Warn when a collected test module was imported too early, and fix three cases where it is

[RonnyPfannschmidt]

AI-authored. I asked Claude Opus 5 (via Claude Code) to dig into #1930. The
investigation, the repros, the fixes, the tests and this text are the agent's
work; I read it and I am posting it. Commits carry Co-authored-by trailers.
I will answer review questions on it myself.

Closes #1930.

What the agent found

#1930 asked for a warning when assertion rewriting silently doesn't happen. Chasing
it turned up that the warning, once written, fires in three places on main where
rewriting is genuinely missing today — so the fixes come first and the warning last.

The original #1929 repro still reproduces, in the shape the gist actually had it:

# conftest.py
import foo

# foo.py  — only a test module because it is named on the command line
def test_compare():
    assert Foo(1) == Foo(2)

$ pytest foo.py
E       AssertionError          # no introspection, no warning

It works if the module is named test_*.py (_should_rewrite matches python_files
regardless of who imports first), which is why #1930 looked fixed in 2018.

The commits

1. pytest pkg/__init__.py is never rewritten. _early_rewrite_bailout seeds
   _basenames_to_check_rewrite from each initial path's file stem, so an
   __init__.py contributes __init__ while the module is imported as pkg. The
   bailout fires and _should_rewrite — which would have said yes via isinitpath —
   is never reached.

2. --import-mode=importlib never rewrites a package's __init__.py.
   _import_module_using_spec asks sys.meta_path using module_path.parent; for
   pkg/sub/__init__.py that is pkg/sub, so PathFinder hunts for sub inside
   sub/ and finds nothing. The loop falls through to spec_from_file_location,
   which picks SourceFileLoader and bypasses the hook. Same tree under prepend
   gives assert 1 == 2; under importlib, bare AssertionError. Distinct from
   --import-mode=importlib breaks assertion rewriting #12044.

3. --pyargs t.py imports the test module while resolving it. search_pypath
   hands the argument to importlib.util.find_spec, which imports everything up to
   the last dot as a package — for t.py that imports t. The lookup then fails and
   pytest falls back to the path, but the module is in sys.modules and can no
   longer be rewritten.

4. The warning warn when tests raise plain assertion errors without annotation #1930 asked for. After importing a test module, check whether it
   came from the rewrite hook's loader; if not, but _should_rewrite says it should
   have, emit PytestAssertRewriteWarning naming both remedies.

   Re-asking _should_rewrite is what makes this usable. Without that gate the check
   fired ~150 times across pytest's own suite — every package __init__.py, every
   --doctest-modules target — for 87 failures. With it: 8 warnings, 5 failures,
   and all five were the three bugs above. With those fixed the suite is clean and
   the warning is silent.

5. Docs. assert.rst said pytest rewrites the modules it discovers, without
   saying that an earlier import takes that away. The pytest_assertrepr_compare
   example there does the same top-level conftest import that bit the no output from pytest_assertrepr_compare in conftest #1929 reporter.

Notes for review

• All four changelog fragments are named 1930.*; the three bugfixes have no issue
  numbers of their own. Happy to open separate issues and rename if you'd rather.
• warn_if_not_rewritten reaches for hook._should_rewrite. It lives in
  _pytest/assertion/__init__.py, next to the hook, so it stays inside the package —
  but say the word if you'd rather that became a public method on the hook.
• The --pyargs fix keys off a .py suffix. That matches the existing
  "looks like package module, but actually filename" comment and the documented
  contract (--pyargs takes module names), at the cost of no longer resolving a
  hypothetical real module named t.py.

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