Add BlackBox

Adds a `VolatileCell`-like struct which introduces an optimization
barrier on all accesses.

The `*Cell`-like `get()` method is the only accessor for the inner value
and uses a more generalized (i.e. for all `Copy` types) `black_box`
to preclude optimizations.

This is useful for things like bitwise mask values to ensure LLVM
doesn't try to optimize around special cases like `0`, especially in the
context of loops.

For an example use case, see:

https://rustsec.org/advisories/RUSTSEC-2024-0344.html

src/lib.rs

@@ -96,6 +96,9 @@ use core::cmp;
 use core::ops::{BitAnd, BitAndAssign, BitOr, BitOrAssign, BitXor, BitXorAssign, Neg, Not};
 use core::option::Option;
 
+#[cfg(feature = "core_hint_black_box")]
+use core::hint::black_box;
+
 /// The `Choice` struct represents a choice for use in conditional assignment.
 ///
 /// It is a wrapper around a `u8`, which should have the value either `1` (true)
@@ -217,25 +220,23 @@ impl Not for Choice {
 /// code may break in a non-destructive way in the future, “constant-time” code
 /// is a continually moving target, and this is better than doing nothing.
 #[inline(never)]
-fn black_box(input: u8) -> u8 {
-    debug_assert!((input == 0u8) | (input == 1u8));
-
+fn black_box<T: Copy>(input: T) -> T {
     unsafe {
         // Optimization barrier
         //
-        // Unsafe is ok, because:
-        //   - &input is not NULL;
-        //   - size of input is not zero;
-        //   - u8 is neither Sync, nor Send;
-        //   - u8 is Copy, so input is always live;
-        //   - u8 type is always properly aligned.
-        core::ptr::read_volatile(&input as *const u8)
+        // SAFETY:
+        //   - &input is not NULL because we own input;
+        //   - input is Copy and always live;
+        //   - input is always properly aligned.
+        core::ptr::read_volatile(&input)
     }
 }
 
 impl From<u8> for Choice {
     #[inline]
     fn from(input: u8) -> Choice {
+        debug_assert!((input == 0u8) | (input == 1u8));
+
         // Our goal is to prevent the compiler from inferring that the value held inside the
         // resulting `Choice` struct is really an `i1` instead of an `i8`.
         Choice(black_box(input))
@@ -986,3 +987,21 @@ impl ConstantTimeLess for cmp::Ordering {
         (a as u8).ct_lt(&(b as u8))
     }
 }
+
+/// Wrapper type which implements an optimization barrier for all accesses.
+#[derive(Clone, Copy, Debug)]
+pub struct BlackBox<T: Copy>(T);
+
+impl<T: Copy> BlackBox<T> {
+    /// Constructs a new instance of `BlackBox` which will wrap the specified value.
+    ///
+    /// All access to the inner value will be mediated by a `black_box` optimization barrier.
+    pub fn new(value: T) -> Self {
+        Self(value)
+    }
+
+    /// Read the inner value, applying an optimization barrier on access.
+    pub fn get(self) -> T {
+        black_box(self.0)
+    }
+}

tests/mod.rs

@@ -423,3 +423,10 @@ fn less_than_ordering() {
     assert_eq!(cmp::Ordering::Greater.ct_lt(&cmp::Ordering::Less).unwrap_u8(), 0);
     assert_eq!(cmp::Ordering::Less.ct_lt(&cmp::Ordering::Greater).unwrap_u8(), 1);
 }
+
+#[test]
+fn black_box_round_trip() {
+    let n = 42u64;
+    let black_box = BlackBox::new(n);
+    assert_eq!(n, black_box.get());
+}