cargo-mutants changelog


  • Fixed: The auto-set timeout for building mutants is now 2 times the baseline build time times the number of jobs, with a minimum of 20 seconds. This was changed because builds of mutants contend with each other for access to CPUs and may be slower than the baseline build.


  • Fixed: Follow path attributes on mod statements.

  • New: --build-timeout and --build-timeout-multiplier options for setting timeouts for the build and check cargo phases.

  • Changed: --timeout-multiplier now overrides timeout_multiplier from .cargo/mutants.toml.

  • Changed: --timeout and --timeout-multiplier are now conflicting options.


  • Changes: Baselines and mutants are now built with cargo test --no-run rather than cargo build --tests as previously. This avoids wasted build effort if the dev and test Cargo profiles are not the same, and may better distinguish build failures from test failures. With --test-tool=nextest, the corresponding cargo nextest run --no-run is used.

  • Fixed: .ignore files can no longer affect source tree copying, so test files listed in a .ignore (e.g. *.snap for Insta snapshots) are now correctly copied into temporary build directories.

  • Fixed: Don't visit files marked with #![cfg(test)] (or other inner attributes that generally cause code to be skipped.)

  • Fixed: Paths to module files nested within mod blocks are now correctly resolved.

  • Added: Document stability policy in the manual.

  • New: Generate mutations that delete the ! and - unary operators.


  • Fixed: cargo install cargo-mutants without --locked was failing due to breaking API changes in some unstable dependencies.

  • Changed: In globs, * no longer matches path separators, only parts of a filename. For example, src/*.rs will now only match files directly in src/, not in subdirectories. To include subdirectories, use ** as in src/**/*.rs.

    And, patterns that do not contain a path separator match directories at any level, and all files within them. For example, -f db will match src/ and src/db/ and all files in src/db/ or in other/db.

    This may break existing configurations but is considered a bug fix because it brings the behavior in line with other tools and allows more precise expressions.

  • Changed: Minimum Rust version (to build cargo-mutants, not to use it) increased to 1.74.

  • Changed: Removed the count of failure from mutants.out/outcomes.json: it was already the case that every outcome received some other classification, so the count was always zero.


  • New: --features, --no-default-features and --all-features options are passed through to Cargo.

  • Changed: Minimum Rust version (to build cargo-mutants, not to use it) increased to 1.73.

  • New: Warn if nextest returns an exit code indicating some failure other than test failure, such as an internal error in nextest.

  • New: json output includes the exit code of subprocesses, and the signal if it was killed by a signal.

  • Changed: Set INSTA_FORCE_PASS=0 (in addition to previously INSTA_UPDATE=no) when running tests, so that tests that use the Insta library don't write updates back into the source directory, and so don't falsely pass.

  • New: --timeout-multiplier option allows setting the timeout for mutants to be a multiple of the baseline timeout, rather than a fixed time.


  • New: Colored output can be enabled in CI or other noninteractive situations by passing --colors=always, or setting CARGO_TERM_COLOR=always, or CLICOLOR_FORCE=1. Colors can similarly be forced off with --colors=never, CARGO_TERM_COLOR=never, or NO_COLOR=1.


  • New: --in-place option tests mutations in the original source tree, without copying the tree. This is faster and uses less disk space, but it's incompatible with --jobs, and you must be careful not to edit or commit the source tree while tests are running.


  • New: Mutate +, -, *, /, %, &, ^, |, <<, >> binary ops, and their corresponding assignment ops like +=.

  • New: --baseline=skip option to skip running tests in an unmutated tree, when they're already been checked externally.

  • Changed: Stop generating mutations of || and && to != and ||, because it seems to raise too many low-value false positives that may be hard to test.

  • Fixed: Colors in command-line help and error messages.


  • New! cargo mutants --test-tool nextest, or test_tool = "nextest" in .cargo/mutants.toml runs tests under Nextest. Some trees have tests that only work under Nextest, and this allows them to be tested. In other cases Nextest may be significantly faster, because it will exit soon after the first test failure.

