Support custom gate defintions in QASM parser (#6917)

Resolves #3560

* Tests verify parsed vs expected, for unparameterized, parameterized,
and for broadcasted qregs.
* They also use `assert_qiskit_parsed_qasm_consistent_with_unitary` to
ensure consistency of parsed circuit (and sanity check that the provided
QASM definition is actually valid).
* Several tests for invalid gate definitions or invocations and verify
similar error message for Cirq and qiskit parsers.
* One odd instance where qiskit does not throw an error on an invalid
QASM circuit, even though simulating it in qiskit fails; chalking it up
to a qiskit bug.

cirq-core/cirq/circuits/qasm_output.py

@@ -366,10 +366,10 @@ def on_stuck(bad_op):
             if should_annotate:
                 output_line_gap(1)
                 if isinstance(main_op, ops.GateOperation):
-                    x = str(main_op.gate).replace('\n', '\n //')
+                    x = str(main_op.gate).replace('\n', '\n//       ')
                     output(f'// Gate: {x!s}\n')
                 else:
-                    x = str(main_op).replace('\n', '\n //')
+                    x = str(main_op).replace('\n', '\n//            ')
                     output(f'// Operation: {x!s}\n')
 
             for qasm in qasms:

cirq-core/cirq/contrib/qasm_import/_lexer.py

@@ -33,6 +33,7 @@ def __init__(self):
         'creg': 'CREG',
         'measure': 'MEASURE',
         'reset': 'RESET',
+        'gate': 'GATE',
         'if': 'IF',
         '->': 'ARROW',
         '==': 'EQ',
@@ -120,6 +121,10 @@ def t_RESET(self, t):
         r"""reset"""
         return t
 
+    def t_GATE(self, t):
+        r"""gate"""
+        return t
+
     def t_IF(self, t):
         r"""if"""
         return t

cirq-core/cirq/contrib/qasm_import/_lexer_test.py

@@ -159,6 +159,74 @@ def test_creg():
     assert token.value == ";"
 
 
+def test_custom_gate():
+    lexer = QasmLexer()
+    lexer.input('gate name(param1,param2) q1, q2 {X(q1)}')
+    token = lexer.token()
+    assert token.type == "GATE"
+    assert token.value == "gate"
+
+    token = lexer.token()
+    assert token.type == "ID"
+    assert token.value == "name"
+
+    token = lexer.token()
+    assert token.type == "("
+    assert token.value == "("
+
+    token = lexer.token()
+    assert token.type == "ID"
+    assert token.value == "param1"
+
+    token = lexer.token()
+    assert token.type == ","
+    assert token.value == ","
+
+    token = lexer.token()
+    assert token.type == "ID"
+    assert token.value == "param2"
+
+    token = lexer.token()
+    assert token.type == ")"
+    assert token.value == ")"
+
+    token = lexer.token()
+    assert token.type == "ID"
+    assert token.value == "q1"
+
+    token = lexer.token()
+    assert token.type == ","
+    assert token.value == ","
+
+    token = lexer.token()
+    assert token.type == "ID"
+    assert token.value == "q2"
+
+    token = lexer.token()
+    assert token.type == "{"
+    assert token.value == "{"
+
+    token = lexer.token()
+    assert token.type == "ID"
+    assert token.value == "X"
+
+    token = lexer.token()
+    assert token.type == "("
+    assert token.value == "("
+
+    token = lexer.token()
+    assert token.type == "ID"
+    assert token.value == "q1"
+
+    token = lexer.token()
+    assert token.type == ")"
+    assert token.value == ")"
+
+    token = lexer.token()
+    assert token.type == "}"
+    assert token.value == "}"
+
+
 def test_error():
     lexer = QasmLexer()
     lexer.input('θ')