Make Field::double take &self and return Self

zcash_primitives/src/jubjub/edwards.rs

@@ -345,7 +345,7 @@ impl<E: JubjubEngine, Subgroup> Point<E, Subgroup> {
         // C = 2*Z1^2
         let mut c = self.z;
         c.square();
-        c.double();
+        c = c.double();
 
         // D = a*A
         //   = -A

zcash_primitives/src/jubjub/fs.rs

@@ -502,12 +502,16 @@ impl Field for Fs {
     }
 
     #[inline]
-    fn double(&mut self) {
+    fn double(&self) -> Self {
+        let mut ret = *self;
+
         // This cannot exceed the backing capacity.
-        self.0.mul2();
+        ret.0.mul2();
 
         // However, it may need to be reduced.
-        self.reduce();
+        ret.reduce();
+
+        ret
     }
 
     fn inverse(&self) -> Option<Self> {
@@ -680,7 +684,7 @@ impl Fs {
     fn mul_bits<S: AsRef<[u64]>>(&self, bits: BitIterator<S>) -> Self {
         let mut res = Self::zero();
         for bit in bits {
-            res.double();
+            res = res.double();
 
             if bit {
                 res.add_assign(self)
@@ -1466,11 +1470,8 @@ fn test_fs_double() {
 
     for _ in 0..1000 {
         // Ensure doubling a is equivalent to adding a to itself.
-        let mut a = Fs::random(&mut rng);
-        let mut b = a;
-        b.add_assign(&a);
-        a.double();
-        assert_eq!(a, b);
+        let a = Fs::random(&mut rng);
+        assert_eq!(a.double(), a + a);
     }
 }
 

zcash_primitives/src/jubjub/mod.rs

@@ -195,8 +195,7 @@ impl JubjubParams<Bls12> for JubjubBls12 {
 impl JubjubBls12 {
     pub fn new() -> Self {
         let montgomery_a = Fr::from_str("40962").unwrap();
-        let mut montgomery_2a = montgomery_a;
-        montgomery_2a.double();
+        let montgomery_2a = montgomery_a.double();
 
         let mut tmp_params = JubjubBls12 {
             // d = -(10240/10241)

zcash_primitives/src/jubjub/montgomery.rs

@@ -216,19 +216,18 @@ impl<E: JubjubEngine, Subgroup> Point<E, Subgroup> {
         {
             let mut tmp = *params.montgomery_a();
             tmp.mul_assign(&self.x);
-            tmp.double();
+            tmp = tmp.double();
             delta.add_assign(&tmp);
         }
         {
             let mut tmp = self.x;
             tmp.square();
             delta.add_assign(&tmp);
-            tmp.double();
+            tmp = tmp.double();
             delta.add_assign(&tmp);
         }
         {
-            let mut tmp = self.y;
-            tmp.double();
+            let tmp = self.y.double();
             delta.mul_assign(&tmp.inverse().expect("y is nonzero so this must be nonzero"));
         }
 

zcash_primitives/src/jubjub/tests.rs

@@ -314,10 +314,7 @@ fn test_jubjub_params<E: JubjubEngine>(params: &E::Params) {
 
     {
         // Check that 2A is consistent with A
-        let mut tmp = *params.montgomery_a();
-        tmp.double();
-
-        assert_eq!(&tmp, params.montgomery_2a());
+        assert_eq!(&params.montgomery_a().double(), params.montgomery_2a());
     }
 
     {

zcash_primitives/src/pedersen_hash.rs

@@ -58,7 +58,7 @@ where
             if a {
                 tmp.add_assign(&cur);
             }
-            cur.double(); // 2^1 * cur
+            cur = cur.double(); // 2^1 * cur
             if b {
                 tmp.add_assign(&cur);
             }
@@ -75,9 +75,7 @@ where
             if chunks_remaining == 0 {
                 break;
             } else {
-                cur.double(); // 2^2 * cur
-                cur.double(); // 2^3 * cur
-                cur.double(); // 2^4 * cur
+                cur = cur.double().double().double(); // 2^4 * cur
             }
         }