Move from Field::negate to Neg operator

ff/ff_derive/src/lib.rs

@@ -833,6 +833,21 @@ fn prime_field_impl(
             }
         }
 
+        impl ::std::ops::Neg for #name {
+            type Output = #name;
+
+            #[inline]
+            fn neg(self) -> #name {
+                let mut ret = self;
+                if !ret.is_zero() {
+                    let mut tmp = MODULUS;
+                    tmp.sub_noborrow(&ret.0);
+                    ret.0 = tmp;
+                }
+                ret
+            }
+        }
+
         impl<'r> ::std::ops::Add<&'r #name> for #name {
             type Output = #name;
 
@@ -1033,15 +1048,6 @@ fn prime_field_impl(
                 self.reduce();
             }
 
-            #[inline]
-            fn negate(&mut self) {
-                if !self.is_zero() {
-                    let mut tmp = MODULUS;
-                    tmp.sub_noborrow(&self.0);
-                    self.0 = tmp;
-                }
-            }
-
             fn inverse(&self) -> Option<Self> {
                 if self.is_zero() {
                     None

ff/src/lib.rs

@@ -11,7 +11,7 @@ use rand_core::RngCore;
 use std::error::Error;
 use std::fmt;
 use std::io::{self, Read, Write};
-use std::ops::{Add, AddAssign, Mul, MulAssign, Sub, SubAssign};
+use std::ops::{Add, AddAssign, Mul, MulAssign, Neg, Sub, SubAssign};
 
 /// This trait represents an element of a field.
 pub trait Field:
@@ -27,6 +27,7 @@ pub trait Field:
     + Add<Output = Self>
     + Sub<Output = Self>
     + Mul<Output = Self>
+    + Neg<Output = Self>
     + for<'a> Add<&'a Self, Output = Self>
     + for<'a> Mul<&'a Self, Output = Self>
     + for<'a> Sub<&'a Self, Output = Self>
@@ -55,9 +56,6 @@ pub trait Field:
     /// Doubles this element.
     fn double(&mut self);
 
-    /// Negates this element.
-    fn negate(&mut self);
-
     /// Computes the multiplicative inverse of this element, if nonzero.
     fn inverse(&self) -> Option<Self>;