Mutations

Mutations are the individual faults we inject into the program. Think of an accidental multiplication instead of an addition, a negated boolean expression, or something more complex. When a mutation is applied to the valid program, it effectively creates a mutant program, with slightly different behavior. These are used to evaluate the test suite, by seeing whether the test suite is able to “detect” this difference in behavior.

Mutations in mutest-rs are generated upfront, and combined into a single executable program for efficiency, alongside the generic mutation test harness (contained in the mutest-runtime crate).

The mutations mutest-rs produces can be controlled in various ways:

• by changing the mutation depth, the depth in the call graph up to which mutest-rs attempts to generate mutations, using the -d, or --depth argument (default: 3);
• by changing the set of mutation operators during the invocation of mutest-rs, changing the set of transformation rules applied to the program;
• by applying tool attributes to individual code elements (e.g. functions, statements, expressions), giving you fine-grained control over where and how mutation operators are applied.

Mutations in mutest-rs have a unique safety property, matching Rust’s notion of safety. Depending on the nature of your codebase, you may relax the safety requirements mutest-rs operates on, which influences how mutations are generated and evaluated. For more information, see Mutation Safety.

In addition to controlling the generation of mutations, mutest-rs also implements novel techniques for improving the efficiency of evaluating these mutations. By batching mutations, mutest-rs is able to improve the parallelism of mutation-related test case evaluation.