`R1CSVar`: `Option::None` -> `ConstraintSystemRef::None` (#281)

crypto-primitives/src/merkle_tree/constraints.rs

@@ -37,7 +37,7 @@ where
         // proof.
         let leaf_bits = leaf.to_bytes()?;
         let leaf_hash = CRHGadget::evaluate(parameters, &leaf_bits)?;
-        let cs = leaf_hash.cs().or(root.cs()).unwrap();
+        let cs = leaf_hash.cs().or(root.cs());
 
         // Check if leaf is one of the bottom-most siblings.
         let leaf_is_left = Boolean::new_witness(r1cs_core::ns!(cs, "leaf_is_left"), || {

crypto-primitives/src/prf/blake2s/constraints.rs

@@ -350,7 +350,7 @@ impl AllocVar<[u8; 32], ConstraintF> for OutputVar
 impl<F: PrimeField> R1CSVar<F> for OutputVar<F> {
     type Value = [u8; 32];
 
-    fn cs(&self) -> Option<ConstraintSystemRef<F>> {
+    fn cs(&self) -> ConstraintSystemRef<F> {
         self.0.cs()
     }
 

r1cs-std/src/bits/boolean.rs

@@ -232,10 +232,10 @@ pub enum Boolean {
 impl<F: Field> R1CSVar<F> for Boolean<F> {
     type Value = bool;
 
-    fn cs(&self) -> Option<ConstraintSystemRef<F>> {
+    fn cs(&self) -> ConstraintSystemRef<F> {
         match self {
-            Self::Is(a) | Self::Not(a) => Some(a.cs.clone()),
-            _ => None,
+            Self::Is(a) | Self::Not(a) => a.cs.clone(),
+            _ => ConstraintSystemRef::None,
         }
     }
 
@@ -598,11 +598,10 @@ impl Boolean {
         match r {
             Constant(true) => Ok(()),
             Constant(false) => Err(SynthesisError::AssignmentMissing),
-            Is(_) | Not(_) => r.cs().unwrap().enforce_constraint(
-                r.lc(),
-                lc!() + Variable::One,
-                lc!() + Variable::One,
-            ),
+            Is(_) | Not(_) => {
+                r.cs()
+                    .enforce_constraint(r.lc(), lc!() + Variable::One, lc!() + Variable::One)
+            }
         }
     }
 
@@ -778,7 +777,7 @@ impl EqGadget for Boolean {
         };
 
         if condition != &Constant(false) {
-            let cs = self.cs().or(other.cs()).or(condition.cs()).unwrap();
+            let cs = self.cs().or(other.cs()).or(condition.cs());
             cs.enforce_constraint(lc!() + difference, condition.lc(), lc!())?;
         }
         Ok(())
@@ -814,11 +813,7 @@ impl EqGadget for Boolean {
         };
 
         if should_enforce != &Constant(false) {
-            let cs = self
-                .cs()
-                .or(other.cs())
-                .or(should_enforce.cs())
-                .ok_or(SynthesisError::UnconstrainedVariable)?;
+            let cs = self.cs().or(other.cs()).or(should_enforce.cs());
             cs.enforce_constraint(difference, should_enforce.lc(), should_enforce.lc())?;
         }
         Ok(())
@@ -863,7 +858,7 @@ impl CondSelectGadget for Boolean {
                 (&Constant(true), x) => cond.or(x),
                 (x, &Constant(true)) => cond.not().or(x),
                 (a, b) => {
-                    let cs = cond.cs().unwrap();
+                    let cs = cond.cs();
                     let result: Boolean<F> =
                         AllocatedBit::new_witness_without_booleanity_check(cs.clone(), || {
                             let cond = cond.value()?;

r1cs-std/src/bits/uint.rs

@@ -38,7 +38,7 @@ macro_rules! make_uint {
             impl<F: Field> R1CSVar<F> for $name<F> {
                 type Value = $native;
 
