New-test detection ignores positional test-name filters in the run passthrough #39

Problem

New-test detection lists tests with cargo nextest list and compares against the coverage DB. The build args for that listing are extracted from the post--- passthrough by cargo_build_args (src/collect.rs), which is an allowlist of cargo build flags — it deliberately drops everything else, including positional test-name filters and -E/--filterset.

So when the user runs cargo affected run -- some_test_name:

• cargo nextest list enumerates all tests (the positional filter is dropped).
• cargo nextest run receives the positional filter and runs only matching tests.

A new test (absent from the DB) that does not match some_test_name is still flagged new, lands in the generated --config-file selection, and is handed to nextest run — which then skips it because it doesn't match the positional filter. If the selection collapses to only such tests, nextest run exits 4 (error: no tests to run). run_tests in src/run.rs does not pass --no-tests=warn, so that exit propagates.

This is the same failure class as the bugs fixed in #38, via a different trigger. It is pre-existing and was not introduced or worsened by #38 — flagged there as a known follow-up.

Proposed fix

Two coordinated changes:

1. cargo_build_args → a run-only-flag denylist. Instead of allowlisting build flags, forward everything to nextest list except nextest run-only flags (--retries, --no-fail-fast, --no-tests, --max-fail, -j/--test-threads, --status-level, …). That carries positionals and -E through, so the listing reflects the run's full filter config.
2. Key detection off filter-match.status instead of the ignored boolean. cargo nextest list --message-format json tags every testcase with filter-match: {status: matches|mismatch, …}. A test counts toward new/stranded/affected selection only when filter-match.status == "matches". This single predicate subsumes #[ignore]d tests, positional/-E filters, --run-ignored, and the project's own default-filter — unifying the three exclusion checks (new/stranded/affected) that fix: correct new-test detection for ignored tests and feature builds #38 left as separate listing.ignored filters.

Verified: cargo nextest list <positional-filter> still enumerates all testcases (each tagged with its filter-match status), so forwarding filters to list does not shrink Listing.tests and does not break collect --diff's prune.

Tradeoff

The denylist must stay complete against nextest's CLI. A future nextest run-only flag not in the denylist would be forwarded to cargo nextest list, which rejects unknown args and exits non-zero — cargo affected run -- --that-new-flag would break loudly until the denylist is updated. The current allowlist instead fails silently on a future cargo build flag (listing mismatch, no error). The denylist's loud failure is arguably better aligned with the repo's "fail loudly over silently degrading" principle, but it is a maintenance commitment worth noting.

Ref #38.

This was written by Claude Code on behalf of Maximilian Roos