Add unit test

acvm-repo/acvm/src/compiler/optimizers/merge_expressions.rs

@@ -328,6 +328,50 @@ mod tests {
         check_circuit(circuit);
     }
 
+    #[test]
+    fn does_not_eliminate_witnesses_returned_from_circuit() {
+        let opcodes = vec![
+            Opcode::AssertZero(Expression {
+                mul_terms: vec![(FieldElement::from(-1i128), Witness(0), Witness(0))],
+                linear_combinations: vec![(FieldElement::from(1i128), Witness(1))],
+                q_c: FieldElement::zero(),
+            }),
+            Opcode::AssertZero(Expression {
+                mul_terms: Vec::new(),
+                linear_combinations: vec![
+                    (FieldElement::from(-1i128), Witness(1)),
+                    (FieldElement::from(1i128), Witness(2)),
+                ],
+                q_c: FieldElement::zero(),
+            }),
+        ];
+        // Witness(1) could be eliminated because it's only used by 2 opcodes.
+
+        let mut private_parameters = BTreeSet::new();
+        private_parameters.insert(Witness(0));
+
+        let mut return_values = BTreeSet::new();
+        return_values.insert(Witness(1));
+        return_values.insert(Witness(2));
+
+        let circuit = Circuit {
+            current_witness_index: 2,
+            expression_width: ExpressionWidth::Bounded { width: 4 },
+            opcodes,
+            private_parameters,
+            public_parameters: PublicInputs::default(),
+            return_values: PublicInputs(return_values),
+            assert_messages: Default::default(),
+        };
+
+        let mut merge_optimizer = MergeExpressionsOptimizer::new();
+        let acir_opcode_positions = vec![0; 20];
+        let (opcodes, _) =
+            merge_optimizer.eliminate_intermediate_variable(&circuit, acir_opcode_positions);
+
+        assert_eq!(opcodes.len(), 2);
+    }
+
     #[test]
     fn does_not_attempt_to_merge_into_previous_opcodes() {
         let opcodes = vec![