Add `ToConstraintFieldGadget` (#278)

r1cs-std/src/bits/boolean.rs

@@ -1,6 +1,6 @@
-use algebra::{BitIteratorBE, Field};
+use algebra::{BitIteratorBE, Field, PrimeField};
 
-use crate::{prelude::*, Assignment, Vec};
+use crate::{fields::fp::FpVar, prelude::*, Assignment, ToConstraintFieldGadget, Vec};
 use core::borrow::Borrow;
 use r1cs_core::{lc, ConstraintSystemRef, LinearCombination, Namespace, SynthesisError, Variable};
 
@@ -597,6 +597,14 @@ impl ToBytesGadget for Boolean {
     }
 }
 
+impl<F: PrimeField> ToConstraintFieldGadget<F> for Boolean<F> {
+    #[tracing::instrument(target = "r1cs")]
+    fn to_constraint_field(&self) -> Result<Vec<FpVar<F>>, SynthesisError> {
+        let var = From::from(self.clone());
+        Ok(vec![var])
+    }
+}
+
 impl<F: Field> CondSelectGadget<F> for Boolean<F> {
     #[tracing::instrument(target = "r1cs")]
     fn conditionally_select(

r1cs-std/src/fields/cubic_extension.rs

@@ -5,10 +5,11 @@ use algebra::{
 use core::{borrow::Borrow, marker::PhantomData};
 use r1cs_core::{ConstraintSystemRef, Namespace, SynthesisError};
 
+use crate::fields::fp::FpVar;
 use crate::{
     fields::{FieldOpsBounds, FieldVar},
     prelude::*,
-    Assignment, Vec,
+    Assignment, ToConstraintFieldGadget, Vec,
 };
 
 #[derive(Derivative)]
@@ -440,6 +441,25 @@ where
     }
 }
 
+impl<BF, P> ToConstraintFieldGadget<P::BasePrimeField> for CubicExtVar<BF, P>
+where
+    BF: FieldVar<P::BaseField, P::BasePrimeField>,
+    for<'a> &'a BF: FieldOpsBounds<'a, P::BaseField, BF>,
+    P: CubicExtVarParams<BF>,
+    BF: ToConstraintFieldGadget<P::BasePrimeField>,
+{
+    #[tracing::instrument(target = "r1cs")]
+    fn to_constraint_field(&self) -> Result<Vec<FpVar<P::BasePrimeField>>, SynthesisError> {
+        let mut res = Vec::new();
+
+        res.extend_from_slice(&self.c0.to_constraint_field()?);
+        res.extend_from_slice(&self.c1.to_constraint_field()?);
+        res.extend_from_slice(&self.c2.to_constraint_field()?);
+
+        Ok(res)
+    }
+}
+
 impl<BF, P> CondSelectGadget<P::BasePrimeField> for CubicExtVar<BF, P>
 where
     BF: FieldVar<P::BaseField, P::BasePrimeField>,

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

@@ -4,7 +4,7 @@ use r1cs_core::{lc, ConstraintSystemRef, LinearCombination, Namespace, Synthesis
 use core::borrow::Borrow;
 
 use crate::fields::{FieldOpsBounds, FieldVar};
-use crate::{prelude::*, Assignment, Vec};
+use crate::{prelude::*, Assignment, ToConstraintFieldGadget, Vec};
 
 pub mod cmp;
 
@@ -432,6 +432,13 @@ impl ToBytesGadget for AllocatedFp {
     }
 }
 
+impl<F: PrimeField> ToConstraintFieldGadget<F> for AllocatedFp<F> {
+    #[tracing::instrument(target = "r1cs")]
+    fn to_constraint_field(&self) -> Result<Vec<FpVar<F>>, SynthesisError> {
+        Ok(vec![self.clone().into()])
+    }
+}
+
 impl<F: PrimeField> CondSelectGadget<F> for AllocatedFp<F> {
     #[inline]
     #[tracing::instrument(target = "r1cs")]
@@ -845,6 +852,13 @@ impl ToBytesGadget for FpVar {
     }
 }
 
+impl<F: PrimeField> ToConstraintFieldGadget<F> for FpVar<F> {
+    #[tracing::instrument(target = "r1cs")]
+    fn to_constraint_field(&self) -> Result<Vec<FpVar<F>>, SynthesisError> {
+        Ok(vec![self.clone()])
+    }
+}
+
 impl<F: PrimeField> CondSelectGadget<F> for FpVar<F> {
     #[tracing::instrument(target = "r1cs")]
     fn conditionally_select(

r1cs-std/src/fields/quadratic_extension.rs

@@ -5,10 +5,11 @@ use algebra::{
 use core::{borrow::Borrow, marker::PhantomData};
 use r1cs_core::{ConstraintSystemRef, Namespace, SynthesisError};
 