-                fn cs(&self) -> Option<ConstraintSystemRef<F>> {
+                fn cs(&self) -> ConstraintSystemRef<F> {
                     self.bits.as_slice().cs()
                 }
 
@@ -254,7 +254,7 @@ macro_rules! make_uint {
 
                         return Ok($name::constant(modular_value.unwrap()));
                     }
-                    let cs = operands.cs().unwrap();
+                    let cs = operands.cs();
 
                     // Storage area for the resulting bits
                     let mut result_bits = vec![];

r1cs-std/src/bits/uint8.rs

@@ -18,7 +18,7 @@ pub struct UInt8 {
 impl<F: Field> R1CSVar<F> for UInt8<F> {
     type Value = u8;
 
-    fn cs(&self) -> Option<ConstraintSystemRef<F>> {
+    fn cs(&self) -> ConstraintSystemRef<F> {
         self.bits.as_slice().cs()
     }
 

r1cs-std/src/eq.rs

@@ -108,16 +108,16 @@ impl + R1CSVar, F: Field> EqGadget for [T] {
     ) -> Result<(), SynthesisError> {
         assert_eq!(self.len(), other.len());
         let some_are_different = self.is_neq(other)?;
-        if let Some(cs) = some_are_different.cs().or(should_enforce.cs()) {
+        if [&some_are_different, should_enforce].is_constant() {
+            assert!(some_are_different.value().unwrap());
+            Ok(())
+        } else {
+            let cs = [&some_are_different, should_enforce].cs();
             cs.enforce_constraint(
                 some_are_different.lc(),
                 should_enforce.lc(),
                 should_enforce.lc(),
             )
-        } else {
-            // `some_are_different` and `should_enforce` are both constants
-            assert!(some_are_different.value().unwrap());
-            Ok(())
         }
     }
 }

r1cs-std/src/fields/cubic_extension.rs

@@ -9,7 +9,7 @@ use crate::fields::fp::FpVar;
 use crate::{
     fields::{FieldOpsBounds, FieldVar},
     prelude::*,
-    Assignment, ToConstraintFieldGadget, Vec,
+    ToConstraintFieldGadget, Vec,
 };
 
 /// This struct is the `R1CS` equivalent of the cubic extension field type
@@ -89,7 +89,7 @@ where
 {
     type Value = CubicExtField<P>;
 
-    fn cs(&self) -> Option<ConstraintSystemRef<P::BasePrimeField>> {
+    fn cs(&self) -> ConstraintSystemRef<P::BasePrimeField> {
         [&self.c0, &self.c1, &self.c2].cs()
     }
 
@@ -272,7 +272,7 @@ where
             AllocationMode::Witness
         };
         let inverse = Self::new_variable(
-            self.cs().get()?.clone(),
+            self.cs(),
             || {
                 self.value()
                     .map(|f| f.inverse().unwrap_or(CubicExtField::zero()))

r1cs-std/src/fields/fp/cmp.rs

@@ -140,10 +140,11 @@ impl FpVar {
     /// Helper function to enforce `self < other`. This function assumes `self` and `other`
     /// are `<= (p-1)/2` and does not generate constraints to verify that.
     fn enforce_smaller_than_unchecked(&self, other: &FpVar<F>) -> Result<(), SynthesisError> {
-        let cs = [self, other].cs().unwrap();
         let is_smaller_than = self.is_smaller_than_unchecked(other)?;
         let lc_one = lc!() + Variable::One;
-        cs.enforce_constraint(is_smaller_than.lc(), lc_one.clone(), lc_one)
+        [self, other]
+            .cs()
+            .enforce_constraint(is_smaller_than.lc(), lc_one.clone(), lc_one)
     }
 }
 

r1cs-std/src/fields/fp/mod.rs

@@ -46,10 +46,10 @@ pub enum FpVar {
 impl<F: PrimeField> R1CSVar<F> for FpVar<F> {
     type Value = F;
 
