ctutils: CtAssign <=> CtSelect delegating macros (#1366)

Adds/adapts and exports macros for writing impls of `CtSelect` based on
`CtAssign` and vice versa:

- `impl_ct_assign_with_ct_select!`
- `impl_ct_select_with_ct_assign!`

The latter already existed as an internal implementation detail of
`ct_select`, but this re-exports it and makes it more general by
changing it from working on `Copy` types to working on `Clone` types.

Author: tarcieri

ctutils/src/traits/ct_assign.rs

@@ -8,7 +8,27 @@ pub trait CtAssign {
     fn ct_assign(&mut self, other: &Self, choice: Choice);
 }
 
-// Impl `CtAssign` using the `cmov::Cmov` trait
+/// Impl `CtAssign` using the `CtSelect` trait.
+///
+/// In cases where `CtSelect` is more straightforward to implement, but you want to use a provided
+/// implementation of `CtAssign` based on it, you can use this macro to write it for you.
+#[macro_export]
+macro_rules! impl_ct_assign_with_ct_select {
+    ( $($ty:ty),+ ) => {
+        $(
+            impl CtAssign for $ty {
+                #[inline]
+                fn ct_assign(&mut self, other: &Self, choice: Choice) {
+                    *self = Self::ct_select(self, other, choice);
+                }
+            }
+        )+
+    };
+}
+
+impl_ct_assign_with_ct_select!(cmp::Ordering);
+
+/// Impl `CtAssign` using the `cmov::Cmov` trait
 macro_rules! impl_ct_assign_with_cmov {
     ( $($ty:ty),+ ) => {
         $(
@@ -56,13 +76,6 @@ impl CtAssign for usize {
     }
 }
 
-impl CtAssign for cmp::Ordering {
-    #[inline]
-    fn ct_assign(&mut self, other: &Self, choice: Choice) {
-        *self = Self::ct_select(self, other, choice);
-    }
-}
-
 impl<T> CtAssign for [T]
 where
     T: CtAssign,

ctutils/src/traits/ct_select.rs

@@ -21,14 +21,20 @@ pub trait CtSelect: CtAssign + Sized {
     }
 }
 
-// Impl `CtSelect` using the `CtAssign` trait, which in turn calls `cmov::Cmov`
+/// Impl `CtSelect` using the `CtAssign` trait.
+///
+/// In cases where `CtAssign` is more straightforward to implement, but you want to use a provided
+/// implementation of `CtSelect` based on it, you can use this macro to write it for you.
+///
+/// Requires the provided type(s) impl `Clone`.
+#[macro_export]
 macro_rules! impl_ct_select_with_ct_assign {
     ( $($ty:ty),+ ) => {
         $(
             impl CtSelect for $ty {
                 #[inline]
                 fn ct_select(&self, other: &Self, choice: Choice) -> Self {
-                    let mut ret = *self;
+                    let mut ret = self.clone();
                     ret.ct_assign(other, choice);
                     ret
                 }