Add public unsafe evaluate_witness() to the composer (#601)

Resolves: #612

src/constraint_system/composer.rs

@@ -127,8 +127,14 @@ impl TurboComposer {
     }
 
     /// Evaluate the runtime value of a witness
-    #[allow(dead_code)]
-    pub(crate) fn evaluate_witness(&self, witness: &Witness) -> &BlsScalar {
+    ///
+    /// # Safety
+    ///
+    /// Witness evaluation inside a gadget isn't expected and could produce an
+    /// unsound circuit (different circuit representation for the same code).
+    ///
+    /// Calling this function performs operations outside the circuit.
+    pub unsafe fn evaluate_witness(&self, witness: &Witness) -> &BlsScalar {
         &self.witnesses[witness]
     }
 

src/constraint_system/ecc/curve_addition/variable_base_gate.rs

@@ -143,8 +143,9 @@ mod test {
             composer.gate_add(d_x1_x2_y1_y2, zero, zero, constraint);
 
         // Compute the inverse
-        let inv_x_denom =
-            composer.evaluate_witness(&x_denominator).invert().unwrap();
+        let inv_x_denom = unsafe {
+            composer.evaluate_witness(&x_denominator).invert().unwrap()
+        };
         let inv_x_denom = composer.append_witness(inv_x_denom);
 
         let constraint = Constraint::new().mul(1).constant(-BlsScalar::one());
@@ -164,8 +165,9 @@ mod test {
         let y_denominator =
             composer.gate_add(d_x1_x2_y1_y2, zero, zero, constraint);
 
-        let inv_y_denom =
-            composer.evaluate_witness(&y_denominator).invert().unwrap();
+        let inv_y_denom = unsafe {
+            composer.evaluate_witness(&y_denominator).invert().unwrap()
+        };
         let inv_y_denom = composer.append_witness(inv_y_denom);
 
         // Assert that we actually have the inverse