  • Fixed: Fixed spurious "Patch input contains repeated filenames" error when --in-diff is given a patch that deletes multiple files.


  • New: A --shard k/n allows you to split the work across n independent parallel cargo mutants invocations running on separate machines to get a faster overall solution on large suites. You, or your CI system, are responsible for launching all the shards and checking whether any of them failed.

  • Improved: Better documentation about -j, with stronger recommendations not to set it too high.

  • New: Binary releases on GitHub through cargo-dist.


  • Improved progress bars and console output, including putting the outcome of each mutant on the left, and the overall progress bar at the bottom. Improved display of estimated remaining time, and other times.

  • Fixed: Correctly traverse mod statements within package top source files that are not named or, by following the path setting of each target within the manifest.

  • Improved: Don't generate function mutants that have the same AST as the code they're replacing.


An exciting step forward: cargo-mutants can now generate mutations smaller than a whole function. To start with, several binary operators are mutated.

  • New: Mutate == to != and vice versa.

  • New: Mutate && to || and vice versa, and mutate both of them to == and !=.

  • New: Mutate <, <=, >, >=.

  • Changed: If no mutants are generated then cargo mutants now exits successfully, showing a warning. (Previously it would exit with an error.) This works better with --in-diff in CI, where it's normal that some changes may not have any mutants.

  • Changed: Include column numbers in text listings of mutants and output to disambiguate smaller-than-function mutants, for example if there are several operators that can be changed on one line. This also applies to the names used for regex matching, so may break some regexps that match the entire line (sorry). The new option --line-col=false turns them both off in --list output.

  • Changed: In the mutants.json format, replaced the function, line, and return_type fields with a function submessage (including the name and return type) and a span indicating the entire replaced region, to better handle smaller-than-function mutants. Also, the function includes the line-column span of the entire function.


  • Changed: If --file or --exclude are set on the command line, then they replace the corresponding config file options. Similarly, if --re is given then the examine_re config key is ignored, and if --exclude-re is given then exclude_regex is ignored. (Previously the values were combined.) This makes it easier to use the command line to test files or mutants that are normally not tested.

  • Improved: By default, files matching gitignore patterns (including in parent directories, per-user configuration, and info/exclude) are excluded from copying to temporary build directories. This should improve performance in some large trees with many files that are not part of the build. This behavior can be turned off with --gitignore=false.

  • Improved: Run cargo metadata with --no-deps, so that it doesn't download and compute dependency information, which can save time in some situations.

  • Added: Alternative aliases for command line options, so you don't need to remember if it's "regex" or "re": --regex, --examine-re, --examine-regex (all for names to include) and --exclude-regex.

  • Added: Accept --manifest-path as an alternative to -d, for consistency with other cargo commands.


  • New --in-diff FILE option tests only mutants that are in the diff from the given file. This is useful to avoid testing mutants from code that has not changed, either locally or in CI.


  • Changed: cargo mutants now tries to match the behavior of cargo test when run within a workspace. If run in a package directory, it tests only that package. If run in a workspace that is not a package (a "virtual workspace"), it tests the configured default packages, or otherwise all packages. This can all be overridden with the --package or --workspace options.

  • New: generate key-value map values from types like BTreeMap<String, Vec<u8>>.

  • Changed: Send trace messages to stderr rather stdout, in part so that it won't pollute json output.


  • The baseline test (with no mutants) now tests only the packages in which mutants will be generated, subject to any file or regex filters. This should both make baseline tests faster, and allow testing workspaces in which some packages have non-hermetic tests.


  • Mutate the known collection types BinaryHeap, BTreeSet, HashSet, LinkedList, and VecDeque to generate empty and one-element collections using T::new() and T::from_iter(..).

  • Mutate known container types like Arc, Box, Cell, Mutex, Rc, RefCell into T::new(a).

  • Mutate unknown types that look like containers or collections T<A> or T<'a, A>' and try to construct them from an A with T::from_iter, T::new, and T::from.

