fix #12426: adds warning to empty usefixtures.

changelog/12426.improvement.rst

@@ -0,0 +1 @@
+A warning is now issued when :ref:`pytest.mark.usefixtures ref` is used without specifying any fixtures. Previously, empty usefixtures markers were silently ignored.

src/_pytest/fixtures.py

@@ -1535,7 +1535,9 @@ def _getautousenames(self, node: nodes.Node) -> Iterator[str]:
 
     def _getusefixturesnames(self, node: nodes.Item) -> Iterator[str]:
         """Return the names of usefixtures fixtures applicable to node."""
-        for mark in node.iter_markers(name="usefixtures"):
+        for marker_node, mark in node.iter_markers_with_node(name="usefixtures"):
+            if not mark.args:
+                marker_node.warn(Warning("usefixtures() is empty, so it has no effect"))
             yield from mark.args
 
     def getfixtureclosure(