fix subtraction

src/field/crandall_field.rs

@@ -268,7 +268,7 @@ impl Sub for CrandallField {
     #[inline]
     #[allow(clippy::suspicious_arithmetic_impl)]
     fn sub(self, rhs: Self) -> Self {
-        let (diff, under) = self.0.overflowing_sub(rhs.0);
+        let (diff, under) = self.0.overflowing_sub(rhs.to_canonical_u64());
         Self(diff.overflowing_add((under as u64) * Self::ORDER).0)
     }
 }

src/field/field_testing.rs

@@ -305,6 +305,16 @@ macro_rules! test_arithmetic {
                     }
                 }
             }
+
+            #[test]
+            fn subtraction() {
+                type F = $field;
+
+                let (a, b) = (F::from_canonical_u64((F::ORDER + 1) / 2), F::TWO);
+                let x = a * b;
+                assert_eq!(x, F::ONE);
+                assert_eq!(F::ZERO - x, F::NEG_ONE);
+            }
         }
     };
 }