zeroize: mark optimization_barrier as public (#1261)

The function was added in #1252.

Author: newpavlov

zeroize/CHANGELOG.md

@@ -5,10 +5,14 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## 1.9.0 (unreleased)
+### Added
+- `optimization_barrier` function ([#1261])
+
 ### Changed
 - Edition changed to 2024 and MSRV bumped to 1.85 ([#1149])
 
 [#1149]: https://github.com/RustCrypto/utils/pull/1149
+[#1261]: https://github.com/RustCrypto/utils/pull/1261
 
 ## 1.8.2 (2025-09-29)
 ### Changed

zeroize/src/barrier.rs

@@ -11,12 +11,18 @@
 /// implemented using `#[inline(never)]` and `read_volatile`.
 ///
 /// # Examples
-/// ```ignore
+/// ```
 /// use core::num::NonZeroU32;
 /// use zeroize::{ZeroizeOnDrop, zeroize_flat_type};
 ///
+/// # type ThirdPartyType = u32;
+///
 /// struct DataToZeroize {
 ///     buf: [u8; 32],
+///     // `ThirdPartyType` can be a type with private fields
+///     // defined in a third-party crate and which does not implement
+///     // `Zeroize` or zeroization on drop.
+///     data: ThirdPartyType,
 ///     pos: NonZeroU32,
 /// }
 ///
@@ -25,6 +31,7 @@
 /// impl Drop for DataToZeroize {
 ///     fn drop(&mut self) {
 ///         self.buf = [0u8; 32];
+///         self.data = ThirdPartyType::default();
 ///         self.pos = NonZeroU32::new(32).unwrap();
 ///         zeroize::optimization_barrier(self);
 ///     }
@@ -34,12 +41,16 @@
 ///
 /// let mut data = DataToZeroize {
 ///     buf: [3u8; 32],
+///     data: ThirdPartyType::default(),
 ///     pos: NonZeroU32::new(32).unwrap(),
 /// };
 ///
 /// // data gets zeroized when dropped
 /// ```
-pub(crate) fn optimization_barrier<T: ?Sized>(val: &T) {
+///
+/// Note that erasure of `ThirdPartyType` demonstrated above can be fragile if it contains
+/// `MaybeUninit` or `union` data. It also does not perform erasure of types like `Box` or `Vec`.
+pub fn optimization_barrier<T: ?Sized>(val: &T) {
     #[cfg(all(
         not(miri),
         any(

zeroize/src/lib.rs

@@ -251,7 +251,7 @@ mod aarch64;
 mod x86;
 
 mod barrier;
-use barrier::optimization_barrier;
+pub use barrier::optimization_barrier;
 
 use core::{
     marker::{PhantomData, PhantomPinned},

zeroize/tests/alloc.rs

@@ -42,9 +42,32 @@ impl<S: Zeroize> Drop for SecretBox<S> {
 }
 
 #[test]
-fn proxy_alloc_test() {
+fn secret_box_alloc_test() {
     let b1 = SecretBox::new([u128::MAX; 10]);
     core::hint::black_box(&b1);
     let b2 = SecretBox::new([u8::MAX; 160]);
     core::hint::black_box(&b2);
 }
+
+struct ObserveSecretBox<S: Default>(Box<S>);
+
+impl<S: Default> ObserveSecretBox<S> {
+    fn new(val: S) -> Self {
+        Self(Box::new(val))
+    }
+}
+
+impl<S: Default> Drop for ObserveSecretBox<S> {
+    fn drop(&mut self) {
+        *self.0 = Default::default();
+        zeroize::optimization_barrier(&self);
+    }
+}
+
+#[test]
+fn observe_secret_box_alloc_test() {
+    let b1 = ObserveSecretBox::new([u128::MAX; 10]);
+    core::hint::black_box(&b1);
+    let b2 = SecretBox::new([u8::MAX; 160]);
+    core::hint::black_box(&b2);
+}