Skipping untestable code

Some functions may be inherently hard to cover with tests, for example if:

• Generated mutants cause tests to hang.
• You’ve chosen to test the functionality by human inspection or some higher-level integration tests.
• The function has side effects or performance characteristics that are hard to test.
• You’ve decided the function is not important to test.

There are three ways to skip mutating some code:

1. Marking the function with an attribute within the source file.
2. Filtering by path in the config file or command line.
3. Filtering by function and mutant name in the config file or command line.

The results of all these filters can be previewed using the --list option.

Which filtering method to use?

• If some particular functions are hard to test with cargo-mutants, use an attribute, so that the skip is visible in the code.
• If a whole module is untestable, use a filter by path in the config file, so that the filter’s stored in the source tree and covers any new code in that module.
• If you want to permanently ignore a class of functions, such as Debug implementations, use a regex filter in the config file.
• If you want to run cargo-mutants just once, focusing on a subset of files, functions, or mutants, use command line options to filter by name or path.