Handle final comments with no terminating newline in OpenQASM 2

Previously, if an OpenQASM 2 program to be parsed ended in a comment
with no terminating '\n', the lexer would fail to notice the end of the
file and would instead attempt to emit the second '/' of the comment
introduction as a 'Slash` token.

crates/qasm2/src/lex.rs

@@ -665,8 +665,11 @@ impl TokenStream {
             b'}' => TokenType::RBrace,
             b'/' => {
                 if let Some(b'/') = self.peek_byte()? {
-                    self.advance_line()?;
-                    return self.next(context);
+                    return if self.advance_line()? == 0 {
+                        Ok(None)
+                    } else {
+                        self.next(context)
+                    }
                 } else {
                     TokenType::Slash
                 }

releasenotes/notes/fix-qasm2-final-comment-f0904c3e13215a00.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    OpenQASM 2 programs that end in comments with no terminating newline character will now parse
+    successfully.  Fixed `#10770 <https://github.com/Qiskit/qiskit/issues/10770>`__.

test/python/qasm2/test_structure.py

@@ -39,10 +39,22 @@
 from . import gate_builder
 
 
-class TestEmpty(QiskitTestCase):
+@ddt.ddt
+class TestWhitespace(QiskitTestCase):
     def test_allows_empty(self):
         self.assertEqual(qiskit.qasm2.loads(""), QuantumCircuit())
 
+    @ddt.data("", "\n", "\r\n", "\n  ")
+    def test_empty_except_comment(self, terminator):
+        program = "// final comment" + terminator
+        self.assertEqual(qiskit.qasm2.loads(program), QuantumCircuit())
+
+    @ddt.data("", "\n", "\r\n", "\n  ")
+    def test_final_comment(self, terminator):
+        # This is similar to the empty-circuit test, except that we also have an instruction.
+        program = "qreg q[2]; // final comment" + terminator
+        self.assertEqual(qiskit.qasm2.loads(program), QuantumCircuit(QuantumRegister(2, "q")))
+
 
 class TestVersion(QiskitTestCase):
     def test_complete_version(self):