-    fn cs(&self) -> Option<ConstraintSystemRef<F>> {
+    fn cs(&self) -> ConstraintSystemRef<F> {
         match self {
-            Self::Constant(_) => Some(ConstraintSystemRef::None),
-            Self::Var(a) => Some(a.cs.clone()),
+            Self::Constant(_) => ConstraintSystemRef::None,
+            Self::Var(a) => a.cs.clone(),
         }
     }
 
@@ -67,7 +67,7 @@ impl From> for FpVar {
             Self::Constant(F::from(b as u8))
         } else {
             // `other` is a variable
-            let cs = other.cs().unwrap();
+            let cs = other.cs();
             let variable = cs.new_lc(other.lc()).unwrap();
             Self::Var(AllocatedFp::new(
                 other.value().ok().map(|b| F::from(b as u8)),
@@ -90,12 +90,9 @@ impl<'a, F: PrimeField> FieldOpsBounds<'a, F, FpVar> for &'a FpVar {}
 impl<F: PrimeField> AllocatedFp<F> {
     /// Constructs `Self` from a `Boolean`: if `other` is false, this outputs `zero`, else it outputs `one`.
     pub fn from(other: Boolean<F>) -> Self {
-        if let Some(cs) = other.cs() {
-            let variable = cs.new_lc(other.lc()).unwrap();
-            Self::new(other.value().ok().map(|b| F::from(b as u8)), variable, cs)
-        } else {
-            unreachable!("Cannot create a constant value")
-        }
+        let cs = other.cs();
+        let variable = cs.new_lc(other.lc()).unwrap();
+        Self::new(other.value().ok().map(|b| F::from(b as u8)), variable, cs)
     }
 
     /// Returns the value assigned to `self` in the underlying constraint system
@@ -511,7 +508,7 @@ impl CondSelectGadget for AllocatedFp {
             Boolean::Constant(true) => Ok(true_val.clone()),
             Boolean::Constant(false) => Ok(false_val.clone()),
             _ => {
-                let cs = cond.cs().unwrap();
+                let cs = cond.cs();
                 let result = Self::new_witness(cs.clone(), || {
                     cond.value()
                         .and_then(|c| if c { true_val } else { false_val }.value.get())
@@ -541,24 +538,20 @@ impl TwoBitLookupGadget for AllocatedFp {
     fn two_bit_lookup(b: &[Boolean<F>], c: &[Self::TableConstant]) -> Result<Self, SynthesisError> {
         debug_assert_eq!(b.len(), 2);
         debug_assert_eq!(c.len(), 4);
-        if let Some(cs) = b.cs() {
-            let result = Self::new_witness(cs.clone(), || {
-                let lsb = usize::from(b[0].value()?);
-                let msb = usize::from(b[1].value()?);
-                let index = lsb + (msb << 1);
-                Ok(c[index])
-            })?;
-            let one = Variable::One;
-            cs.enforce_constraint(
-                lc!() + b[1].lc() * (c[3] - &c[2] - &c[1] + &c[0]) + (c[1] - &c[0], one),
-                lc!() + b[0].lc(),
-                lc!() + result.variable - (c[0], one) + b[1].lc() * (c[0] - &c[2]),
-            )?;
-
-            Ok(result)
-        } else {
-            unreachable!("must provide a way to obtain a ConstraintSystemRef")
-        }
+        let result = Self::new_witness(b.cs(), || {
+            let lsb = usize::from(b[0].value()?);
+            let msb = usize::from(b[1].value()?);
+            let index = lsb + (msb << 1);
+            Ok(c[index])
+        })?;
+        let one = Variable::One;
+        b.cs().enforce_constraint(
+            lc!() + b[1].lc() * (c[3] - &c[2] - &c[1] + &c[0]) + (c[1] - &c[0], one),
+            lc!() + b[0].lc(),
+            lc!() + result.variable - (c[0], one) + b[1].lc() * (c[0] - &c[2]),
+        )?;
+
+        Ok(result)
     }
 }
 
