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The seeded robustness goldens (tests/robustness/goldens.json, tests::robustness_goldens) recorded what truck built for seeds 0-95, so they now fail on every seed SMLib builds differently. The kernel-cases ladder (tests/kernel-cases/ladder, cargo xtask kernel-cases --ladder), whose cases each carry their own right answer, supersedes them. Gone with them: fuzz::golden, the CI goldens lane (the goldens output, SLOW_SEEDS, the re-record hint), their nextest filter and priority entries, the mutants sample and the tests/robustness/ test-data mapping in affected.py. The robustness seeds and tests::robustness stay. The docs point at the ladder. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
70 lines
3.5 KiB
Markdown
70 lines
3.5 KiB
Markdown
# Mutation testing
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SolidCraft will move from truck to solidcore (#1597). The tests that have to catch regressions
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across that switch are the ones that don't depend on the kernel: the reference tests
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([docs/oracle.md](oracle.md)) and the engine's command tests. The reference for what the kernel
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builds is the kernel-cases ladder (`tests/kernel-cases/ladder/`,
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[docs/kernel-ladder.md](kernel-ladder.md)). Most of the kernel's own unit tests call
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truck-specific code and go away with it.
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Mutation testing checks those suites. [cargo-mutants](https://mutants.rs) makes small changes to
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the source, one at a time (`*` becomes `/`, a condition is negated, a function returns a default
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value), rebuilds, and runs the tests. A mutant the tests don't catch is a change in behaviour
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nobody would notice, and solidcore could get the same thing wrong without anyone noticing.
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## What judges a mutant
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`.cargo/mutants.toml` judges mutants by the black-box suites only:
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- `oracle_suite` (`apps/solidcraft-cli/tests/oracle_cases.rs`): every case in `tests/oracle` with
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expected values and no `known_failure` must pass, as `cargo xtask oracle` requires. It is
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`#[ignore]`d so CI doesn't replay the oracle twice.
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The kernel's unit tests are left out on purpose. A mutant that only they catch marks a behaviour
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the black-box suites would miss once the kernel is swapped.
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## Running it
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```
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cargo install cargo-mutants --locked
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CARGO_TARGET_DIR=$PWD/target cargo mutants --in-place \
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-f crates/kernel/src/measure.rs \
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--test-package solidcraft-cli --test-package solidcraft-engine \
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--baseline skip --timeout 900 --build-timeout 900
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```
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- Use a worktree you don't otherwise touch: `--in-place` writes each mutant into the source tree
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and restores it afterwards.
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- `--shard 0/200 --sharding round-robin` with `-d crates/kernel` takes an even sample (every 200th
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mutant) of the whole crate.
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- The `mutants` profile in `Cargo.toml` is the `ci` profile with incremental builds. It cuts the
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rebuild after a kernel change from about 3 minutes to about 15 seconds.
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- A mutant costs 1.5–5 minutes on a 12-core Mac, mostly the tests. The kernel has about 40,000
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mutants, so run a sample or one file at a time, never the whole crate. This is a tool for
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finding gaps, not a CI gate.
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Two problems with cargo-mutants 27.1 are behind the flags above:
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- The baseline (the unmutated run) ignores `--test-package` and tests the mutated crate, which
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fails. Skip it with `--baseline skip`, set `--timeout` yourself, and check the unmutated tree
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passes first.
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- `--re` and `--exclude-re` don't filter "delete field" mutants. Saved diffs in
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`mutants.out/diff/` name the mutation, not the file, in their `+++` line: apply one with
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`patch <file> < <diff>`.
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## Pilot results (October 2026, `cb857f1f`)
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| Sample | Mutants that build | Caught by the black-box suites |
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|---|---:|---:|
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| `crates/kernel/src/measure.rs`, every mutant | 85 | 85% |
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| Whole kernel, every 200th mutant | 173 | 15% (95% interval 10–21%) |
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Out of 49 of the kernel sample's survivors, the kernel's own unit tests caught 21 (43%) and
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nothing caught 28 (57%). About 11% of survivors swap `>` and `>=` on floating-point values that
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are never exactly equal, so they don't change behaviour.
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The black-box suites are weakest on tessellation and the display mesh (0 of 19 mutants caught),
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offset, shell, emboss and lip (0 of 14), blends (15%) and STEP/IGES exchange (17%). For a
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survivor that a kernel unit test catches, that test's design is the starting point for a new
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oracle case or engine test.
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