Add parameter 'inputs_already_allocated' to CircomCircuit

To allow allocating the inputs in previous stages.

src/circom/builder.rs

@@ -58,6 +58,7 @@ impl<F: PrimeField> CircomBuilder<F> {
         let mut circom = CircomCircuit {
             r1cs: self.cfg.r1cs.clone(),
             witness: None,
+            inputs_already_allocated: false,
         };
 
         // Disable the wire mapping

src/circom/circuit.rs

@@ -11,6 +11,7 @@ use color_eyre::Result;
 pub struct CircomCircuit<F: PrimeField> {
     pub r1cs: R1CS<F>,
     pub witness: Option<Vec<F>>,
+    pub inputs_already_allocated: bool,
 }
 
 impl<F: PrimeField> CircomCircuit<F> {
@@ -31,6 +32,7 @@ impl<F: PrimeField> ConstraintSynthesizer<F> for CircomCircuit<F> {
         let wire_mapping = &self.r1cs.wire_mapping;
 
         // Start from 1 because Arkworks implicitly allocates One for the first input
+        if !self.inputs_already_allocated {
             for i in 1..self.r1cs.num_inputs {
                 cs.new_input_variable(|| {
                     Ok(match witness {
@@ -42,6 +44,7 @@ impl<F: PrimeField> ConstraintSynthesizer<F> for CircomCircuit<F> {
                     })
                 })?;
             }
+        }
 
         for i in 0..self.r1cs.num_aux {
             cs.new_witness_variable(|| {
@@ -69,7 +72,12 @@ impl<F: PrimeField> ConstraintSynthesizer<F> for CircomCircuit<F> {
             )
         };
 
-        for constraint in &self.r1cs.constraints {
+        let skip = if self.inputs_already_allocated {
+            self.r1cs.num_inputs
+        } else {
+            0
+        };
+        for constraint in self.r1cs.constraints.iter().skip(skip) {
             cs.enforce_constraint(
                 make_lc(&constraint.0),
                 make_lc(&constraint.1),