Make Field::double take &self and return Self

bellman/src/gadgets/multipack.rs

@@ -20,7 +20,7 @@ where
         for bit in bits {
             num = num.add_bool_with_coeff(CS::one(), bit, coeff);
 
-            coeff.double();
+            coeff = coeff.double();
         }
 
         let input = cs.alloc_input(|| format!("input {}", i), || Ok(*num.get_value().get()?))?;
@@ -63,7 +63,7 @@ pub fn compute_multipacking<E: ScalarEngine>(bits: &[bool]) -> Vec<E::Fr> {
                 cur.add_assign(&coeff);
             }
 
-            coeff.double();
+            coeff = coeff.double();
         }
 
         result.push(cur);

bellman/src/gadgets/num.rs

@@ -177,7 +177,7 @@ impl<E: ScalarEngine> AllocatedNum<E> {
         for bit in result.iter().rev() {
             lc = lc + (coeff, bit.get_variable());
 
-            coeff.double();
+            coeff = coeff.double();
         }
 
         lc = lc - self.variable;
@@ -203,7 +203,7 @@ impl<E: ScalarEngine> AllocatedNum<E> {
         for bit in bits.iter() {
             lc = lc + (coeff, bit.get_variable());
 
-            coeff.double();
+            coeff = coeff.double();
         }
 
         lc = lc - self.variable;

bellman/src/gadgets/uint32.rs

@@ -330,7 +330,7 @@ impl UInt32 {
 
                 all_constants &= bit.is_constant();
 
-                coeff.double();
+                coeff = coeff.double();
             }
         }
 
@@ -368,7 +368,7 @@ impl UInt32 {
 
             max_value >>= 1;
             i += 1;
-            coeff.double();
+            coeff = coeff.double();
         }
 
         // Enforce equality between the sum and result

bellman/src/groth16/tests/dummy_engine.rs

@@ -144,8 +144,8 @@ impl Field for Fr {
         self.0 = (self.0 * self.0) % MODULUS_R;
     }
 
-    fn double(&mut self) {
-        self.0 = (self.0 << 1) % MODULUS_R;
+    fn double(&self) -> Self {
+        Fr((self.0 << 1) % MODULUS_R)
     }
 
     fn inverse(&self) -> Option<Self> {
@@ -406,7 +406,7 @@ impl CurveProjective for Fr {
     }
 
     fn double(&mut self) {
-        <Fr as Field>::double(self);
+        self.0 = <Fr as Field>::double(self).0;
     }
 
     fn add_assign(&mut self, other: &Self) {

ff/ff_derive/src/lib.rs

@@ -1040,12 +1040,16 @@ fn prime_field_impl(
             }
 
             #[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> {

ff/src/lib.rs

@@ -54,7 +54,8 @@ pub trait Field:
     fn square(&mut self);
 
     /// Doubles this element.
-    fn double(&mut self);
+    #[must_use]
+    fn double(&self) -> Self;
 
     /// Computes the multiplicative inverse of this element, if nonzero.
     fn inverse(&self) -> Option<Self>;

pairing/src/bls12_381/ec.rs

@@ -322,11 +322,10 @@ macro_rules! curve_impl {
                 d.square();
                 d.sub_assign(&a);
                 d.sub_assign(&c);
-                d.double();
+                d = d.double();
 
                 // E = 3*A
-                let mut e = a;
-                e.double();
+                let mut e = a.double();
                 e.add_assign(&a);
 
                 // F = E^2
@@ -335,7 +334,7 @@ macro_rules! curve_impl {
 
                 // Z3 = 2*Y1*Z1
                 self.z.mul_assign(&self.y);
-                self.z.double();
+                self.z = self.z.double();
 
                 // X3 = F-2*D
                 self.x = f;
@@ -346,9 +345,7 @@ macro_rules! curve_impl {
                 self.y = d;
                 self.y.sub_assign(&self.x);
                 self.y.mul_assign(&e);
-                c.double();
-                c.double();
-                c.double();
+                c = c.double().double().double();
                 self.y.sub_assign(&c);
             }
 
@@ -401,8 +398,7 @@ macro_rules! curve_impl {
                     h.sub_assign(&u1);
 
                     // I = (2*H)^2
-                    let mut i = h;
-                    i.double();
+                    let mut i = h.double();
                     i.square();
 
