legacypath: only add `testdir` and `tmpdir` fixtures if corresponding plugins are registered

This preserves the existing behavior and gives a proper error message
in case e.g. `testdir` is requested without the `pytester` plugin being
loaded.

doc/en/reference/reference.rst

@@ -638,7 +638,7 @@ tmpdir
 
 :ref:`tmpdir and tmpdir_factory`
 
-.. autofunction:: _pytest.legacypath.tmpdir()
+.. autofunction:: _pytest.legacypath.LegacyTmpdirPlugin.tmpdir()
     :no-auto-options:
 
 

src/_pytest/legacypath.py

@@ -248,15 +248,17 @@ class Testdir:
 pytest.Testdir = Testdir  # type: ignore[attr-defined]
 
 
-@pytest.fixture
-def testdir(pytester: pytest.Pytester) -> Testdir:
-    """
-    Identical to :fixture:`pytester`, and provides an instance whose methods return
-    legacy ``LEGACY_PATH`` objects instead when applicable.
+class LegacyTestdirPlugin:
+    @staticmethod
+    @pytest.fixture
+    def testdir(pytester: pytest.Pytester) -> Testdir:
+        """
+        Identical to :fixture:`pytester`, and provides an instance whose methods return
+        legacy ``LEGACY_PATH`` objects instead when applicable.
 
-    New code should avoid using :fixture:`testdir` in favor of :fixture:`pytester`.
-    """
-    return Testdir(pytester, _ispytest=True)
+        New code should avoid using :fixture:`testdir` in favor of :fixture:`pytester`.
+        """
+        return Testdir(pytester, _ispytest=True)
 
 
 @final
@@ -285,29 +287,31 @@ class TempdirFactory:
 pytest.TempdirFactory = TempdirFactory  # type: ignore[attr-defined]
 
 
-@pytest.fixture(scope="session")
-def tmpdir_factory(request: pytest.FixtureRequest) -> TempdirFactory:
-    """Return a :class:`pytest.TempdirFactory` instance for the test session."""
-    # Set dynamically by pytest_configure().
-    return request.config._tmpdirhandler  # type: ignore
+class LegacyTmpdirPlugin:
+    @staticmethod
+    @pytest.fixture(scope="session")
+    def tmpdir_factory(request: pytest.FixtureRequest) -> TempdirFactory:
+        """Return a :class:`pytest.TempdirFactory` instance for the test session."""
+        # Set dynamically by pytest_configure().
+        return request.config._tmpdirhandler  # type: ignore
 
+    @staticmethod
+    @pytest.fixture
+    def tmpdir(tmp_path: Path) -> LEGACY_PATH:
+        """Return a temporary directory path object which is unique to each test
+        function invocation, created as a sub directory of the base temporary
+        directory.
 
-@pytest.fixture
-def tmpdir(tmp_path: Path) -> LEGACY_PATH:
-    """Return a temporary directory path object which is unique to each test
-    function invocation, created as a sub directory of the base temporary
-    directory.
+        By default, a new base temporary directory is created each test session,
+        and old bases are removed after 3 sessions, to aid in debugging. If
+        ``--basetemp`` is used then it is cleared each session. See :ref:`base
+        temporary directory`.
 
-    By default, a new base temporary directory is created each test session,
-    and old bases are removed after 3 sessions, to aid in debugging. If
-    ``--basetemp`` is used then it is cleared each session. See :ref:`base
-    temporary directory`.
+        The returned object is a `legacy_path`_ object.
 
-    The returned object is a `legacy_path`_ object.
-
-    .. _legacy_path: https://py.readthedocs.io/en/latest/path.html
-    """
-    return legacy_path(tmp_path)
+        .. _legacy_path: https://py.readthedocs.io/en/latest/path.html
+        """
+        return legacy_path(tmp_path)
 
 
 def Cache_makedir(self: pytest.Cache, name: str) -> LEGACY_PATH:
@@ -400,19 +404,25 @@ def pytest_configure(config: pytest.Config) -> None:
     mp = pytest.MonkeyPatch()
     config.add_cleanup(mp.undo)
 
-    # Create TmpdirFactory and attach it to the config object.
-    #
-    # This is to comply with existing plugins which expect the handler to be
-    # available at pytest_configure time, but ideally should be moved entirely
-    # to the tmpdir_factory session fixture.
-    try:
-        tmp_path_factory = config._tmp_path_factory  # type: ignore[attr-defined]
-    except AttributeError:
-        # tmpdir plugin is blocked.
-        pass
-    else:
-        _tmpdirhandler = TempdirFactory(tmp_path_factory, _ispytest=True)
-        mp.setattr(config, "_tmpdirhandler", _tmpdirhandler, raising=False)
+    if config.pluginmanager.has_plugin("pytester"):
+        config.pluginmanager.register(LegacyTestdirPlugin, "legacypath-pytester")
+
+    if config.pluginmanager.has_plugin("tmpdir"):
+        # Create TmpdirFactory and attach it to the config object.
+        #
+        # This is to comply with existing plugins which expect the handler to be
+        # available at pytest_configure time, but ideally should be moved entirely
+        # to the tmpdir_factory session fixture.
+        try:
+            tmp_path_factory = config._tmp_path_factory  # type: ignore[attr-defined]
+        except AttributeError:
+            # tmpdir plugin is blocked.
+            pass
+        else:
+            _tmpdirhandler = TempdirFactory(tmp_path_factory, _ispytest=True)
+            mp.setattr(config, "_tmpdirhandler", _tmpdirhandler, raising=False)
+
+        config.pluginmanager.register(LegacyTmpdirPlugin, "legacypath-tmpdir")
 
     # Add Cache.makedir().
     mp.setattr(pytest.Cache, "makedir", Cache_makedir, raising=False)