Scoping a run to a diff

run -in-diff runs only the mutants a unified diff is relevant to, which is what a pull request needs:

git diff --unified=0 --merge-base origin/main > pr.diff
go run ./cmd/mutrim run -test-bin pkg.test -mutants mutants.json -in-diff pr.diff -out report.json

The scope is the commit-relevant mutants, found through the per-test trace: the mutants on the lines the diff adds (changed: true in report.json), the tests reaching any of them (they exercise the change), and every mutant those tests reach, in whichever file of the package. Most commit-relevant mutants lie outside the changed lines (Ojdanić et al., TOSEM 2023), so the lines alone miss them; -diff-expand=false keeps the lines alone.

Every other mutant is reported SKIPPED: it is not executed and counts towards no score.

Only the new side of the diff is read. A removed line holds no mutant, and a context line is code the diff did not change, so a mutant is kept when its span overlaps an added line; _test.go files are skipped, since mutants never live there. The diff’s paths are matched against a mutant’s file by path suffix, so a repository-relative diff matches both the absolute paths of a go list run and the execroot-relative ones of a Bazel run.

gen is unchanged: mutant IDs are content hashes, so mutants.json is the same either way and a scoped run stays comparable with a full one. The diff must be taken against the sources the test binary was built from; mutrim does not verify that, and a diff of another tree simply selects the wrong lines.

Under Bazel the diff is passed by path, which keeps the runner hermetic:

git diff --unified=0 --merge-base origin/main > "$PWD/pr.diff"
bazel test --test_env=MUTRIM_IN_DIFF="$PWD/pr.diff" //...:all
# the changed lines alone
bazel test --test_env=MUTRIM_IN_DIFF="$PWD/pr.diff" --test_arg=-diff-expand=false //...:all