fixtures: avoid FixtureDef <-> FixtureManager reference cycle

There is no need to store the FixtureManager on each FixtureDef.

src/_pytest/fixtures.py

@@ -970,7 +970,7 @@ class FixtureDef(Generic[FixtureValue]):
 
     def __init__(
         self,
-        fixturemanager: "FixtureManager",
+        config: Config,
         baseid: Optional[str],
         argname: str,
         func: "_FixtureFunc[FixtureValue]",
@@ -984,7 +984,6 @@ class FixtureDef(Generic[FixtureValue]):
         _ispytest: bool = False,
     ) -> None:
         check_ispytest(_ispytest)
-        self._fixturemanager = fixturemanager
         # The "base" node ID for the fixture.
         #
         # This is a node ID prefix. A fixture is only available to a node (e.g.
@@ -1010,7 +1009,7 @@ class FixtureDef(Generic[FixtureValue]):
         if scope is None:
             scope = Scope.Function
         elif callable(scope):
-            scope = _eval_scope_callable(scope, argname, fixturemanager.config)
+            scope = _eval_scope_callable(scope, argname, config)
         if isinstance(scope, str):
             scope = Scope.from_user(
                 scope, descr=f"Fixture '{func.__name__}'", where=baseid
@@ -1657,7 +1656,7 @@ class FixtureManager:
             Set this if this is a unittest fixture.
         """
         fixture_def = FixtureDef(
-            fixturemanager=self,
+            config=self.config,
             baseid=nodeid,
             argname=name,
             func=func,

src/_pytest/python.py

@@ -1323,7 +1323,7 @@ class Metafunc:
                 fixturedef = name2pseudofixturedef[argname]
             else:
                 fixturedef = FixtureDef(
-                    fixturemanager=self.definition.session._fixturemanager,
+                    config=self.config,
                     baseid="",
                     argname=argname,
                     func=get_direct_param_fixture_func,

src/_pytest/setuponly.py

@@ -47,20 +47,23 @@ def pytest_fixture_setup(
                 else:
                     param = request.param
                 fixturedef.cached_param = param  # type: ignore[attr-defined]
-            _show_fixture_action(fixturedef, "SETUP")
+            _show_fixture_action(fixturedef, request.config, "SETUP")
 
 
-def pytest_fixture_post_finalizer(fixturedef: FixtureDef[object]) -> None:
+def pytest_fixture_post_finalizer(
+    fixturedef: FixtureDef[object], request: SubRequest
+) -> None:
     if fixturedef.cached_result is not None:
-        config = fixturedef._fixturemanager.config
+        config = request.config
         if config.option.setupshow:
-            _show_fixture_action(fixturedef, "TEARDOWN")
+            _show_fixture_action(fixturedef, request.config, "TEARDOWN")
             if hasattr(fixturedef, "cached_param"):
                 del fixturedef.cached_param  # type: ignore[attr-defined]
 
 
-def _show_fixture_action(fixturedef: FixtureDef[object], msg: str) -> None:
-    config = fixturedef._fixturemanager.config
+def _show_fixture_action(
+    fixturedef: FixtureDef[object], config: Config, msg: str
+) -> None:
     capman = config.pluginmanager.getplugin("capturemanager")
     if capman:
         capman.suspend_global_capture()