@@ -573,37 +566,32 @@ impl ThreeBitCondNegLookupGadget for AllocatedFp {
     ) -> Result<Self, SynthesisError> {
         debug_assert_eq!(b.len(), 3);
         debug_assert_eq!(c.len(), 4);
+        let result = Self::new_witness(b.cs(), || {
+            let lsb = usize::from(b[0].value()?);
+            let msb = usize::from(b[1].value()?);
+            let index = lsb + (msb << 1);
+            let intermediate = c[index];
 
-        if let Some(cs) = b.cs() {
-            let result = Self::new_witness(cs.clone(), || {
-                let lsb = usize::from(b[0].value()?);
-                let msb = usize::from(b[1].value()?);
-                let index = lsb + (msb << 1);
-                let intermediate = c[index];
-
-                let is_negative = b[2].value()?;
-                let y = if is_negative {
-                    -intermediate
-                } else {
-                    intermediate
-                };
-                Ok(y)
-            })?;
-
-            let y_lc = b0b1.lc() * (c[3] - &c[2] - &c[1] + &c[0])
-                + b[0].lc() * (c[1] - &c[0])
-                + b[1].lc() * (c[2] - &c[0])
-                + (c[0], Variable::One);
-            cs.enforce_constraint(
-                y_lc.clone() + y_lc.clone(),
-                b[2].lc(),
-                y_lc.clone() - result.variable,
-            )?;
-
-            Ok(result)
-        } else {
-            unreachable!("must provide a way to obtain a ConstraintSystemRef")
-        }
+            let is_negative = b[2].value()?;
+            let y = if is_negative {
+                -intermediate
+            } else {
+                intermediate
+            };
+            Ok(y)
+        })?;
+
+        let y_lc = b0b1.lc() * (c[3] - &c[2] - &c[1] + &c[0])
+            + b[0].lc() * (c[1] - &c[0])
+            + b[1].lc() * (c[2] - &c[0])
+            + (c[0], Variable::One);
+        b.cs().enforce_constraint(
+            y_lc.clone() + y_lc.clone(),
+            b[2].lc(),
+            y_lc.clone() - result.variable,
+        )?;
+
+        Ok(result)
     }
 }
 
@@ -938,7 +926,7 @@ impl CondSelectGadget for FpVar {
                         Ok(is.mul_constant(*t).add(&not.mul_constant(*f)).into())
                     }
                     (_, _) => {
-                        let cs = cond.cs().unwrap();
+                        let cs = cond.cs();
                         let true_value = match true_value {
                             Self::Constant(f) => AllocatedFp::new_constant(cs.clone(), f)?,
                             Self::Var(v) => v.clone(),
@@ -964,13 +952,13 @@ impl TwoBitLookupGadget for FpVar {
     fn two_bit_lookup(b: &[Boolean<F>], c: &[Self::TableConstant]) -> Result<Self, SynthesisError> {
         debug_assert_eq!(b.len(), 2);
         debug_assert_eq!(c.len(), 4);
-        if b.cs().is_some() {
-            AllocatedFp::two_bit_lookup(b, c).map(Self::Var)
-        } else {
+        if b.is_constant() {
             let lsb = usize::from(b[0].value()?);
             let msb = usize::from(b[1].value()?);
             let index = lsb + (msb << 1);
             Ok(Self::Constant(c[index]))
+        } else {
+            AllocatedFp::two_bit_lookup(b, c).map(Self::Var)
         }
     }
 }
@@ -987,9 +975,9 @@ impl ThreeBitCondNegLookupGadget for FpVar {
         debug_assert_eq!(b.len(), 3);
         debug_assert_eq!(c.len(), 4);
 
