Make Field::double take &self and return Self

zcash_proofs/src/circuit/ecc.rs

@@ -340,7 +340,7 @@ impl<E: JubjubEngine> EdwardsPoint<E> {
         // Compute x3 = (2.A) / (1 + C)
         let x3 = AllocatedNum::alloc(cs.namespace(|| "x3"), || {
             let mut t0 = *a.get_value().get()?;
-            t0.double();
+            t0 = t0.double();
 
             let mut t1 = E::Fr::one();
             t1.add_assign(c.get_value().get()?);
@@ -366,8 +366,7 @@ impl<E: JubjubEngine> EdwardsPoint<E> {
         // Compute y3 = (U - 2.A) / (1 - C)
         let y3 = AllocatedNum::alloc(cs.namespace(|| "y3"), || {
             let mut t0 = *a.get_value().get()?;
-            t0.double();
-            t0 = t0.neg();
+            t0 = t0.double().neg();
             t0.add_assign(t.get_value().get()?);
 
             let mut t1 = E::Fr::one();

zcash_proofs/src/circuit/sapling.rs

@@ -245,7 +245,7 @@ impl<'a, E: JubjubEngine> Circuit<E> for Spend<'a, E> {
             let mut coeff = E::Fr::one();
             for bit in &value_bits {
                 value_num = value_num.add_bool_with_coeff(CS::one(), bit, coeff);
-                coeff.double();
+                coeff = coeff.double();
             }
 
             // Place the value in the note

zcash_proofs/src/circuit/sprout/mod.rs

@@ -268,7 +268,7 @@ impl NoteValue {
         let mut coeff = E::Fr::one();
         for b in &self.bits {
             tmp = tmp + (coeff, b.get_variable());
-            coeff.double();
+            coeff = coeff.double();
         }
 
         tmp