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

@@ -248,8 +248,10 @@ class Testdir:
 pytest.Testdir = Testdir  # type: ignore[attr-defined]
 
 
-@pytest.fixture
-def testdir(pytester: pytest.Pytester) -> Testdir:
+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.
@@ -285,15 +287,17 @@ class TempdirFactory:
 pytest.TempdirFactory = TempdirFactory  # type: ignore[attr-defined]
 
 
-@pytest.fixture(scope="session")
-def tmpdir_factory(request: pytest.FixtureRequest) -> TempdirFactory:
+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
 
-
-@pytest.fixture
-def tmpdir(tmp_path: Path) -> LEGACY_PATH:
+    @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.
@@ -400,6 +404,10 @@ def pytest_configure(config: pytest.Config) -> None:
     mp = pytest.MonkeyPatch()
     config.add_cleanup(mp.undo)
 
+    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
@@ -414,6 +422,8 @@ def pytest_configure(config: pytest.Config) -> None:
             _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)