1 files changed, 44 insertions, 0 deletions
diff --git a/third_party/rust/neqo-crypto/src/once.rs b/third_party/rust/neqo-crypto/src/once.rs
new file mode 100644
index 0000000000..80657cfe26
--- /dev/null
+++ b/third_party/rust/neqo-crypto/src/once.rs
@@ -0,0 +1,44 @@
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::sync::Once;
+
+#[allow(clippy::module_name_repetitions)]
+pub struct OnceResult<T> {
+    once: Once,
+    v: Option<T>,
+}
+
+impl<T> OnceResult<T> {
+    #[must_use]
+    pub const fn new() -> Self {
+        Self {
+            once: Once::new(),
+            v: None,
+        }
+    }
+
+    pub fn call_once<F: FnOnce() -> T>(&mut self, f: F) -> &T {
+        let v = &mut self.v;
+        self.once.call_once(|| {
+            *v = Some(f());
+        });
+        self.v.as_ref().unwrap()
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::OnceResult;
+
+    static mut STATIC_ONCE_RESULT: OnceResult<u64> = OnceResult::new();
+
+    #[test]
+    fn static_update() {
+        assert_eq!(*unsafe { STATIC_ONCE_RESULT.call_once(|| 23) }, 23);
+        assert_eq!(*unsafe { STATIC_ONCE_RESULT.call_once(|| 24) }, 23);
+    }
+}