Merge pull request #303 from tarcieri/constant-traits

Add `ConstZero` and `ConstOne` traits

src/identities.rs

@@ -28,6 +28,13 @@ pub trait Zero: Sized + Add<Self, Output = Self> {
     fn is_zero(&self) -> bool;
 }
 
+/// Defines an associated constant representing the additive identity element
+/// for `Self`.
+pub trait ConstZero: Zero {
+    /// The additive identity element of `Self`, `0`.
+    const ZERO: Self;
+}
+
 macro_rules! zero_impl {
     ($t:ty, $v:expr) => {
         impl Zero for $t {
@@ -40,6 +47,10 @@ macro_rules! zero_impl {
                 *self == $v
             }
         }
+
+        impl ConstZero for $t {
+            const ZERO: Self = $v;
+        }
     };
 }
 
@@ -77,6 +88,13 @@ where
     }
 }
 
+impl<T: ConstZero> ConstZero for Wrapping<T>
+where
+    Wrapping<T>: Add<Output = Wrapping<T>>,
+{
+    const ZERO: Self = Wrapping(T::ZERO);
+}
+
 /// Defines a multiplicative identity element for `Self`.
 ///
 /// # Laws
@@ -115,6 +133,13 @@ pub trait One: Sized + Mul<Self, Output = Self> {
     }
 }
 
+/// Defines an associated constant representing the multiplicative identity
+/// element for `Self`.
+pub trait ConstOne: One {
+    /// The multiplicative identity element of `Self`, `1`.
+    const ONE: Self;
+}
+
 macro_rules! one_impl {
     ($t:ty, $v:expr) => {
         impl One for $t {
@@ -127,6 +152,10 @@ macro_rules! one_impl {
                 *self == $v
             }
         }
+
+        impl ConstOne for $t {
+            const ONE: Self = $v;
+        }
     };
 }
 
@@ -160,6 +189,13 @@ where
     }
 }
 
+impl<T: ConstOne> ConstOne for Wrapping<T>
+where
+    Wrapping<T>: Mul<Output = Wrapping<T>>,
+{
+    const ONE: Self = Wrapping(T::ONE);
+}
+
 // Some helper functions provided for backwards compatibility.
 
 /// Returns the additive identity, `0`.

src/lib.rs

@@ -33,7 +33,7 @@ pub use crate::float::Float;
 pub use crate::float::FloatConst;
 // pub use real::{FloatCore, Real}; // NOTE: Don't do this, it breaks `use num_traits::*;`.
 pub use crate::cast::{cast, AsPrimitive, FromPrimitive, NumCast, ToPrimitive};
-pub use crate::identities::{one, zero, One, Zero};
+pub use crate::identities::{one, zero, ConstOne, ConstZero, One, Zero};
 pub use crate::int::PrimInt;
 pub use crate::ops::bytes::{FromBytes, ToBytes};
 pub use crate::ops::checked::{