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ctutils: add CtSelectUsingCtAssign #1391

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Merged
tarcieri merged 1 commit into from
Jan 18, 2026
Merged

ctutils: add CtSelectUsingCtAssign #1391

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2 changes: 1 addition & 1 deletion ctutils/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -110,5 +110,5 @@ pub use traits::{
ct_lookup::CtLookup,
ct_lt::CtLt,
ct_neg::CtNeg,
ct_select::{CtSelect, CtSelectArray},
ct_select::{CtSelect, CtSelectArray, CtSelectUsingCtAssign},
};
4 changes: 0 additions & 4 deletions ctutils/src/traits/ct_assign.rs
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Expand Up @@ -95,10 +95,6 @@ impl CtAssignSlice for isize {}
impl CtAssignSlice for usize {}

/// 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),+ ) => {
$(
Expand Down
73 changes: 22 additions & 51 deletions ctutils/src/traits/ct_select.rs
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Expand Up @@ -10,10 +10,10 @@ use core::{
#[cfg(feature = "subtle")]
use crate::CtOption;
#[cfg(feature = "alloc")]
use alloc::{boxed::Box, vec::Vec};

#[cfg(doc)]
use crate::CtAssignSlice;
use {
crate::CtAssignSlice,
alloc::{boxed::Box, vec::Vec},
};

/// Constant-time selection: choose between two values based on a given [`Choice`].
///
Expand Down Expand Up @@ -79,26 +79,22 @@ where
}
}

/// 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]
/// Marker trait which enables a blanket impl of [`CtSelect`] for types which also impl
/// [`Clone`] + [`CtAssign`].
pub trait CtSelectUsingCtAssign: Clone + CtAssign {}

impl<T: CtSelectUsingCtAssign> CtSelect for T {
#[inline]
fn ct_select(&self, other: &Self, choice: Choice) -> Self {
let mut ret = self.clone();
ret.ct_assign(other, choice);
ret
}
}

/// Macro to write impls of `CtSelectUsingCtAssign`.
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.clone();
ret.ct_assign(other, choice);
ret
}
}
)+
};
( $($ty:ty),+ ) => { $(impl CtSelectUsingCtAssign for $ty {})+ };
}

impl_ct_select_with_ct_assign!(
Expand Down Expand Up @@ -159,43 +155,18 @@ impl CtSelect for cmp::Ordering {
}

#[cfg(feature = "alloc")]
impl<T> CtSelect for Box<T>
where
T: CtSelect,
{
#[inline]
fn ct_select(&self, other: &Self, choice: Choice) -> Self {
Box::new(T::ct_select(&**self, &**other, choice))
}
}
impl<T: Clone + CtAssign> CtSelectUsingCtAssign for Box<T> {}

#[cfg(feature = "alloc")]
impl<T> CtSelect for Box<[T]>
impl<T> CtSelectUsingCtAssign for Box<[T]>
where
T: Clone,
[T]: CtAssign,
{
#[inline]
fn ct_select(&self, other: &Self, choice: Choice) -> Self {
let mut ret = self.clone();
ret.ct_assign(other, choice);
ret
}
}

#[cfg(feature = "alloc")]
impl<T> CtSelect for Vec<T>
where
T: Clone,
[T]: CtAssign,
{
#[inline]
fn ct_select(&self, other: &Self, choice: Choice) -> Self {
let mut ret = self.clone();
ret.ct_assign(other, choice);
ret
}
}
impl<T: Clone + CtAssignSlice> CtSelectUsingCtAssign for Vec<T> {}

#[cfg(feature = "subtle")]
impl CtSelect for subtle::Choice {
Expand Down