Environment

• Qiskit version: 2.3.1
• Python version: 3.13.2
• Operating system: macOS

What is happening?

When attempting to transpile a quantum circuit using the sabre routing method and optimization level 3, the transpiler crashes with a Rust thread panic if the provided CouplingMap contains disconnected subgraphs (islands) and the circuit contains a 2-qubit gate across these disconnected components.

Instead of gracefully raising a Python-level TranspilerError (e.g., indicating that routing is impossible due to a disconnected coupling map), the underlying Rust implementation panics with an index out of bounds error in crates/circuit/src/nlayout.rs.

How can we reproduce the issue?

import sys
from qiskit import QuantumCircuit, transpile
from qiskit.transpiler import CouplingMap

def generate_poisonous_circuit():
    # 1. Create a disconnected "island" coupling map
    edge_list = [[0, 1], [1, 0], [2, 3], [3, 2]]
    cmap = CouplingMap(edge_list)
    
    # 2. Create a circuit with a gate spanning the disconnected islands
    qc = QuantumCircuit(4)
    qc.h(0)
    qc.h(2)
    qc.cx(0, 2) # Fatal operation across islands
    
    return qc, cmap

if __name__ == "__main__":
    qc, cmap = generate_poisonous_circuit()
    
    # This will trigger the Rust panic
    transpiled_qc = transpile(qc, coupling_map=cmap, routing_method='sabre', optimization_level=3)

What should happen?

The transpiler should catch the invalid routing requirement at the Python/Rust boundary and raise a clear qiskit.transpiler.exceptions.TranspilerError (e.g., "Cannot route circuit: Qubits 0 and 2 are in disconnected components of the coupling map"), rather than panicking the Rust thread.

Any suggestions?

No response