Revert "Replace black_box with std::hint::black_box"

This reverts commit 4978f42.

README.md

@@ -43,7 +43,7 @@ Documentation is available [here][docs].
 
 ## Minimum Supported Rust Version
 
-Rust **1.66** or higher.
+Rust **1.41** or higher.
 
 Minimum supported Rust version can be changed in the future, but it will be done with a minor version bump.
 

src/lib.rs

@@ -206,12 +206,39 @@ impl Not for Choice {
     }
 }
 
+/// This function is a best-effort attempt to prevent the compiler from knowing
+/// anything about the value of the returned `u8`, other than its type.
+///
+/// Because we want to support stable Rust, we don't have access to inline
+/// assembly or test::black_box, so we use the fact that volatile values will
+/// never be elided to register values.
+///
+/// Note: Rust's notion of "volatile" is subject to change over time. While this
+/// 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));
+
+    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)
+    }
+}
+
 impl From<u8> for Choice {
     #[inline]
     fn from(input: u8) -> Choice {
         // 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(core::hint::black_box(input))
+        Choice(black_box(input))
     }
 }