  • Minimum Rust version updated to 1.70.

  • Mutate Cow<'_, T> into Owned and Borrowed variants.

  • Mutate functions returning &[T] and &mut [T] to return leaked vecs of values.

  • Mutate (A, B, C, ...) into the product of all replacements for a, b, c, ...

  • The combination of options --list --diff --json is now supported, and emits a diff key in the JSON.

  • Mutate -> impl Iterator<Item = A> to produce empty and one-element iterators of the item type.


  • Fixed a bug causing an assertion failure when cargo-mutants was run from a subdirectory of a workspace. Thanks to Adam Chalmers!

  • Generate HttpResponse::Ok().finish() as a mutation of an Actix HttpResponse.


  • Generate Box::leak(Box::new(...)) as a mutation of functions returning &mut.

  • Add a concept of mutant "genre", which is included in the json listing of mutants. The only genre today is FnValue, in which a function body is replaced by a value. This will in future allow filtering by genre.

  • Recurse into return types, so that for example Result<bool> can generate Ok(true) and Ok(false), and Some<T> generates None and every generated value of T. Similarly for Box<T>, Vec<T>, Rc<T>, Arc<T>.

  • Generate specific values for integers: [0, 1] for unsigned integers, [0, 1, -1] for signed integers; [1] for NonZero unsigned integers and [1, -1] for NonZero signed integers.

  • Generate specific values for floats: [0.0, 1.0, -1.0].

  • Generate (fixed-length) array values, like [0; 256], [1; 256] using every recursively generated value for the element type.


"Pickled crab"

Released 2023-05-27

  • cargo mutants can now successfully test packages that transitively depend on a different version of themselves, such as itertools. Previously, cargo-mutants used the cargo --package option, which is ambiguous in this case, and now it uses --manifest-path instead.

  • Mutate functions returning &'_ str (whether a lifetime is named or not) to return "xyzzy" and "".

  • Switch to CalVer numbering.


Released 2023-05-05

  • Mutate functions returning String to String::new() rather than "".into(): same result but a bit more idiomatic.

  • New --leak-dirs option, for debugging cargo-mutants.

  • Update to syn 2.0, adding support for new Rust syntax.

  • Minimum supported Rust version increased to 1.65 due to changes in dependencies.

  • New --error option, to cause functions returning Result to be mutated to return the specified error.

  • New --no-config option, to disable reading .cargo/mutants.toml.


Released 2023-04-01

  • Don't mutate unsafe fns.

  • Don't mutate functions that never return (i.e. -> !).

  • Minimum supported Rust version increased to 1.64 due to changes in dependencies.

  • Some command-line options can now also be configured through environment variables: CARGO_MUTANTS_JOBS, CARGO_MUTANTS_TRACE_LEVEL.

  • New command line option --minimum-test-timeout and config file variable minimum_test_timeout join existing environment variable CARGO_MUTANTS_MINIMUM_TEST_TIMEOUT, to allow boosting the minimum, especially for test environments with poor or uneven throughput.

  • Changed: Renamed fields in outcomes.json from cargo_result to process_status and from command to argv.

  • Warn if no mutants were generated or if all mutants were unviable.


Released 2023-01-05

  • Converted most of the docs to a book available at

  • Fixed: Correctly find submodules that don't use mmod.rsnaming, e.g. when descending fromsrc/foo.rstosrc/foo/ Also handle module names that are raw identifiers usingr#`. (Thanks to @kpreid for the report.)


Thankful mutants!

  • Fixed: Files that are excluded by filters are also excluded from --list-files.

  • Fixed: --exclude-re and --re can match against the return type as shown in --list.

  • New: A .cargo/mutants.toml file can be used to configure standard filters and cargo args for a project.


Released 2022-10-31

Spooky mutants!

  • Fixed support for the Mold linker, or for other options passed via RUSTFLAGS or CARGO_ENCODED_RUSTFLAGS. (See the instructions in

  • Source trees are walked by following mod statements rather than globbing the directory. This is more correct if there are files that are not referenced by mod statements. Once attributes on modules are stable in Rust ( this opens a path to skip mods using attributes.


