Add --last-failed functionality to rerun only failed tests

[andrewgazelka]

Warning: I made this with AI. If you think this is a good design, I can manually refactor the code to make it more readable.

Inspiration: https://x.com/mitsuhiko/status/1939105797448872265 mentions how LLMs perform better when they only need to rerun failed tests and there is not an easy way to do this in Rust.

Similar to pytest's --last-failed feature, this adds the ability to rerun only tests that failed in the previous run. This helps developers iterate faster when fixing failing tests.

New CLI options:

• --last-failed / --lf: Run only tests that failed in the previous run
• --failed-last / --fl: Run all tests, but prioritize failed tests first
• --clear-failed: Clear the failed test history

Failed tests are stored in target/nextest//-last-failed.json and are automatically updated after each test run. Tests that pass are removed from the failed list.

This implementation:

• Adds a new last_failed module in nextest-runner for data persistence
• Integrates with the test execution flow to track failures
• Uses the existing test filtering mechanism for --last-failed
• Updates documentation to describe the new feature

[codecov]

Codecov Report

Attention: Patch coverage is 75.58685% with 52 lines in your changes missing coverage. Please review.

Project coverage is 79.38%. Comparing base (ec6ce90) to head (197ac74).

Files with missing lines	Patch %	Lines
cargo-nextest/src/dispatch.rs	80.15%	25 Missing ⚠️
nextest-runner/src/reporter/last_failed.rs	71.08%	24 Missing ⚠️
cargo-nextest/src/errors.rs	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2472      +/-   ##
==========================================
- Coverage   79.42%   79.38%   -0.04%     
==========================================
  Files         107      108       +1     
  Lines       23964    24171     +207     
==========================================
+ Hits        19033    19188     +155     
- Misses       4931     4983      +52     

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[sunshowers]

Thank you for the contribution! This is definitely a feature I would like to add to nextest. However I would go about this in a more composable manner:

1. Do a full log of events received by the reporter (see [WIP] [nextest-runner] initial support for recording runs #1265 for an initial attempt that can probably be resurrected).
2. Allow loading the event dump in.
3. Add a cargo nextest rerun command which looks at the current list of tests + what's completed, and does a set difference to find out which tests to run.

The goal is to use the event log not just for rerunning tests but also for replaying test runs, etc.

I know it's more work, but would you be willing to do it? It would be really valuable.

[sunshowers]

Actually I like passing in --last-failed because it lets users say !! --last-failed or !! --lf in their shells. Something to consider though is what we expect to happen if the set of tests to run changes. It would be worth making a table to ensure we're reasoning about all the cases.

[andrewgazelka]

Actually I like passing in --last-failed because it lets users say !! --last-failed or !! --lf in their shells. Something to consider though is what we expect to happen if the set of tests to run changes. It would be worth making a table to ensure we're reasoning about all the cases.

I do not really have capacity for this at this time but perhaps I can work on it a bit later. I'd love to see something like this at sometime though. I'd love bundling cargo nextest into default tools our agent can use. 🥲