-        if b.cs().or(b0b1.cs()).is_some() {
-            AllocatedFp::three_bit_cond_neg_lookup(b, b0b1, c).map(Self::Var)
-        } else {
+        if !b.cs().or(b0b1.cs()).is_none() {
+            // We only have constants
+
             let lsb = usize::from(b[0].value()?);
             let msb = usize::from(b[1].value()?);
             let index = lsb + (msb << 1);
@@ -1002,6 +990,8 @@ impl ThreeBitCondNegLookupGadget for FpVar {
                 intermediate
             };
             Ok(Self::Constant(y))
+        } else {
+            AllocatedFp::three_bit_cond_neg_lookup(b, b0b1, c).map(Self::Var)
         }
     }
 }

r1cs-std/src/fields/mod.rs

@@ -152,7 +152,7 @@ pub trait FieldVar:
     /// It is up to the caller to ensure that denominator is non-zero,
     /// since in that case the result is unconstrained.
     fn mul_by_inverse(&self, denominator: &Self) -> Result<Self, SynthesisError> {
-        let result = Self::new_witness(self.cs().unwrap(), || {
+        let result = Self::new_witness(self.cs(), || {
             let denominator_inv_native = denominator.value()?.inverse().get()?;
             let result = self.value()? * &denominator_inv_native;
             Ok(result)

r1cs-std/src/fields/quadratic_extension.rs

@@ -9,7 +9,7 @@ use crate::fields::fp::FpVar;
 use crate::{
     fields::{FieldOpsBounds, FieldVar},
     prelude::*,
-    Assignment, ToConstraintFieldGadget, Vec,
+    ToConstraintFieldGadget, Vec,
 };
 
 /// This struct is the `R1CS` equivalent of the quadratic extension field type
@@ -122,7 +122,7 @@ where
 {
     type Value = QuadExtField<P>;
 
-    fn cs(&self) -> Option<ConstraintSystemRef<P::BasePrimeField>> {
+    fn cs(&self) -> ConstraintSystemRef<P::BasePrimeField> {
         [&self.c0, &self.c1].cs()
     }
 
@@ -279,7 +279,7 @@ where
             AllocationMode::Witness
         };
         let inverse = Self::new_variable(
-            self.cs().get()?.clone(),
+            self.cs(),
             || {
                 self.value()
                     .map(|f| f.inverse().unwrap_or(QuadExtField::zero()))

r1cs-std/src/groups/curves/short_weierstrass/mod.rs

@@ -119,7 +119,7 @@ where
 {
     type Value = SWProjective<P>;
 
-    fn cs(&self) -> Option<ConstraintSystemRef<<P::BaseField as Field>::BasePrimeField>> {
+    fn cs(&self) -> ConstraintSystemRef<<P::BaseField as Field>::BasePrimeField> {
         self.x.cs().or(self.y.cs()).or(self.z.cs())
     }
 
@@ -152,7 +152,7 @@ where
     /// Convert this point into affine form.
     #[tracing::instrument(target = "r1cs")]
     pub fn to_affine(&self) -> Result<AffineVar<P, F>, SynthesisError> {
-        let cs = self.cs().unwrap_or(ConstraintSystemRef::None);
+        let cs = self.cs();
         let mode = if self.is_constant() {
             let point = self.value()?.into_affine();
             let x = F::new_constant(ConstraintSystemRef::None, point.x)?;

r1cs-std/src/groups/curves/twisted_edwards/mod.rs

@@ -49,7 +49,7 @@ mod montgomery_affine_impl {
     {
         type Value = (P::BaseField, P::BaseField);
 
-        fn cs(&self) -> Option<ConstraintSystemRef<<P::BaseField as Field>::BasePrimeField>> {
+        fn cs(&self) -> ConstraintSystemRef<<P::BaseField as Field>::BasePrimeField> {
             self.x.cs().or(self.y.cs())
         }
 
