issue a warning when Item and Collector are used in diamond inheritance (#8447)

* issue a warning when Items and Collector form a diamond

addresses #8435

* Apply suggestions from code review

Co-authored-by: Ran Benita <ran@unusedvar.com>

* Return support for the broken File/Item hybrids

* adds deprecation
* ads necessary support code in node construction

* fix incorrect mypy based assertions

* add docs for deprecation of Item/File inheritance

* warn when a non-cooperative ctor is encountered

* use getattr instead of cast to get the class __init__ for legacy ctors

* update documentation references for node inheritance

* clean up file+item inheritance test

enhance docs
move import upwards

Co-authored-by: Ran Benita <ran@unusedvar.com>

testing/test_nodes.py

@@ -5,6 +5,7 @@ from typing import Type
 
 import pytest
 from _pytest import nodes
+from _pytest.compat import legacy_path
 from _pytest.pytester import Pytester
 from _pytest.warning_types import PytestWarning
 
@@ -39,6 +40,36 @@ def test_node_from_parent_disallowed_arguments() -> None:
         nodes.Node.from_parent(None, config=None)  # type: ignore[arg-type]
 
 
+def test_subclassing_both_item_and_collector_deprecated(
+    request, tmp_path: Path
+) -> None:
+    """
+    Verifies we warn on diamond inheritance
+    as well as correctly managing legacy inheritance ctors with missing args
+    as found in plugins
+    """
+
+    with pytest.warns(
+        PytestWarning,
+        match=(
+            "(?m)SoWrong is an Item subclass and should not be a collector, however its bases File are collectors.\n"
+            "Please split the Collectors and the Item into separate node types.\n.*"
+        ),
+    ):
+
+        class SoWrong(nodes.File, nodes.Item):
+            def __init__(self, fspath, parent):
+                """Legacy ctor with legacy call # don't wana see"""
+                super().__init__(fspath, parent)
+
+    with pytest.warns(
+        PytestWarning, match=".*SoWrong.* not using a cooperative constructor.*"
+    ):
+        SoWrong.from_parent(
+            request.session, fspath=legacy_path(tmp_path / "broken.txt")
+        )
+
+
 @pytest.mark.parametrize(
     "warn_type, msg", [(DeprecationWarning, "deprecated"), (PytestWarning, "pytest")]
 )