--list: Show what mutants could be generated, without running them.
--list, also include a diff of the source change for each mutant.
--list, show the list in json for easier processing by other programs.
(The same format is written to
mutants.out/mutants.json when running tests.)
cargo check on all generated mutants to find out which ones are viable, but don't actually run the tests. (This is primarily useful when debugging cargo-mutants.)