Switch QuantumCircuit.from_qasm_str and from_qasm_file to new parser

qiskit/circuit/quantumcircuit.py

@@ -43,7 +43,6 @@
 from qiskit.circuit.instruction import Instruction
 from qiskit.circuit.gate import Gate
 from qiskit.circuit.parameter import Parameter
-from qiskit.qasm.qasm import Qasm
 from qiskit.qasm.exceptions import QasmError
 from qiskit.circuit.exceptions import CircuitError
 from .parameterexpression import ParameterExpression, ParameterValueType
@@ -2504,11 +2503,23 @@ def from_qasm_file(path: str) -> "QuantumCircuit":
 
         Args:
           path (str): Path to the file for a QASM program
+
         Return:
           QuantumCircuit: The QuantumCircuit object for the input QASM
+
+        See also:
+            :func:`.qasm2.load`: the complete interface to the OpenQASM 2 importer.
         """
-        qasm = Qasm(filename=path)
-        return _circuit_from_qasm(qasm)
+        # pylint: disable=cyclic-import
+        from qiskit import qasm2
+
+        return qasm2.load(
+            path,
+            include_path=qasm2.LEGACY_INCLUDE_PATH,
+            custom_instructions=qasm2.LEGACY_CUSTOM_INSTRUCTIONS,
+            custom_classical=qasm2.LEGACY_CUSTOM_CLASSICAL,
+            strict=False,
+        )
 
     @staticmethod
     def from_qasm_str(qasm_str: str) -> "QuantumCircuit":
@@ -2518,9 +2529,20 @@ def from_qasm_str(qasm_str: str) -> "QuantumCircuit":
           qasm_str (str): A QASM program string
         Return:
           QuantumCircuit: The QuantumCircuit object for the input QASM
+
+        See also:
+            :func:`.qasm2.loads`: the complete interface to the OpenQASM 2 importer.
         """
-        qasm = Qasm(data=qasm_str)
-        return _circuit_from_qasm(qasm)
+        # pylint: disable=cyclic-import
+        from qiskit import qasm2
+
+        return qasm2.loads(
+            qasm_str,
+            include_path=qasm2.LEGACY_INCLUDE_PATH,
+            custom_instructions=qasm2.LEGACY_CUSTOM_INSTRUCTIONS,
+            custom_classical=qasm2.LEGACY_CUSTOM_CLASSICAL,
+            strict=False,
+        )
 
     @property
     def global_phase(self) -> ParameterValueType:
@@ -4858,16 +4880,6 @@ def qubit_stop_time(self, *qubits: Union[Qubit, int]) -> float:
         return 0  # If there are no instructions over bits
 
 
-def _circuit_from_qasm(qasm: Qasm) -> "QuantumCircuit":
-    # pylint: disable=cyclic-import
-    from qiskit.converters import ast_to_dag
-    from qiskit.converters import dag_to_circuit
-
-    ast = qasm.parse()
-    dag = ast_to_dag(ast)
-    return dag_to_circuit(dag, copy_operations=False)
-
-
 def _standard_compare(value1, value2):
     if value1 < value2:
         return -1

releasenotes/notes/new-circuit-qasm2-methods-b1a06ee2859e2cce.yaml

@@ -0,0 +1,18 @@
+---
+upgrade:
+  - |
+    The OpenQASM 2 constructor methods on :class:`.QuantumCircuit`
+    (:meth:`~.QuantumCircuit.from_qasm_str` and :meth:`~.QuantumCircuit.from_qasm_file`) have been
+    switched to use the Rust-based parser added in Qiskit Terra 0.24.  This should result in
+    significantly faster parsing times (10 times or more is not uncommon) and massively reduced
+    intermediate memory usage.
+
+    The :class:`.QuantumCircuit` methods are kept with the same interface for continuity; the
+    preferred way to access the OpenQASM 2 importer is to use :func:`.qasm2.load` and
+    :func:`.qasm2.loads`, which offer an expanded interface to control the parsing and construction.
+fixes:
+  - |
+    The OpenQASM 2 circuit-constructor methods (:meth:`.QuantumCircuit.from_qasm_str` and
+    :meth:`~.QuantumCircuit.from_qasm_file`) will no longer error when encountering a ``gate``
+    definition that contains ``U`` or ``CX`` instructions. See `#5536
+    <https://github.com/Qiskit/qiskit-terra/issues/5536>`__.

test/python/circuit/test_circuit_load_from_qasm.py → test/python/qasm2/test_circuit_methods.py

@@ -120,14 +120,6 @@ def test_fail_qasm_file(self):
         """
         self.assertRaises(QiskitError, QuantumCircuit.from_qasm_file, "")
 
-    def test_fail_qasm_string(self):
-        """
-        Test fail_qasm_string.
-
-        If all is correct we should get a QiskitError
-        """
-        self.assertRaises(QiskitError, QuantumCircuit.from_qasm_str, "")
-
     def test_qasm_text(self):
         """
         Test qasm_text and get_circuit.
@@ -492,6 +484,21 @@ def test_from_qasm_str_delay(self):
         expected.delay(172, qr[0])
         self.assertEqualUnroll("u", circuit, expected)
 
+    def test_definition_with_u_cx(self):
+        """Test that gate-definition bodies can use U and CX."""
+        qasm_string = """
+OPENQASM 2.0;
+gate bell q0, q1 { U(pi/2, 0, pi) q0; CX q0, q1; }
+qreg q[2];
+bell q[0], q[1];
+"""
+        circuit = QuantumCircuit.from_qasm_str(qasm_string)
+        qr = QuantumRegister(2, "q")
+        expected = QuantumCircuit(qr)
+        expected.h(0)
+        expected.cx(0, 1)
+        self.assertEqualUnroll(["u", "cx"], circuit, expected)
+
     def assertEqualUnroll(self, basis, circuit, expected):
         """Compares the dags after unrolling to basis"""
         circuit_dag = circuit_to_dag(circuit)