[libclang/python] Return None instead of null cursors from Token.cursor (llvm#163183)

Since llvm#138103 , the `Cursor`
class throws an error when any of its methods is called on a null
cursor. Simultaneously, we adapted all methods to return `None` instead
of a null cursor, so users should not encounter these. We have
overlooked one way to end up with null cursors, namely the
`Token.cursor` property, which may return null cursors under some
circumstances.

Fixes llvm#163180

Author: DeinAlptraum

clang/bindings/python/clang/cindex.py

@@ -3853,6 +3853,8 @@ def cursor(self):
         cursor._tu = self._tu
 
         conf.lib.clang_annotateTokens(self._tu, byref(self), 1, byref(cursor))
+        if cursor.is_null():
+            return None
 
         return cursor
 

clang/bindings/python/tests/cindex/test_tokens.py

@@ -53,3 +53,9 @@ def test_token_extent(self):
 
         self.assertEqual(extent.start.offset, 4)
         self.assertEqual(extent.end.offset, 7)
+
+    def test_null_cursor(self):
+        """Ensure that the cursor property converts null cursors to None"""
+        tu = get_tu("int i = 5;")
+        tokens = list(tu.get_tokens(extent=tu.cursor.extent))
+        self.assertEqual(tokens[-1].cursor, None)

clang/docs/ReleaseNotes.rst

@@ -94,6 +94,7 @@ Clang Frontend Potentially Breaking Changes
 
 Clang Python Bindings Potentially Breaking Changes
 --------------------------------------------------
+- Return ``None`` instead of null cursors from ``Token.cursor``
 - ``Cursor.from_location`` now returns ``None`` instead of a null cursor.
   This eliminates the last known source of null cursors.
 - Almost all ``Cursor`` methods now assert that they are called on non-null cursors.