Revert linter speedup; use authoritative pytest selection

The prefix-matching shortcut was a hand-rolled approximation of
pytest's selector. Go back to invoking pytest.main() per pattern so
selection matches CI exactly, even though it's slower (~89s). Keep the
exit-code assertion so collection errors fail loudly.

Author: adamtheturtle

ci/test_custom_linters.py

@@ -34,9 +34,9 @@ def pytest_collection_modifyitems(
         self.nodeids.update(item.nodeid for item in items)
 
 
-@pytest.fixture(scope="module")
-def all_tests() -> frozenset[str]:
-    """Collect every test node ID in the suite, exactly once.
+@beartype
+def _tests_from_pattern(*, ci_pattern: str) -> set[str]:
+    """From a CI pattern, get all tests ``pytest`` would collect.
 
     Uses a collection-hook plugin instead of parsing stdout: an in-process
     ``pytest.main()`` installs its own output capture, so reading from
@@ -57,42 +57,20 @@ def all_tests() -> frozenset[str]:
             # Unknown config option: retry_delay
             # ```
             "--disable-warnings",
-            ".",
+            ci_pattern,
         ],
         plugins=[plugin],
     )
     # Fail loudly on collection errors (import failures, syntax errors, etc.)
     # rather than silently using whatever items were captured before the
     # crash.
     assert exit_code == pytest.ExitCode.OK, (
-        f"Collection failed with exit code {exit_code}."
+        f"Collection for {ci_pattern!r} failed with exit code {exit_code}."
     )
-    return frozenset(plugin.nodeids)
-
+    return plugin.nodeids
 
-@beartype
-def _matches(*, nodeid: str, ci_pattern: str) -> bool:
-    """Whether ``pytest <ci_pattern>`` would have collected ``nodeid``.
-
-    The patterns in the CI matrix are all of the form ``path[/]`` or
-    ``path::Class[::method]``. A node ID matches if it equals the pattern,
-    is a directory child of a pattern ending with ``/``, or extends the
-    pattern at a ``::`` (sub-item), ``/`` (path), or ``[`` (parametrize)
-    boundary.
-    """
-    if nodeid == ci_pattern:
-        return True
-    if not nodeid.startswith(ci_pattern):
-        return False
-    if ci_pattern.endswith("/"):
-        return True
-    return nodeid[len(ci_pattern)] in {":", "/", "["}
 
-
-def test_ci_patterns_valid(
-    request: pytest.FixtureRequest,
-    all_tests: frozenset[str],
-) -> None:
+def test_ci_patterns_valid(request: pytest.FixtureRequest) -> None:
     """
     All of the CI patterns in the CI configuration match at least one
     test in
@@ -101,17 +79,14 @@ def test_ci_patterns_valid(
     ci_patterns = _ci_patterns(repository_root=request.config.rootpath)
 
     for ci_pattern in ci_patterns:
-        matched = {
-            n for n in all_tests if _matches(nodeid=n, ci_pattern=ci_pattern)
-        }
+        tests = _tests_from_pattern(ci_pattern=ci_pattern)
         message = f'"{ci_pattern}" does not match any tests.'
-        assert matched, message
+        assert tests, message
 
 
 def test_tests_collected_once(
     *,
     request: pytest.FixtureRequest,
-    all_tests: frozenset[str],
 ) -> None:
     """Each test in the test suite is collected exactly once.
 
@@ -121,9 +96,9 @@ def test_tests_collected_once(
     tests_to_patterns: dict[str, set[str]] = {}
 
     for pattern in ci_patterns:
-        for test in all_tests:
-            if _matches(nodeid=test, ci_pattern=pattern):
-                tests_to_patterns.setdefault(test, set()).add(pattern)
+        tests = _tests_from_pattern(ci_pattern=pattern)
+        for test in tests:
+            tests_to_patterns.setdefault(test, set()).add(pattern)
 
     for test_name, patterns in tests_to_patterns.items():
         message = (
@@ -133,5 +108,6 @@ def test_tests_collected_once(
         )
         assert len(patterns) == 1, message
 
+    all_tests = _tests_from_pattern(ci_pattern=".")
     assert tests_to_patterns.keys() - all_tests == set()
     assert all_tests - tests_to_patterns.keys() == set()