                     // J = H*I
@@ -412,7 +408,7 @@ macro_rules! curve_impl {
                     // r = 2*(S2-S1)
                     let mut r = s2;
                     r.sub_assign(&s1);
-                    r.double();
+                    r = r.double();
 
                     // V = U1*I
                     let mut v = u1;
@@ -430,7 +426,7 @@ macro_rules! curve_impl {
                     self.y.sub_assign(&self.x);
                     self.y.mul_assign(&r);
                     s1.mul_assign(&j); // S1 = S1 * J * 2
-                    s1.double();
+                    s1 = s1.double();
                     self.y.sub_assign(&s1);
 
                     // Z3 = ((Z1+Z2)^2 - Z1Z1 - Z2Z2)*H
@@ -484,9 +480,7 @@ macro_rules! curve_impl {
                     hh.square();
 
                     // I = 4*HH
-                    let mut i = hh;
-                    i.double();
-                    i.double();
+                    let i = hh.double().double();
 
                     // J = H*I
                     let mut j = h;
@@ -495,7 +489,7 @@ macro_rules! curve_impl {
                     // r = 2*(S2-Y1)
                     let mut r = s2;
                     r.sub_assign(&self.y);
-                    r.double();
+                    r = r.double();
 
                     // V = X1*I
                     let mut v = self.x;
@@ -510,7 +504,7 @@ macro_rules! curve_impl {
 
                     // Y3 = r*(V-X3)-2*Y1*J
                     j.mul_assign(&self.y); // J = 2*Y1*J
-                    j.double();
+                    j = j.double();
                     self.y = v;
                     self.y.sub_assign(&self.x);
                     self.y.mul_assign(&r);

pairing/src/bls12_381/fq.rs

@@ -2001,11 +2001,8 @@ fn test_fq_double() {
 
     for _ in 0..1000 {
         // Ensure doubling a is equivalent to adding a to itself.
-        let mut a = Fq::random(&mut rng);
-        let mut b = a;
-        b.add_assign(&a);
-        a.double();
-        assert_eq!(a, b);
+        let a = Fq::random(&mut rng);
+        assert_eq!(a.double(), a + a);
     }
 }
 

pairing/src/bls12_381/fq12.rs

@@ -183,9 +183,11 @@ impl Field for Fq12 {
         self.c0.is_zero() && self.c1.is_zero()
     }
 
-    fn double(&mut self) {
-        self.c0.double();
-        self.c1.double();
+    fn double(&self) -> Self {
+        Fq12 {
+            c0: self.c0.double(),
+            c1: self.c1.double(),
+        }
     }
 
     fn frobenius_map(&mut self, power: usize) {

pairing/src/bls12_381/fq2.rs

@@ -213,9 +213,11 @@ impl Field for Fq2 {
         self.c0 = c0;
     }
 
-    fn double(&mut self) {
-        self.c0.double();
-        self.c1.double();
+    fn double(&self) -> Self {
+        Fq2 {
+            c0: self.c0.double(),
+            c1: self.c1.double(),
+        }
     }
 
     fn inverse(&self) -> Option<Self> {
@@ -741,7 +743,7 @@ fn test_fq2_doubling() {
     use super::fq::FqRepr;
     use ff::PrimeField;
 
-    let mut a = Fq2 {
+    let a = Fq2 {
         c0: Fq::from_repr(FqRepr([
             0x2d0078036923ffc7,
             0x11e59ea221a3b6d2,
@@ -761,9 +763,8 @@ fn test_fq2_doubling() {
         ]))
         .unwrap(),
     };
-    a.double();
     assert_eq!(
-        a,
+        a.double(),
         Fq2 {
             c0: Fq::from_repr(FqRepr([
                 0x5a00f006d247ff8e,

pairing/src/bls12_381/fq6.rs

@@ -286,10 +286,12 @@ impl Field for Fq6 {
         self.c0.is_zero() && self.c1.is_zero() && self.c2.is_zero()
     }
 
