fixtures: remove dirty optimization for request.getfixturevalue()

Currently for each `Function` we copy the `FunctionDefinition`'s
`_arg2fixturedefs`. This is done due to an ineffective optimization for
a dynamic `request.getfixturevalue()` when the fixture name wasn't
statically requested. In this case, we would save the dynamically-found
`FixtureDef` in `_arg2fixturedef`, such that if it is requested again in
the same item, it is returned immediately instead of doing a
`_matchfactories` check again. But this case is already covered by the
`_fixture_defs` optimization.

I've always disliked this copy and mutation. The `_arg2fixturedefs`
shenanigans performed during collection are hard enough to follow, and
this only adds to the complexity, due to the mutability and having
multiple different `_arg2fixturedefs` with different contents.

So summing up:

Pros: faster repeated `request.getfixturevalue()` in same test (ineffective)
Cons: complexity (reasoning about mutability), extra copy

Even without the `_fixture_defs` optimization, since
`request.getfixturevalue()` is mostly a last-resort thing, so shouldn't
be too common, and *repeated* calls to it in the same test should be
even less common, and if so `_matchfactories` shouldn't be *that* slow,
it should be fine to remove it.

Author: bluetech

src/_pytest/fixtures.py

@@ -363,7 +363,7 @@ def __init__(
         self,
         pyfuncitem: Function,
         fixturename: str | None,
-        arg2fixturedefs: dict[str, Sequence[FixtureDef[Any]]],
+        arg2fixturedefs: Mapping[str, Sequence[FixtureDef[Any]]],
         fixture_defs: dict[str, FixtureDef[Any]],
         *,
         _ispytest: bool = False,
@@ -372,10 +372,9 @@ def __init__(
         #: Fixture for which this request is being performed.
         self.fixturename: Final = fixturename
         self._pyfuncitem: Final = pyfuncitem
-        # The FixtureDefs for each fixture name requested by this item.
-        # Starts from the statically-known fixturedefs resolved during
-        # collection. Dynamically requested fixtures (using
-        # `request.getfixturevalue("foo")`) are added dynamically.
+        # The FixtureDefs for each fixture name statically requested by this
+        # item (computed during collection). Dynamically requested fixtures
+        # (using `request.getfixturevalue("foo")`) are not included here.
         self._arg2fixturedefs: Final = arg2fixturedefs
         # The evaluated argnames so far, mapping to the FixtureDef they resolved
         # to.
@@ -564,8 +563,6 @@ def _get_active_fixturedef(self, argname: str) -> FixtureDef[object]:
             # getfixturevalue(argname) which was naturally
             # not known at parsing/collection time.
             fixturedefs = self._fixturemanager.getfixturedefs(argname, self._pyfuncitem)
-            if fixturedefs is not None:
-                self._arg2fixturedefs[argname] = fixturedefs
         # No fixtures defined with this name.
         if fixturedefs is None:
             raise FixtureLookupError(argname, self)
@@ -669,7 +666,7 @@ def __init__(self, pyfuncitem: Function, *, _ispytest: bool = False) -> None:
         super().__init__(
             fixturename=None,
             pyfuncitem=pyfuncitem,
-            arg2fixturedefs=pyfuncitem._fixtureinfo.name2fixturedefs.copy(),
+            arg2fixturedefs=pyfuncitem._fixtureinfo.name2fixturedefs,
             fixture_defs={},
             _ispytest=_ispytest,
         )