State of Mutation Testing at Google

Mutation testing assesses test suite efficacy by inserting small faults into programs and measuring the ability of the test suite to detect them. It is widely considered the strongest test criterion in terms of finding the most faults and it subsumes a number of other coverage criteria. Traditional mutation analysis is computationally prohibitive which hinders its adoption as an industry standard. In order to alleviate the computational issues, we present a diff-based probabilistic approach to mutation analysis that drastically reduces the number of mutants by omitting lines of code without statement coverage and lines that are determined to be uninteresting - we dub these arid lines. Furthermore, by reducing the number of mutants and carefully selecting only the most interesting ones we make it easier for humans to understand and evaluate the result of mutation analysis. We propose a heuristic for judging whether a node is arid or not, conditioned on the programming language. We focus on a code-review based approach and consider the effects of surfacing mutation results on developer attention. The described system is used by 6,000 engineers in Google on all code changes they author or review, affecting in total more than 13,000 code authors as part of the mandatory code review process. The system processes about 30% of all diffs across Google that have statement coverage calculated. About 15% of coverage statement calculations fail across Google.