+use crate::fields::fp::FpVar;
 use crate::{
     fields::{FieldOpsBounds, FieldVar},
     prelude::*,
-    Assignment, Vec,
+    Assignment, ToConstraintFieldGadget, Vec,
 };
 
 #[derive(Derivative)]
@@ -430,6 +431,24 @@ where
     }
 }
 
+impl<BF, P> ToConstraintFieldGadget<P::BasePrimeField> for QuadExtVar<BF, P>
+where
+    BF: FieldVar<P::BaseField, P::BasePrimeField>,
+    for<'a> &'a BF: FieldOpsBounds<'a, P::BaseField, BF>,
+    P: QuadExtVarParams<BF>,
+    BF: ToConstraintFieldGadget<P::BasePrimeField>,
+{
+    #[tracing::instrument(target = "r1cs")]
+    fn to_constraint_field(&self) -> Result<Vec<FpVar<P::BasePrimeField>>, SynthesisError> {
+        let mut res = Vec::new();
+
+        res.extend_from_slice(&self.c0.to_constraint_field()?);
+        res.extend_from_slice(&self.c1.to_constraint_field()?);
+
+        Ok(res)
+    }
+}
+
 impl<BF, P> CondSelectGadget<P::BasePrimeField> for QuadExtVar<BF, P>
 where
     BF: FieldVar<P::BaseField, P::BasePrimeField>,

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

@@ -8,7 +8,8 @@ use algebra::{
 use core::{borrow::Borrow, marker::PhantomData};
 use r1cs_core::{ConstraintSystemRef, Namespace, SynthesisError};
 
-use crate::{prelude::*, Vec};
+use crate::fields::fp::FpVar;
+use crate::{prelude::*, ToConstraintFieldGadget, Vec};
 
 pub mod bls12;
 pub mod mnt4;
@@ -80,6 +81,25 @@ where
     }
 }
 
+impl<P, F> ToConstraintFieldGadget<<P::BaseField as Field>::BasePrimeField> for AffineVar<P, F>
+where
+    P: SWModelParameters,
+    F: FieldVar<P::BaseField, <P::BaseField as Field>::BasePrimeField>,
+    for<'a> &'a F: FieldOpsBounds<'a, P::BaseField, F>,
+    F: ToConstraintFieldGadget<<P::BaseField as Field>::BasePrimeField>,
+{
+    fn to_constraint_field(
+        &self,
+    ) -> Result<Vec<FpVar<<P::BaseField as Field>::BasePrimeField>>, SynthesisError> {
+        let mut res = Vec::<FpVar<<P::BaseField as Field>::BasePrimeField>>::new();
+
+        res.extend_from_slice(&self.x.to_constraint_field()?);
+        res.extend_from_slice(&self.y.to_constraint_field()?);
+
+        Ok(res)
+    }
+}
+
 impl<P, F> R1CSVar<<P::BaseField as Field>::BasePrimeField> for ProjectiveVar<P, F>
 where
     P: SWModelParameters,

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

@@ -8,8 +8,9 @@ use algebra::{
 
 use r1cs_core::{ConstraintSystemRef, Namespace, SynthesisError};
 
-use crate::{prelude::*, Vec};
+use crate::{prelude::*, ToConstraintFieldGadget, Vec};
 
+use crate::fields::fp::FpVar;
 use core::{borrow::Borrow, marker::PhantomData};
 
 #[derive(Derivative)]
@@ -646,6 +647,25 @@ where
     }
 }
 
+impl<P, F> ToConstraintFieldGadget<<P::BaseField as Field>::BasePrimeField> for AffineVar<P, F>
+where
+    P: TEModelParameters,
+    F: FieldVar<P::BaseField, <P::BaseField as Field>::BasePrimeField>,
+    for<'a> &'a F: FieldOpsBounds<'a, P::BaseField, F>,
+    F: ToConstraintFieldGadget<<P::BaseField as Field>::BasePrimeField>,
+{
+    fn to_constraint_field(
+        &self,
+    ) -> Result<Vec<FpVar<<P::BaseField as Field>::BasePrimeField>>, SynthesisError> {
+        let mut res = Vec::new();
+
+        res.extend_from_slice(&self.x.to_constraint_field()?);
+        res.extend_from_slice(&self.y.to_constraint_field()?);
+
+        Ok(res)
+    }
+}
+
 #[inline]
 fn div2(limbs: &mut [u64]) {
     let mut t = 0;

r1cs-std/src/lib.rs

@@ -154,3 +154,10 @@ impl Assignment for Option {
         self.ok_or(r1cs_core::SynthesisError::AssignmentMissing)
     }
 }
+
+/// Obtains the field variables
+pub trait ToConstraintFieldGadget<ConstraintF: algebra::PrimeField> {
+    fn to_constraint_field(
+        &self,
+    ) -> Result<Vec<crate::fields::fp::FpVar<ConstraintF>>, r1cs_core::SynthesisError>;
+}