Revert new black_box optimization barrier

.travis.yml

@@ -26,10 +26,8 @@ script:
   - cargo test && cargo test --no-default-features &&
     cargo test --no-default-features --features std &&
     cargo test --no-default-features --features "std i128" &&
-    cargo test --no-default-features --features "std core_hint_black_box" &&
     cargo test --no-default-features --features "std const-generics" &&
-    cargo test --no-default-features --features "std i128 core_hint_black_box" &&
-    cargo test --no-default-features --features "std i128 core_hint_black_box const-generics"
+    cargo test --no-default-features --features "std i128 const-generics"
 
 notifications:
   slack:

Cargo.toml

@@ -30,6 +30,7 @@ rand = { version = "0.8" }
 
 [features]
 const-generics = []
+# DEPRECATED: As of 2.5.1, this feature does nothing.
 core_hint_black_box = []
 default = ["std", "i128"]
 std = []

README.md

@@ -26,10 +26,6 @@ prevent this refinement, the crate tries to hide the value of a `Choice`'s
 inner `u8` by passing it through a volatile read. For more information, see
 the _About_ section below.
 
-Rust versions from 1.66 or higher support a new best-effort optimization
-barrier ([`core::hint::black_box`]).  To use the new optimization barrier,
-enable the `core_hint_black_box` feature.
-
 Rust versions from 1.51 or higher have const generics support. You may enable
 `const-generics` feautre to have `subtle` traits implemented for arrays `[T; N]`.
 
@@ -59,11 +55,8 @@ Old versions of the optimization barrier in `impl From<u8> for Choice` were
 based on Tim Maclean's [work on `rust-timing-shield`][rust-timing-shield],
 which attempts to provide a more comprehensive approach for preventing
 software side-channels in Rust code.
-
 From version `2.2`, it was based on Diane Hosfelt and Amber Sprenkels' work on
-"Secret Types in Rust".  Version `2.5` adds the `core_hint_black_box` feature,
-which uses the original method through the [`core::hint::black_box`] function
-from the Rust standard library.
+"Secret Types in Rust".
 
 `subtle` is authored by isis agora lovecruft and Henry de Valence.
 
@@ -78,5 +71,4 @@ effort is fundamentally limited.
 **USE AT YOUR OWN RISK**
 
 [docs]: https://docs.rs/subtle
-[`core::hint::black_box`]: https://doc.rust-lang.org/core/hint/fn.black_box.html
 [rust-timing-shield]: https://www.chosenplaintext.ca/open-source/rust-timing-shield/security

src/lib.rs

@@ -41,10 +41,6 @@
 //! inner `u8` by passing it through a volatile read. For more information, see
 //! the _About_ section below.
 //!
-//! Rust versions from 1.66 or higher support a new best-effort optimization
-//! barrier ([`core::hint::black_box`]).  To use the new optimization barrier,
-//! enable the `core_hint_black_box` feature.
-//!
 //! Rust versions from 1.51 or higher have const generics support. You may enable
 //! `const-generics` feautre to have `subtle` traits implemented for arrays `[T; N]`.
 //!
@@ -74,11 +70,8 @@
 //! based on Tim Maclean's [work on `rust-timing-shield`][rust-timing-shield],
 //! which attempts to provide a more comprehensive approach for preventing
 //! software side-channels in Rust code.
-//!
 //! From version `2.2`, it was based on Diane Hosfelt and Amber Sprenkels' work on
-//! "Secret Types in Rust". Version `2.5` adds the `core_hint_black_box` feature,
-//! which uses the original method through the [`core::hint::black_box`] function
-//! from the Rust standard library.
+//! "Secret Types in Rust".
 //!
 //! `subtle` is authored by isis agora lovecruft and Henry de Valence.
 //!
@@ -93,7 +86,6 @@
 //! **USE AT YOUR OWN RISK**
 //!
 //! [docs]: https://docs.rs/subtle
-//! [`core::hint::black_box`]: https://doc.rust-lang.org/core/hint/fn.black_box.html
 //! [rust-timing-shield]: https://www.chosenplaintext.ca/open-source/rust-timing-shield/security
 
 #[cfg(feature = "std")]
@@ -224,7 +216,6 @@ impl Not for Choice {
 /// 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.
-#[cfg(not(feature = "core_hint_black_box"))]
 #[inline(never)]
 fn black_box(input: u8) -> u8 {
     debug_assert!((input == 0u8) | (input == 1u8));
@@ -242,13 +233,6 @@ fn black_box(input: u8) -> u8 {
     }
 }
 
-#[cfg(feature = "core_hint_black_box")]
-#[inline(never)]
-fn black_box(input: u8) -> u8 {
-    debug_assert!((input == 0u8) | (input == 1u8));
-    core::hint::black_box(input)
-}
-
 impl From<u8> for Choice {
     #[inline]
     fn from(input: u8) -> Choice {