Detect partial reductions in expected compiler warnings #159340

get_unexpected_improvements() ignores a file while it still has warnings, even when the count is below its finite ignore rule. It also reports an improvement for a zero-count rule with no warnings.

Reproducer, run with the interpreter built from the CPython checkout:

import sys
sys.path.insert(0, 'Tools/build')
import check_warnings

rules = {check_warnings.IgnoreRule('source.c', 2)}
print(check_warnings.get_unexpected_improvements(
    rules, {'source.c': [{}]}))

rules = {check_warnings.IgnoreRule('source.c', 0)}
print(check_warnings.get_unexpected_improvements(rules, {}))

Expected: The first call reports the reduction and returns 1; the second returns 0.

Observed: The first call returns 0. The second reports an improvement and returns 1.

Environment: CPython main, Windows debug build.

Linked PRs

• gh-159340: Detect partial reductions in expected compiler warnings #159341