Diff-based mutation testing is a mutation testing approach that only mutates lines affected by a code change under review. This approach scales independently of the code-base size and introduces test goals (mutants) that are directly relevant to an engineer’s goal such as fixing a bug, adding a new feature, or refactoring existing functionality. Google’s mutation testing service integrates diff-based mutation testing into the code review process and continuously gathers developer feedback on mutants surfaced during code review. To enhance the developer experience, the mutation testing service uses a number of manually-written rules that suppress not-useful mutants—mutants that have consistently received negative developer feedback. However, while effective, manually implementing suppression rules requires significant engineering time.

This paper proposes and evaluates MuRS, an automated approach that groups mutants by patterns in the source code under test and uses these patterns to rank and suppress future mutants based on historical developer feedback on mutants in the same group. To evaluate MuRS, we conducted an A/B testing study, comparing MuRS to the existing mutation testing service. Despite the strong baseline, which uses manually-written suppression rules, the results show a statistically significantly lower negative feedback ratio of 11.45% for MuRS versus 12.41% for the baseline. The results also show that MuRS is able to recover existing suppression rules implemented in the baseline. Finally, the results show that statement-deletion mutant groups received both the most positive and negative developer feedback, suggesting a need for additional context that can distinguish between useful and not-useful mutants in these groups. Overall, MuRS is able to recover existing suppression rules and automatically learn additional, finer-grained suppression rules from developer feedback.