chore(acir): Unify how we display witness indices (#8192)

Author: vezenovm

acvm-repo/acir/src/circuit/mod.rs

@@ -323,13 +323,13 @@ impl<F: AcirField + for<'a> Deserialize<'a>> Program<F> {
 
 impl<F: AcirField> std::fmt::Display for Circuit<F> {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        writeln!(f, "current witness index : {}", self.current_witness_index)?;
+        writeln!(f, "current witness index : _{}", self.current_witness_index)?;
 
         let write_witness_indices =
             |f: &mut std::fmt::Formatter<'_>, indices: &[u32]| -> Result<(), std::fmt::Error> {
                 write!(f, "[")?;
                 for (index, witness_index) in indices.iter().enumerate() {
-                    write!(f, "{witness_index}")?;
+                    write!(f, "_{witness_index}")?;
                     if index != indices.len() - 1 {
                         write!(f, ", ")?;
                     }
@@ -509,6 +509,44 @@ mod tests {
         assert!(deserialization_result.is_err());
     }
 
+    #[test]
+    fn circuit_display_snapshot() {
+        let circuit = Circuit {
+            current_witness_index: 3,
+            expression_width: ExpressionWidth::Unbounded,
+            opcodes: vec![
+                Opcode::AssertZero(crate::native_types::Expression {
+                    mul_terms: vec![],
+                    linear_combinations: vec![(FieldElement::from(2u128), Witness(1))],
+                    q_c: FieldElement::from(8u128),
+                }),
+                range_opcode(),
+                and_opcode(),
+                keccakf1600_opcode(),
+            ],
+            private_parameters: BTreeSet::new(),
+            public_parameters: PublicInputs(BTreeSet::from_iter(vec![Witness(2)])),
+            return_values: PublicInputs(BTreeSet::from_iter(vec![Witness(2)])),
+            assert_messages: Default::default(),
+        };
+
+        // We want to make sure that we witness indices are displayed in a unified format.
+        // All witnesses are expected to be formatted as `_{witness_index}`.
+        insta::assert_snapshot!(
+            circuit.to_string(),
+            @r"
+            current witness index : _3
+            private parameters indices : []
+            public parameters indices : [_2]
+            return value indices : [_2]
+            EXPR [ (2, _1) 8 ]
+            BLACKBOX::RANGE [(_1, 8)] []
+            BLACKBOX::AND [(_1, 4), (_2, 4)] [_3]
+            BLACKBOX::KECCAKF1600 [(_1, 8), (_2, 8), (_3, 8), (_4, 8), (_5, 8), (_6, 8), (_7, 8), (_8, 8), (_9, 8), (_10, 8), (_11, 8), (_12, 8), (_13, 8), (_14, 8), (_15, 8), (_16, 8), (_17, 8), (_18, 8), (_19, 8), (_20, 8), (_21, 8), (_22, 8), (_23, 8), (_24, 8), (_25, 8)] [_26, _27, _28, _29, _30, _31, _32, _33, _34, _35, _36, _37, _38, _39, _40, _41, _42, _43, _44, _45, _46, _47, _48, _49, _50]
+            "
+        );
+    }
+
     /// Property based testing for serialization
     mod props {
         use acir_field::FieldElement;