Fix PytestPluginManager._is_in_confcutdir on Windows

On Windows two paths might be in different drives, so if a path is not one of the parents of the confcutdir, it does not necessarily mean it is inside the confcutdir either.

Use `Path.is_relative_to` instead which directly answers that question for us.

The previous implementation could cause pytest to inspect all the parent paths of a path from one drive if pytest was invoked from another drive.

Moved the implementation to a function because it would be too convoluted to test the `PytestPluginManager._is_in_confcutdir` method directly.

src/_pytest/config/__init__.py

@@ -580,11 +580,9 @@ class PytestPluginManager(PluginManager):
     def _is_in_confcutdir(self, path: Path) -> bool:
         """Whether a path is within the confcutdir.
 
-        When false, should not load conftest.
+        When false, should not load conftest or recurse into path for collection.
         """
-        if self._confcutdir is None:
+        return path_within_confcutdir(path=path, confcutdir=self._confcutdir)
-            return True
-        return path not in self._confcutdir.parents
 
     def _try_load_conftest(
         self, anchor: Path, importmode: Union[str, ImportMode], rootpath: Path
@@ -609,9 +607,6 @@ class PytestPluginManager(PluginManager):
         if directory in self._dirpath2confmods:
             return
 
-        # XXX these days we may rather want to use config.rootpath
-        # and allow users to opt into looking into the rootdir parent
-        # directories instead of requiring to specify confcutdir.
         clist = []
         for parent in reversed((directory, *directory.parents)):
             if self._is_in_confcutdir(parent):
@@ -1908,3 +1903,10 @@ def apply_warning_filters(
 
     for arg in cmdline_filters:
         warnings.filterwarnings(*parse_warning_filter(arg, escape=True))
+
+
+def path_within_confcutdir(*, path: Path, confcutdir: Optional[Path]) -> bool:
+    # Extracted into a function for unit-testing.
+    if confcutdir is None:
+        return True
+    return path.is_relative_to(confcutdir)

testing/test_pluginmanager.py

@@ -1,12 +1,15 @@
 # mypy: allow-untyped-defs
 import os
+from pathlib import Path
 import shutil
 import sys
 import types
 from typing import List
+from typing import Optional
 
 from _pytest.config import Config
 from _pytest.config import ExitCode
+from _pytest.config import path_within_confcutdir
 from _pytest.config import PytestPluginManager
 from _pytest.config.exceptions import UsageError
 from _pytest.main import Session
@@ -404,7 +407,7 @@ class TestPytestPluginManager:
             pytestpm.consider_conftest(mod, registration_name="unused")
 
 
-class TestPytestPluginManagerBootstrapming:
+class TestPytestPluginManagerBootstrapping:
     def test_preparse_args(self, pytestpm: PytestPluginManager) -> None:
         pytest.raises(
             ImportError, lambda: pytestpm.consider_preparse(["xyz", "-p", "hello123"])
@@ -464,3 +467,53 @@ class TestPytestPluginManagerBootstrapming:
         assert pytestpm.has_plugin("abc")
         assert not pytestpm.is_blocked("abc")
         assert not pytestpm.is_blocked("pytest_abc")
+
+
+skip_if_win = pytest.mark.skipif(
+    not sys.platform.startswith("win"), reason="Windows only"
+)
+
+
+@pytest.mark.parametrize(
+    "path, confcutdir, expected",
+    [
+        (Path("/projects/app/tests"), Path("/projects/app"), True),
+        (Path("/projects/app"), Path("/projects/app"), True),
+        (Path("/projects"), Path("/projects/app"), False),
+        (Path("/"), Path("/projects/app"), False),
+        pytest.param(
+            Path("e:/projects/app/tests"),
+            Path("e:/projects/app"),
+            True,
+            marks=skip_if_win,
+        ),
+        pytest.param(
+            Path("e:/projects/app"),
+            Path("e:/projects/app"),
+            True,
+            marks=skip_if_win,
+        ),
+        pytest.param(
+            Path("e:/"),
+            Path("e:/projects/app"),
+            False,
+            marks=skip_if_win,
+        ),
+        pytest.param(
+            Path("c:/testing"),
+            Path("e:/projects/app"),
+            False,
+            marks=skip_if_win,
+        ),
+        pytest.param(
+            Path("c:/projects/app"),
+            Path("e:/projects/app"),
+            False,
+            marks=skip_if_win,
+        ),
+    ],
+)
+def test_path_within_confcutdir(
+    path: Path, confcutdir: Optional[Path], expected: bool
+) -> None:
+    assert path_within_confcutdir(path=path, confcutdir=confcutdir) == expected