BREAKING: seal the Integer trait (#1227)

Note: this is a breaking change in a stable release, and hopefully the
last such change we make. As a result, the v0.7.1 release will be
yanked. We suspect this will not have an impact on users as it's
unlikely any downstream crates have impl'd the `Integer` trait.

Sealing the `Integer` trait allows us to make additive changes to its
bounds, e.g. #1226

Author: tarcieri

src/int.rs

@@ -1,16 +1,5 @@
 //! Stack-allocated big signed integers.
 
-use crate::{
-    Bounded, Choice, ConstOne, ConstZero, Constants, CtEq, CtOption, FixedInteger, Integer, Limb,
-    NonZero, Odd, One, Signed, Uint, Word, Zero,
-};
-use core::fmt;
-
-#[cfg(feature = "serde")]
-use crate::Encoding;
-#[cfg(feature = "serde")]
-use serdect::serde::{Deserialize, Deserializer, Serialize, Serializer};
-
 mod add;
 mod bit_and;
 mod bit_not;
@@ -39,6 +28,17 @@ pub(crate) mod types;
 #[cfg(feature = "rand_core")]
 mod rand;
 
+use crate::{
+    Bounded, Choice, ConstOne, ConstZero, Constants, CtEq, CtOption, FixedInteger, Integer, Limb,
+    NonZero, Odd, One, Signed, Uint, Word, Zero, sealed::Sealed,
+};
+use core::fmt;
+
+#[cfg(feature = "serde")]
+use crate::Encoding;
+#[cfg(feature = "serde")]
+use serdect::serde::{Deserialize, Deserializer, Serialize, Serializer};
+
 /// Stack-allocated big _signed_ integer.
 /// See [`Uint`] for _unsigned_ integers.
 ///
@@ -264,6 +264,8 @@ impl<const LIMBS: usize> Integer for Int<LIMBS> {
     }
 }
 
+impl<const LIMBS: usize> Sealed for Int<LIMBS> {}
+
 impl<const LIMBS: usize> Signed for Int<LIMBS> {
     type Unsigned = Uint<LIMBS>;
 

src/limb.rs

@@ -24,7 +24,7 @@ mod rand;
 
 use crate::{
     Bounded, Choice, ConstOne, ConstZero, Constants, CtEq, CtOption, Integer, NonZero, One,
-    UintRef, Unsigned, WideWord, Word, Zero, primitives::u32_bits, word,
+    UintRef, Unsigned, WideWord, Word, Zero, primitives::u32_bits, traits::sealed::Sealed, word,
 };
 use core::{fmt, ptr, slice};
 
@@ -260,6 +260,8 @@ impl Integer for Limb {
     }
 }
 
+impl Sealed for Limb {}
+
 impl Unsigned for Limb {
     #[inline]
     fn as_uint_ref(&self) -> &UintRef {

src/traits.rs

@@ -32,6 +32,7 @@ pub trait Bounded {
 /// Integer trait: represents common functionality of integer types provided by this crate.
 pub trait Integer:
     'static
+    + sealed::Sealed
     + Add<Output = Self>
     + for<'a> Add<&'a Self, Output = Self>
     + AddAssign<Self>
@@ -1286,6 +1287,12 @@ pub(crate) trait AmmMultiplier<'a>: MontyMultiplier<'a> {
     fn square_amm_assign(&mut self, a: &mut <Self::Monty as MontyForm>::Integer);
 }
 
+/// Support for sealing traits defined in this module.
+pub(crate) mod sealed {
+    /// Sealed trait.
+    pub trait Sealed {}
+}
+
 #[cfg(test)]
 pub(crate) mod tests {
     use super::{Integer, Signed, ToUnsigned, Unsigned};

src/uint.rs

@@ -8,7 +8,7 @@ pub(crate) use ref_type::UintRef;
 use crate::{
     Bounded, Choice, ConstOne, ConstZero, Constants, CtEq, CtOption, EncodedUint, FixedInteger,
     Int, Integer, Limb, NonZero, Odd, One, Unsigned, UnsignedWithMontyForm, Word, Zero,
-    limb::nlimbs, modular::FixedMontyForm, primitives::u32_bits,
+    limb::nlimbs, modular::FixedMontyForm, primitives::u32_bits, traits::sealed::Sealed,
 };
 use core::fmt;
 
@@ -344,6 +344,8 @@ impl<const LIMBS: usize> Integer for Uint<LIMBS> {
     }
 }
 
+impl<const LIMBS: usize> Sealed for Uint<LIMBS> {}
+
 impl<const LIMBS: usize> Unsigned for Uint<LIMBS> {
     fn as_uint_ref(&self) -> &UintRef {
         self.as_uint_ref()

src/uint/boxed.rs

@@ -31,7 +31,7 @@ mod rand;
 
 use crate::{
     Choice, CtAssign, CtEq, CtOption, Integer, Limb, NonZero, Odd, One, Resize, UintRef, Unsigned,
-    UnsignedWithMontyForm, Word, Zero, modular::BoxedMontyForm,
+    UnsignedWithMontyForm, Word, Zero, modular::BoxedMontyForm, traits::sealed::Sealed,
 };
 use alloc::{boxed::Box, vec, vec::Vec};
 use core::{
@@ -458,6 +458,8 @@ impl Integer for BoxedUint {
     }
 }
 
+impl Sealed for BoxedUint {}
+
 impl Unsigned for BoxedUint {
     fn as_uint_ref(&self) -> &UintRef {
         self.as_uint_ref()