primefield: add shr methods/trait impls to macro (#1159)

Updates `primefield::field_element_type` to add `shr` methods and trait
impls, replacing the copy/pasted definition to one provided by the macro

Author: tarcieri

bign256/src/arithmetic/scalar.rs

@@ -96,13 +96,6 @@ impl Scalar {
         ]);
         CtOption::new(sqrt, (sqrt * sqrt).ct_eq(self))
     }
-
-    /// Right shifts the scalar.
-    ///
-    /// Note: not constant-time with respect to the `shift` parameter.
-    pub const fn shr_vartime(&self, shift: u32) -> Scalar {
-        Self(self.0.wrapping_shr_vartime(shift))
-    }
 }
 
 impl AsRef<Scalar> for Scalar {

p384/src/arithmetic/scalar.rs

@@ -152,13 +152,6 @@ impl Scalar {
         }
         x
     }
-
-    /// Right shifts the scalar.
-    ///
-    /// Note: not constant-time with respect to the `shift` parameter.
-    pub const fn shr_vartime(&self, shift: u32) -> Scalar {
-        Self(self.0.wrapping_shr_vartime(shift))
-    }
 }
 
 impl AsRef<Scalar> for Scalar {

primefield/src/lib.rs

@@ -61,6 +61,8 @@ mod fiat;
 /// - `Mul`
 /// - `MulAssign`
 /// - `Neg`
+/// - `Shr`
+/// - `ShrAssign`
 #[macro_export]
 macro_rules! field_element_type {
     (
@@ -201,6 +203,22 @@ macro_rules! field_element_type {
 
                 res
             }
+
+            /// Right shifts the [`
+            #[doc = stringify!($fe)]
+            /// `].
+            pub const fn shr(&self, shift: u32) -> Self {
+                Self(self.0.wrapping_shr(shift))
+            }
+
+            /// Right shifts the [`
+            #[doc = stringify!($fe)]
+            /// `].
+            ///
+            /// Note: not constant-time with respect to the `shift` parameter.
+            pub const fn shr_vartime(&self, shift: u32) -> Self {
+                Self(self.0.wrapping_shr_vartime(shift))
+            }
         }
 
         impl $crate::ff::Field for $fe {
@@ -293,6 +311,31 @@ macro_rules! field_element_type {
             }
         }
 
+        impl ::core::ops::Shr<u32> for $fe {
+            type Output = Self;
+
+            #[inline]
+            fn shr(self, rhs: u32) -> Self {
+                Self::shr(&self, rhs)
+            }
+        }
+
+        impl ::core::ops::Shr<u32> for &$fe {
+            type Output = Self;
+
+            #[inline]
+            fn shr(self, rhs: u32) -> Self {
+                Self::shr(self, rhs)
+            }
+        }
+
+        impl ::core::ops::ShrAssign<u32> for $fe {
+            #[inline]
+            fn shr_assign(&mut self, rhs: u32) {
+                *self = Self::shr(self, rhs)
+            }
+        }
+
         impl Default for $fe {
             fn default() -> Self {
                 Self::ZERO