ctutils: make Choice::{to_bool, to_u8} into const fn; MSRV 1.86 (#1547)

We have a stable release for MSRV 1.85 users of that version can rely
on, and 1.86 was released nearly a year and half ago.

We've also received reports of codegen problems in `const fn`s that can
only be addressed this way.

Author: tarcieri

.github/workflows/ctutils.yml

@@ -33,7 +33,7 @@ jobs:
     strategy:
       matrix:
         rust:
-          - 1.85.0 # MSRV
+          - 1.86.0 # MSRV
           - stable
         target:
           - thumbv7em-none-eabi
@@ -58,7 +58,7 @@ jobs:
     strategy:
       matrix:
         rust:
-          - 1.85.0 # MSRV
+          - 1.86.0 # MSRV
           - stable
     steps:
       - uses: actions/checkout@v7

ctutils/Cargo.toml

@@ -14,7 +14,7 @@ categories = ["cryptography", "no-std"]
 keywords = ["constant-time", "crypto", "intrinsics"]
 readme = "README.md"
 edition = "2024"
-rust-version = "1.85"
+rust-version = "1.86"
 
 [dependencies]
 cmov = "0.5.3"

ctutils/README.md

@@ -79,7 +79,7 @@ dual licensed as above, without any additional terms or conditions.
 [docs-image]: https://docs.rs/ctutils/badge.svg
 [docs-link]: https://docs.rs/ctutils/
 [license-image]: https://img.shields.io/badge/license-Apache2.0/MIT-blue.svg
-[msrv-image]: https://img.shields.io/badge/rustc-1.85+-blue.svg
+[msrv-image]: https://img.shields.io/badge/rustc-1.86+-blue.svg
 [build-image]: https://github.com/RustCrypto/utils/actions/workflows/ctutils.yml/badge.svg
 [build-link]: https://github.com/RustCrypto/utils/actions/workflows/ctutils.yml
 [chat-image]: https://img.shields.io/badge/zulip-join_chat-blue.svg

ctutils/src/choice.rs

@@ -386,44 +386,42 @@ impl Choice {
     /// the small amount of timing variability it introduces can potentially be exploited. Whenever
     /// possible, prefer fully constant-time approaches instead.
     /// </div>
-    // TODO(tarcieri): `const fn` when MSRV 1.86
     #[must_use]
-    pub fn to_bool(self) -> bool {
+    pub const fn to_bool(self) -> bool {
         self.to_u8() != 0
     }
 
     /// Convert [`Choice`] to a `u8`, attempting to apply a "best effort" optimization barrier.
-    // TODO(tarcieri): `const fn` when MSRV 1.86
     #[must_use]
-    pub fn to_u8(self) -> u8 {
+    pub const fn to_u8(self) -> u8 {
         // `black_box` is documented as working on a "best effort" basis. That's fine, this type is
         // likewise documented as only working on a "best effort" basis itself. The only way we
         // rely on `black_box` for correctness is it behaving as the identity function.
         core::hint::black_box(self.0)
     }
 
-    /// HACK: workaround to allow `const fn` boolean support on Rust 1.85.
+    /// DEPRECATED: previously a workaround to allow `const fn` boolean support on Rust 1.85.
     ///
-    /// This does not apply `black_box` to the output.
+    /// Use [`Choice::to_bool`] instead.
     ///
     /// <div class = "warning">
     /// <b>Security Warning</b>
     ///
     /// See the security warnings for [`Choice::to_bool`].
     /// </div>
-    // TODO(tarcieri): deprecate/remove this in favor of `to_bool` when MSRV is Rust 1.86
+    #[deprecated(since = "0.4.3", note = "use `Choice::to_bool` instead")]
     #[must_use]
     pub const fn to_bool_vartime(self) -> bool {
-        self.0 != 0
+        self.to_bool()
     }
 
-    /// HACK: workaround to allow `const fn` boolean support on Rust 1.85.
+    /// DEPRECATED: previously a workaround to allow `const fn` boolean support on Rust 1.85.
     ///
-    /// This does not apply `black_box` to the output.
-    // TODO(tarcieri): deprecate/remove this in favor of `to_u8` when MSRV is Rust 1.86
+    /// Use [`Choice::to_u8`] instead.
+    #[deprecated(since = "0.4.3", note = "use `Choice::to_u8` instead")]
     #[must_use]
     pub const fn to_u8_vartime(self) -> u8 {
-        self.0
+        self.to_u8()
     }
 
     /// Create a `u8` bitmask.

ctutils/src/ct_option.rs

@@ -158,8 +158,7 @@ impl<T> CtOption<T> {
     #[must_use]
     #[track_caller]
     pub const fn expect_ref(&self, msg: &str) -> &T {
-        // TODO(tarcieri): use `self.is_some().to_bool()` when MSRV is 1.86
-        assert!(self.is_some.to_bool_vartime(), "{}", msg);
+        assert!(self.is_some.to_bool(), "{}", msg);
         self.as_inner_unchecked()
     }
 
@@ -222,8 +221,7 @@ impl<T> CtOption<T> {
     where
         T: Copy,
     {
-        // TODO(tarcieri): use `self.is_some().to_bool()` when MSRV is 1.86
-        if self.is_some.to_bool_vartime() {
+        if self.is_some.to_bool() {
             Some(self.value)
         } else {
             None
@@ -693,7 +691,7 @@ mod tests {
         // Don't actually use this! It's just a test function implemented in variable-time
         #[allow(clippy::trivially_copy_pass_by_ref)]
         const fn select_vartime(a: &u8, b: &u8, choice: Choice) -> u8 {
-            if choice.to_bool_vartime() { *b } else { *a }
+            if choice.to_bool() { *b } else { *a }
         }
 
         assert_eq!(