warnings: speed up work done in catch_warnings_for_item()

When setting up the warnings capture, filter strings (with the general
form `action:message:category:module:line`) are collected from the
cmdline, ini and item and applied. This happens for every test and other
cases.

To apply a string it needs to be parsed into a tuple, and it turns out
this is slow. Since we already vendor the parsing code from Python's
warnings.py, we can speed it up by caching the result. After splitting
the parsing part from the applying part, the parsing is pure and is
straightforward to cache.

An alternative is to parse ahead of time and reuse the result, however
the caching solution turns out cleaner and more general in this case.

On this benchmark:

    import pytest
    @pytest.mark.parametrize("x", range(5000))
    def test_foo(x): pass

Before:

============================ 5000 passed in 14.11s =============================
         14365646 function calls (13450775 primitive calls) in 14.536 seconds

After:

============================ 5000 passed in 13.61s =============================
         13290372 function calls (12375498 primitive calls) in 14.034 seconds

src/_pytest/warnings.py

@@ -1,34 +1,52 @@
+import re
 import sys
 import warnings
 from contextlib import contextmanager
+from functools import lru_cache
 from typing import Generator
+from typing import Tuple
 
 import pytest
+from _pytest.compat import TYPE_CHECKING
 from _pytest.main import Session
 
+if TYPE_CHECKING:
+    from typing_extensions import Type
 
-def _setoption(wmod, arg):
-    """
-    Copy of the warning._setoption function but does not escape arguments.
+
+@lru_cache(maxsize=50)
+def _parse_filter(
+    arg: str, *, escape: bool
+) -> "Tuple[str, str, Type[Warning], str, int]":
+    """Parse a warnings filter string.
+
+    This is copied from warnings._setoption, but does not apply the filter,
+    only parses it, and makes the escaping optional.
     """
     parts = arg.split(":")
     if len(parts) > 5:
-        raise wmod._OptionError("too many fields (max 5): {!r}".format(arg))
+        raise warnings._OptionError("too many fields (max 5): {!r}".format(arg))
     while len(parts) < 5:
         parts.append("")
-    action, message, category, module, lineno = [s.strip() for s in parts]
-    action = wmod._getaction(action)
-    category = wmod._getcategory(category)
-    if lineno:
+    action_, message, category_, module, lineno_ = [s.strip() for s in parts]
+    action = warnings._getaction(action_)  # type: str # type: ignore[attr-defined]
+    category = warnings._getcategory(
+        category_
+    )  # type: Type[Warning] # type: ignore[attr-defined]
+    if message and escape:
+        message = re.escape(message)
+    if module and escape:
+        module = re.escape(module) + r"\Z"
+    if lineno_:
         try:
-            lineno = int(lineno)
+            lineno = int(lineno_)
             if lineno < 0:
                 raise ValueError
         except (ValueError, OverflowError):
-            raise wmod._OptionError("invalid lineno {!r}".format(lineno))
+            raise warnings._OptionError("invalid lineno {!r}".format(lineno_))
     else:
         lineno = 0
-    wmod.filterwarnings(action, message, category, module, lineno)
+    return (action, message, category, module, lineno)
 
 
 def pytest_addoption(parser):
@@ -79,15 +97,15 @@ def catch_warnings_for_item(config, ihook, when, item):
         # filters should have this precedence: mark, cmdline options, ini
         # filters should be applied in the inverse order of precedence
         for arg in inifilters:
-            _setoption(warnings, arg)
+            warnings.filterwarnings(*_parse_filter(arg, escape=False))
 
         for arg in cmdline_filters:
-            warnings._setoption(arg)
+            warnings.filterwarnings(*_parse_filter(arg, escape=True))
 
         if item is not None:
             for mark in item.iter_markers(name="filterwarnings"):
                 for arg in mark.args:
-                    _setoption(warnings, arg)
+                    warnings.filterwarnings(*_parse_filter(arg, escape=False))
 
         yield