+    impl<T, F: FnOnce() -> T> Lazy<T, F> {
+        /// Forces the evaluation of this lazy value and returns a reference to
+        /// the result.
+        ///
+        /// This is equivalent to the `Deref` impl, but is explicit.
+        pub(crate) fn force(this: &Lazy<T, F>) -> &T {
+            this.cell.call_once(|| match this.init.take() {
+                Some(f) => f(),
+                None => panic!("Lazy instance has previously been poisoned"),
+            })
+        }
+    }
+
+    impl<T, F: FnOnce() -> T> Deref for Lazy<T, F> {
+        type Target = T;
+        fn deref(&self) -> &T {
+            Lazy::force(self)
+        }
+    }
+
+    impl<T: Default> Default for Lazy<T> {
+        /// Creates a new lazy value using `Default` as the initializing function.
+        fn default() -> Lazy<T> {
+            Lazy::new(T::default)
+        }
+    }
+}