Filtering functions and mutants

You can filter mutants by name, using the --re and --exclude-re command line options and the corresponding examine_re and exclude_re config file options.

These options are useful if you want to run cargo-mutants just once, focusing on a subset of functions or mutants.

These options filter mutants by the full name of the mutant, which includes the function name, file name, and a description of the change, as shown in the output of cargo mutants --list.

For example, one mutant name might be:

src/ replace <impl Serialize for ScenarioOutcome>::serialize -> Result<S::Ok, S::Error> with Ok(Default::default())

Within this name, your regex can match any substring, including for example:

  • The filename
  • The trait, impl Serialize
  • The struct name, ScenarioOutcome
  • The function name, serialize
  • The mutated return value, with Ok(Defualt::default()), or any part of it.

The regex matches a substring, but can be anchored with ^ and $ to require that it match the whole name.

The regex syntax is defined by the regex crate.

These filters are applied after filtering by filename, and --re is applied before --exclude-re.


  • -E 'impl Debug' -- don't test impl Debug methods, because coverage of them might be considered unimportant.

  • -F 'impl Serialize' -F 'impl Deserialize' -- test implementations of these two traits.

Configuring filters by name

Mutants can be filtered by name in the .cargo/mutants.toml file. The exclude_re and examine_re keys are each a list of strings.

This can be helpful if you want to systematically skip testing implementations of certain traits, or functions with certain names.

From cargo-mutants 23.11.2 onwards, if the command line options are given then the corresponding config file option is ignored.

For example:

exclude_re = ["impl Debug"] # same as -E