-    fn double(&mut self) {
-        self.c0.double();
-        self.c1.double();
-        self.c2.double();
+    fn double(&self) -> Self {
+        Fq6 {
+            c0: self.c0.double(),
+            c1: self.c1.double(),
+            c2: self.c2.double(),
+        }
     }
 
     fn frobenius_map(&mut self, power: usize) {
@@ -306,16 +308,14 @@ impl Field for Fq6 {
         s0.square();
         let mut ab = self.c0;
         ab.mul_assign(&self.c1);
-        let mut s1 = ab;
-        s1.double();
+        let s1 = ab.double();
         let mut s2 = self.c0;
         s2.sub_assign(&self.c1);
         s2.add_assign(&self.c2);
         s2.square();
         let mut bc = self.c1;
         bc.mul_assign(&self.c2);
-        let mut s3 = bc;
-        s3.double();
+        let s3 = bc.double();
         let mut s4 = self.c2;
         s4.square();
 

pairing/src/bls12_381/fr.rs

@@ -760,11 +760,8 @@ fn test_fr_double() {
 
     for _ in 0..1000 {
         // Ensure doubling a is equivalent to adding a to itself.
-        let mut a = Fr::random(&mut rng);
-        let mut b = a;
-        b.add_assign(&a);
-        a.double();
-        assert_eq!(a, b);
+        let a = Fr::random(&mut rng);
+        assert_eq!(a.double(), a + a);
     }
 }
 

pairing/src/bls12_381/mod.rs

@@ -199,10 +199,9 @@ impl G2Prepared {
             tmp3.square();
             tmp3.sub_assign(&tmp0);
             tmp3.sub_assign(&tmp2);
-            tmp3.double();
+            tmp3 = tmp3.double();
 
-            let mut tmp4 = tmp0;
-            tmp4.double();
+            let mut tmp4 = tmp0.double();
             tmp4.add_assign(&tmp0);
 
             let mut tmp6 = r.x;
@@ -227,29 +226,25 @@ impl G2Prepared {
             r.y.sub_assign(&r.x);
             r.y.mul_assign(&tmp4);
 
-            tmp2.double();
-            tmp2.double();
-            tmp2.double();
+            tmp2 = tmp2.double().double().double();
 
             r.y.sub_assign(&tmp2);
 
             tmp3 = tmp4;
             tmp3.mul_assign(&zsquared);
-            tmp3.double();
-            tmp3 = tmp3.neg();
+            tmp3 = tmp3.double().neg();
 
             tmp6.square();
             tmp6.sub_assign(&tmp0);
             tmp6.sub_assign(&tmp5);
 
-            tmp1.double();
-            tmp1.double();
+            tmp1 = tmp1.double().double();
 
             tmp6.sub_assign(&tmp1);
 
             tmp0 = r.z;
             tmp0.mul_assign(&zsquared);
-            tmp0.double();
+            tmp0 = tmp0.double();
 
             (tmp0, tmp3, tmp6)
         }
@@ -278,9 +273,7 @@ impl G2Prepared {
             let mut t3 = t2;
             t3.square();
 
-            let mut t4 = t3;
-            t4.double();
-            t4.double();
+            let t4 = t3.double().double();
 
             let mut t5 = t4;
             t5.mul_assign(&t2);
@@ -315,7 +308,7 @@ impl G2Prepared {
 
             t0 = r.y;
             t0.mul_assign(&t5);
-            t0.double();
+            t0 = t0.double();
 
             r.y = t8;
             r.y.sub_assign(&t0);
@@ -328,16 +321,14 @@ impl G2Prepared {
 
             t10.sub_assign(&ztsquared);
 
-            t9.double();
+            t9 = t9.double();
             t9.sub_assign(&t10);
 
-            t10 = r.z;
-            t10.double();
+            t10 = r.z.double();
 
             t6 = t6.neg();
 
-            t1 = t6;
-            t1.double();
+            t1 = t6.double();
 
             (t10, t1, t9)
         }

pairing/src/tests/field.rs

@@ -211,12 +211,8 @@ fn random_negation_tests<F: Field, R: RngCore>(rng: &mut R) {
 
 fn random_doubling_tests<F: Field, R: RngCore>(rng: &mut R) {
     for _ in 0..10000 {
-        let mut a = F::random(rng);
-        let mut b = a;
-        a.add_assign(&b);
-        b.double();
-
-        assert_eq!(a, b);
+        let a = F::random(rng);
+        assert_eq!(a + a, a.double());
     }
 }
 

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
             }
         }
 

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