Create a secure internal path for custom prefixes in registers. (Qiskit#14005)

* FIx: Create an internal path for custom prefixes in registers.

Prior implementations would replace the Register's prefix attribute inplace which is an unsafe operation. The following commits add a secure path for a provisional replacement of a register's prefix name to fix changed unsafe behavior from Qiskit#13860.

* Fix: Address review comments
- Add test-case

* Apply suggestions from code review

Co-authored-by: Kevin Hartman <[email protected]>

* Fix: Address more review comments

* Fix: Lint error

* Update test/python/circuit/test_circuit_operations.py

Co-authored-by: Elena Peña Tapia <[email protected]>

---------

Co-authored-by: Kevin Hartman <[email protected]>
Co-authored-by: Elena Peña Tapia <[email protected]>

Author: 3 people

crates/circuit/src/bit.rs

@@ -771,6 +771,25 @@ macro_rules! create_bit_object {
                 })
             }
 
+            /// Allows for the creation of a new register with a temporary prefix and the
+            /// same instance counter.
+            #[pyo3(signature=(size=None, name=None, bits=None))]
+            #[staticmethod]
+            fn _new_with_prefix(
+                py: Python,
+                size: Option<isize>,
+                name: Option<String>,
+                bits: Option<Vec<$bit_struct>>,
+            ) -> PyResult<Py<Self>> {
+                let name =
+                    format!(
+                        "{}{}",
+                        name.unwrap_or(Self::prefix().to_string()),
+                        $reg_struct::anonymous_instance_count().fetch_add(1, Ordering::Relaxed)
+                    );
+                Py::new(py, Self::py_new(size, Some(name), bits)?)
+            }
+
             #[classattr]
             fn prefix() -> &'static str {
                 $pyreg_prefix

qiskit/circuit/quantumcircuit.py

@@ -3889,15 +3889,15 @@ def clear(self) -> None:
     def _create_creg(self, length: int, name: str) -> ClassicalRegister:
         """Creates a creg, checking if ClassicalRegister with same name exists"""
         if name in [creg.name for creg in self.cregs]:
-            new_creg = ClassicalRegister(length, name=f"{name}{ClassicalRegister.instance_count}")
+            new_creg = ClassicalRegister._new_with_prefix(length, name)
         else:
             new_creg = ClassicalRegister(length, name)
         return new_creg
 
     def _create_qreg(self, length: int, name: str) -> QuantumRegister:
         """Creates a qreg, checking if QuantumRegister with same name exists"""
         if name in [qreg.name for qreg in self.qregs]:
-            new_qreg = QuantumRegister(length, name=f"{name}{QuantumRegister.instance_count}")
+            new_qreg = QuantumRegister._new_with_prefix(length, name)
         else:
             new_qreg = QuantumRegister(length, name)
         return new_qreg

qiskit/transpiler/passes/layout/full_ancilla_allocation.py

@@ -92,9 +92,8 @@ def run(self, dag):
 
         if idle_physical_qubits:
             if self.ancilla_name in dag.qregs:
-                qreg = QuantumRegister(
-                    len(idle_physical_qubits),
-                    name=f"{self.ancilla_name}{QuantumRegister.instances_count + 1}",
+                qreg = QuantumRegister._new_with_prefix(
+                    len(idle_physical_qubits), self.ancilla_name
                 )
             else:
                 qreg = QuantumRegister(len(idle_physical_qubits), name=self.ancilla_name)

test/python/circuit/test_circuit_operations.py

@@ -852,6 +852,32 @@ def test_measure_all_repetition(self):
         self.assertEqual(len(circuit.cregs[0]), 2)  # Both length 2
         self.assertEqual(len(circuit.cregs[1]), 2)
 
+    def test_measure_all_with_multiple_regs_creation(self):
+        """Test measure_all in a circuit where the method is called
+        multiple times consecutively and checks that a register of
+        a different name is created on each call."""
+
+        circuit = QuantumCircuit(1)
+
+        # First call should create a new register
+        circuit.measure_all()
+        self.assertEqual(len(circuit.cregs), 1)  # One creg
+        self.assertEqual(len(circuit.cregs[0]), 1)  # Of length 1
+
+        # Second call should also create a new register
+        circuit.measure_all()
+        self.assertEqual(len(circuit.cregs), 2)  # Now two cregs
+        self.assertTrue(all(len(reg) == 1 for reg in circuit.cregs))  # All of length 1
+        # Check that no name is the same
+        self.assertEqual(len({reg.name for reg in circuit.cregs}), 2)
+
+        # Third call should also create a new register
+        circuit.measure_all()
+        self.assertEqual(len(circuit.cregs), 3)  # Now three cregs
+        self.assertTrue(all(len(reg) == 1 for reg in circuit.cregs))  # All of length 1
+        # Check that no name is the same
+        self.assertEqual(len({reg.name for reg in circuit.cregs}), 3)
+
     def test_remove_final_measurements(self):
         """Test remove_final_measurements
         Removes all measurements at end of circuit.