@@ -112,7 +112,7 @@ mod montgomery_affine_impl {
         /// Converts `self` into a Twisted Edwards curve point variable.
         #[tracing::instrument(target = "r1cs")]
         pub fn into_edwards(&self) -> Result<AffineVar<P, F>, SynthesisError> {
-            let cs = self.cs().unwrap_or(ConstraintSystemRef::None);
+            let cs = self.cs();
             // Compute u = x / y
             let u = F::new_witness(r1cs_core::ns!(cs, "u"), || {
                 let y_inv = self
@@ -153,12 +153,11 @@ mod montgomery_affine_impl {
         #[tracing::instrument(target = "r1cs")]
         fn add(self, other: &'a Self) -> Self::Output {
             let cs = [&self, other].cs();
-            let mode = if cs.is_none() || matches!(cs, Some(ConstraintSystemRef::None)) {
+            let mode = if cs.is_none() {
                 AllocationMode::Constant
             } else {
                 AllocationMode::Witness
             };
-            let cs = cs.unwrap_or(ConstraintSystemRef::None);
 
             let coeff_b = P::MontgomeryModelParameters::COEFF_B;
             let coeff_a = P::MontgomeryModelParameters::COEFF_A;
@@ -378,7 +377,7 @@ where
 {
     type Value = TEProjective<P>;
 
-    fn cs(&self) -> Option<ConstraintSystemRef<<P::BaseField as Field>::BasePrimeField>> {
+    fn cs(&self) -> ConstraintSystemRef<<P::BaseField as Field>::BasePrimeField> {
         self.x.cs().or(self.y.cs())
     }
 
@@ -465,7 +464,7 @@ where
             let value = self.value()?;
             *self = Self::constant(value.double());
         } else {
-            let cs = self.cs().unwrap();
+            let cs = self.cs();
             let a = P::COEFF_A;
 
             // xy
@@ -714,7 +713,7 @@ impl_bounded_ops!(
             assert!(this.is_constant() && other.is_constant());
             AffineVar::constant(this.value().unwrap() + &other.value().unwrap())
         } else {
-            let cs = [this, other].cs().unwrap();
+            let cs = [this, other].cs();
             let a = P::COEFF_A;
             let d = P::COEFF_D;
 

r1cs-std/src/lib.rs

@@ -119,12 +119,13 @@ pub trait R1CSVar {
     type Value: core::fmt::Debug + Eq + Clone;
 
     /// Returns the underlying `ConstraintSystemRef`.
-    fn cs(&self) -> Option<r1cs_core::ConstraintSystemRef<F>>;
+    ///
+    /// If `self` is a constant value, then this *must* return `r1cs_core::ConstraintSystemRef::None`.
+    fn cs(&self) -> r1cs_core::ConstraintSystemRef<F>;
 
     /// Returns `true` if `self` is a circuit-generation-time constant.
     fn is_constant(&self) -> bool {
-        self.cs()
-            .map_or(true, |cs| cs == r1cs_core::ConstraintSystemRef::None)
+        self.cs().is_none()
     }
 
     /// Returns the value that is assigned to `self` in the underlying
@@ -135,8 +136,8 @@ pub trait R1CSVar {
 impl<F: Field, T: R1CSVar<F>> R1CSVar<F> for [T] {
     type Value = Vec<T::Value>;
 
-    fn cs(&self) -> Option<r1cs_core::ConstraintSystemRef<F>> {
-        let mut result = None;
+    fn cs(&self) -> r1cs_core::ConstraintSystemRef<F> {
+        let mut result = r1cs_core::ConstraintSystemRef::None;
         for var in self {
             result = var.cs().or(result);
         }
@@ -155,7 +156,7 @@ impl> R1CSVar for [T] {
 impl<'a, F: Field, T: 'a + R1CSVar<F>> R1CSVar<F> for &'a T {
     type Value = T::Value;
 
-    fn cs(&self) -> Option<r1cs_core::ConstraintSystemRef<F>> {
+    fn cs(&self) -> r1cs_core::ConstraintSystemRef<F> {
         (*self).cs()
     }