[nextest-runner] skip loading output for replays unless necessary

[sunshowers]

Measured this against a 727 test run locally -- time cargo nextest replay > /dev/null dropped from 0.15s to 0.135s.

We use a pure function with a conservative approximation to avoid having to track the cancel reason and other less relevant facts.

The property-based tests ensure the invariants.

[Copilot]

Pull request overview

This PR optimizes cargo nextest replay by skipping decompression/loading of recorded test output when reporter configuration guarantees the output will never be displayed, while keeping the decision logic conservative and validated via property tests.

Changes:

• Introduces OutputLoadDecider to conservatively decide whether a replayed output event needs its output bytes loaded.
• Adds a NotLoaded output variant during replay conversion and wires a per-event LoadOutput decision through replay conversion.
• Refactors output description types: OutputSpec now uses ChildOutputDesc, and recordings use the new ZipStoreOutputDescription (separate from live output).

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
nextest-runner/src/reporter/structured/libtest.rs	Handles the new NotLoaded output variant defensively (unreachable in live libtest reporting).
nextest-runner/src/reporter/mod.rs	Re-exports OutputLoadDecider for replay callers.
nextest-runner/src/reporter/events.rs	Refactors output description generics (ChildOutputDesc) and adds ChildOutputDescription::NotLoaded.
nextest-runner/src/reporter/error_description.rs	Adds NotLoaded match arm with invariant-based unreachable!.
nextest-runner/src/reporter/displayer/unit_output.rs	Factors output display overrides and adds NotLoaded match arm with invariant-based unreachable!.
nextest-runner/src/reporter/displayer/status_level.rs	Adds proptests establishing safety invariants for skipping output loads during replay.
nextest-runner/src/reporter/displayer/mod.rs	Re-exports OutputLoadDecider.
nextest-runner/src/reporter/displayer/imp.rs	Implements OutputLoadDecider and wires overrides through displayer logic.
nextest-runner/src/reporter/aggregator/junit.rs	Handles NotLoaded output variant defensively (unreachable in live JUnit reporting).
nextest-runner/src/record/summary.rs	Introduces ZipStoreOutputDescription for recorded output metadata and updates generic bounds to ChildOutputDesc.
nextest-runner/src/record/rerun.rs	Updates tests to use ZipStoreOutputDescription.
nextest-runner/src/record/replay.rs	Adds LoadOutput, threads it through conversion, and returns NotLoaded when skipping output loads.
nextest-runner/src/record/recorder.rs	Converts live ChildOutputDescription into ZipStoreOutputDescription and asserts NotLoaded is impossible during recording.
nextest-runner/src/record/mod.rs	Re-exports LoadOutput and ZipStoreOutputDescription.
nextest-runner/src/output_spec.rs	Renames associated type to ChildOutputDesc and maps Live/Recording specs to the new description types.
cargo-nextest/src/dispatch/core/replay.rs	Computes LoadOutput per event via OutputLoadDecider and passes it into replay conversion.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

In cancellation_only_hides_output, the final-output monotonicity check only covers the baseline.store_final == OutputStoreFinal::No case. Consider also asserting that if baseline.store_final is Yes { display_output: false } then with_cancel.store_final never becomes Yes { display_output: true } (i.e. cancellation never causes output bytes to be displayed at the end). This would better enforce the stated invariant and protect against future changes to compute_output_on_test_finished that might accidentally flip display_output from false to true.

[codecov]

Codecov Report

❌ Patch coverage is 93.86503% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.27%. Comparing base (78c6cbd) to head (929829e).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
nextest-runner/src/record/recorder.rs	78.57%	3 Missing ⚠️
nextest-runner/src/record/replay.rs	92.85%	2 Missing ⚠️
nextest-runner/src/reporter/aggregator/junit.rs	0.00%	1 Missing ⚠️
...xtest-runner/src/reporter/displayer/unit_output.rs	97.56%	1 Missing ⚠️
nextest-runner/src/reporter/error_description.rs	0.00%	1 Missing ⚠️
nextest-runner/src/reporter/events.rs	0.00%	1 Missing ⚠️
nextest-runner/src/reporter/structured/libtest.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3090      +/-   ##
==========================================
- Coverage   84.27%   84.27%   -0.01%     
==========================================
  Files         156      156              
  Lines       42446    42536      +90     
==========================================
+ Hits        35772    35847      +75     
- Misses       6674     6689      +15     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[trunk-io]

     

View Full Report ↗︎ ⋅ Docs