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 #13860.

Author: raynelfss

crates/circuit/src/bit.rs

@@ -771,6 +771,25 @@ macro_rules! create_bit_object {
                 })
             }
 
+            /// Allows for the creation of a new register with a provisional 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)