Warn when a mark is applied to a fixture

Fixes pytest-dev#3664

src/_pytest/deprecated.py

@@ -95,6 +95,8 @@
     "see https://docs.pytest.org/en/latest/deprecations.html#node-fspath-in-favor-of-pathlib-and-node-path",
 )
 
+MARKED_FIXTURE = PytestDeprecationWarning("Marks cannot be applied to fixtures")
+
 # You want to make some `__init__` or function "private".
 #
 #   def my_private_function(some, args):

src/_pytest/fixtures.py

@@ -54,6 +54,7 @@
 from _pytest.config.argparsing import Parser
 from _pytest.deprecated import check_ispytest
 from _pytest.deprecated import FILLFUNCARGS
+from _pytest.deprecated import MARKED_FIXTURE
 from _pytest.deprecated import NODE_FSPATH
 from _pytest.deprecated import YIELD_FIXTURE
 from _pytest.mark import Mark
@@ -1222,6 +1223,9 @@ def __call__(self, function: _FixtureFunction) -> _FixtureFunction:
                 "fixture is being applied more than once to the same function"
             )
 
+        if hasattr(function, "pytestmark"):
+            warnings.warn(MARKED_FIXTURE, stacklevel=2)
+
         function = wrap_function_to_error_out_if_called_directly(function, self)
 
         name = self.name or function.__name__

src/_pytest/mark/structures.py

@@ -29,6 +29,7 @@
 from ..compat import NotSetType
 from _pytest.config import Config
 from _pytest.deprecated import check_ispytest
+from _pytest.deprecated import MARKED_FIXTURE
 from _pytest.outcomes import fail
 from _pytest.warning_types import PytestUnknownMarkWarning
 
@@ -399,6 +400,9 @@ def store_mark(obj, mark: Mark) -> None:
     assert isinstance(mark, Mark), mark
     # Always reassign name to avoid updating pytestmark in a reference that
     # was only borrowed.
+    if hasattr(obj, "_pytestfixturefunction"):
+        warnings.warn(MARKED_FIXTURE, stacklevel=2)
+
     obj.pytestmark = get_unpacked_marks(obj) + [mark]