ctutils: add *Slice trait impls to Choice/CtOption (#1394)

Adds impls of `CtAssignSlice` and `CtEqSlice` to the `Choice` and
`CtOption` types.

Author: tarcieri

ctutils/src/choice.rs

@@ -1,6 +1,9 @@
-use crate::{CtAssign, CtEq, CtSelect};
+use crate::{CtAssign, CtAssignSlice, CtEq, CtEqSlice, CtSelectUsingCtAssign};
 use core::ops::{BitAnd, BitAndAssign, BitOr, BitOrAssign, BitXor, BitXorAssign, Not};
 
+#[cfg(feature = "subtle")]
+use crate::CtSelect;
+
 /// Bitwise less-than-or equal: returns `1` if `x <= y`, and otherwise returns `0`.
 ///
 /// See "Hacker's Delight" 2nd edition, section 2-12 (Comparison predicates)
@@ -529,20 +532,16 @@ impl CtAssign for Choice {
         self.0.ct_assign(&other.0, choice);
     }
 }
+impl CtAssignSlice for Choice {}
+impl CtSelectUsingCtAssign for Choice {}
 
 impl CtEq for Choice {
     #[inline]
     fn ct_eq(&self, other: &Self) -> Self {
         self.0.ct_eq(&other.0)
     }
 }
-
-impl CtSelect for Choice {
-    #[inline]
-    fn ct_select(&self, other: &Self, choice: Choice) -> Self {
-        Choice(self.0.ct_select(&other.0, choice))
-    }
-}
+impl CtEqSlice for Choice {}
 
 /// DEPRECATED: this exists to aid migrating code from `subtle`. Use `Choice::from_u8_lsb` instead.
 ///

ctutils/src/ct_option.rs

@@ -1,4 +1,4 @@
-use crate::{Choice, CtAssign, CtEq, CtSelect};
+use crate::{Choice, CtAssign, CtAssignSlice, CtEq, CtEqSlice, CtSelect};
 use core::ops::{Deref, DerefMut};
 
 /// Helper macro for providing behavior like the [`CtOption::map`] combinator that works in
@@ -565,6 +565,7 @@ impl<T: CtAssign> CtAssign for CtOption<T> {
         self.is_some.ct_assign(&other.is_some, choice);
     }
 }
+impl<T: CtAssign> CtAssignSlice for CtOption<T> {}
 
 impl<T: CtEq> CtEq for CtOption<T> {
     #[inline]
@@ -574,11 +575,14 @@ impl<T: CtEq> CtEq for CtOption<T> {
     }
 }
 
+impl<T: CtEq> CtEqSlice for CtOption<T> {}
+
 impl<T: CtSelect> CtSelect for CtOption<T> {
     fn ct_select(&self, other: &Self, choice: Choice) -> Self {
-        let value = self.value.ct_select(&other.value, choice);
-        let is_some = self.is_some.ct_select(&other.is_some, choice);
-        CtOption::new(value, is_some)
+        Self {
+            value: self.value.ct_select(&other.value, choice),
+            is_some: self.is_some.ct_select(&other.is_some, choice),
+        }
     }
 }