zeroize: add observe function

zeroize/src/lib.rs

@@ -850,6 +850,78 @@ pub unsafe fn zeroize_flat_type<F: Sized>(data: *mut F) {
     atomic_fence()
 }
 
+/// Observe the referenced data and prevent the compiler from removing previous writes to it.
+///
+/// This function acts like [`core::hint::black_box`] but takes a reference and
+/// does not return the passed value.
+///
+/// It's implemented using the [`core::arch::asm!`] macro on targets where `asm!` is stable,
+/// while on all other targets it's implemented using [`core::hint::black_box`].
+///
+/// # Examples
+/// ```
+/// use core::num::NonZeroU32;
+/// use zeroize::{ZeroizeOnDrop, zeroize_flat_type};
+///
+/// struct DataToZeroize {
+///     buf: [u8; 32],
+///     pos: NonZeroU32,
+/// }
+///
+/// struct SomeMoreFlatData(u64);
+///
+/// impl Drop for DataToZeroize {
+///     fn drop(&mut self) {
+///         self.buf = [0u8; 32];
+///         self.pos = NonZeroU32::new(32).unwrap();
+///         zeroize::observe(self);
+///     }
+/// }
+///
+/// impl zeroize::ZeroizeOnDrop for DataToZeroize {}
+///
+/// let mut data = DataToZeroize {
+///     buf: [3u8; 32],
+///     pos: NonZeroU32::new(32).unwrap(),
+/// };
+///
+/// // data gets zeroized when dropped
+/// ```
+pub fn observe<R: ?Sized>(val: &R) {
+    #[cfg(any(
+        target_arch = "aarch64",
+        target_arch = "arm",
+        target_arch = "arm64ec",
+        target_arch = "loongarch32",
+        target_arch = "loongarch64",
+        target_arch = "riscv32",
+        target_arch = "riscv64",
+        target_arch = "s390x",
+        target_arch = "x86",
+        target_arch = "x86_64",
+    ))]
+    unsafe {
+        core::arch::asm!(
+            "# {}",
+            in(reg) val as *const R as *const (),
+            options(readonly, preserves_flags, nostack),
+        );
+    }
+    #[cfg(not(any(
+        target_arch = "aarch64",
+        target_arch = "arm",
+        target_arch = "arm64ec",
+        target_arch = "loongarch32",
+        target_arch = "loongarch64",
+        target_arch = "riscv32",
+        target_arch = "riscv64",
+        target_arch = "s390x",
+        target_arch = "x86",
+        target_arch = "x86_64",
+    )))]
+    core::hint::black_box(val);
+}
+
 /// Internal module used as support for `AssertZeroizeOnDrop`.
 #[doc(hidden)]
 pub mod __internal {