Released 2022-10-30

Fearless concurrency!

  • cargo-mutants can now run multiple cargo build and test tasks in parallel, to make better use of machine resources and find mutants faster, controlled by --jobs.

  • The minimum Rust version to build cargo-mutants is now 1.63.0. It can still be used to test code under older toolchains.


Released 2022-09-29

  • cargo-mutants is now finds no uncaught mutants in itself! Various tests were added and improved, particularly around handling timeouts.

  • New: --re and --exclude-re options to filter by mutant name, including the path. The regexps match against the strings printed by --list.


Released 2022-09-24

  • New: cargo mutants --completions SHELL to generate shell completions using clap_complete.

  • Changed: cargo-mutants no longer builds in the source directory, and no longer copies the target/ directory to the scratch directory. Since cargo-mutants now sets RUSTFLAGS to avoid false failures from warnings, it is unlikely to match the existing build products in the source directory target/, and in fact building there is just likely to cause rebuilds in the source. The behavior now is as if --no-copy-target was always passed. That option is still accepted, but it has no effect.

  • Changed: cargo-mutants finds all possible mutations before doing the baseline test, so that you can see earlier how many there will be.

  • New: Set INSTA_UPDATE=no so that tests that use the Insta library don't write updates back into the source directory, and so don't falsely pass.


Released 2022-09-12

  • Fixed: Don't try to mutate functions within test targets, e.g. within tests/**/*.rs.

  • New: missed.txt, caught.txt, timeout.txt and unviable.txt files are written in to the output directory to make results easier to review later.

  • New: --output creates the specified directory if it does not exist.

  • Internal: Switched from Argh to Clap for command-line parsing. There may be some small changes in CLI behavior and help formatting.


Released 2022-08-21

A 1.0 release to celebrate that with the addition of workspace handling, cargo-mutants gives useful results on many Rust projects.

  • New: Supports workspaces containing multiple packages. Mutants are generated for all relevant targets in all packages, and mutants are subject to the tests of their own package. cargo mutants --list-files --json and cargo mutants --list --json now includes package names for each file or mutant.

  • Improved: Generate mutations in cdylib, rlib, and ever other *lib target. For example, this correctly exercises Wasm projects.

  • Improved: Write mutants.out/outcomes.json after the source-tree build and baseline tests so that it can be observed earlier on.

  • Improved: mutants.out/outcomes.json includes the commands run.


Released 2022-08-20

  • New --exclude command line option to exclude source files from mutants generation, matching a glob.

  • New: CARGO_MUTANTS_MINIMUM_TEST_TIMEOUT sets a minimum timeout for cargo tests, in seconds. This can be used to allow more time on slow CI builders. If unset the default is still 20s.

  • Added: A new mutants.out/debug.log with internal debugging information.

  • Improved: The time for check, build, and test is now shown separately in progress bars and output, to give a better indication of which is taking more time in the tree under test. Also, times are show in seconds with one decimal place, and they are styled more consistently.

  • Improved: More consistent use of 'unviable' and other terms for outcomes in the UI.


Released 2022-08-07

cargo-mutants 0.2.10 comes with improved docs, and the new -C option can be used to pass options like --release or --all-features to cargo.

  • Added: --cargo-arg (or -C for short) allows passing arguments to cargo commands (check, build, and test), for example to set --release or --features.

  • Improved: Works properly if run from a subdirectory of a crate, or if -d points to a subdirectory of a crate.

  • Improved: Various docs.

  • Improved: Relative dependencies within the source tree are left as relative paths, and will be built within the scratch directory. Relative dependencies outside the source tree are still rewritten as absolute paths.


Released 2022-07-30

  • Faster: cargo mutants no longer runs cargo check before building, in cases where the build products are wanted or tests will be run. This saves a significant amount of work in build phases; in some trees cargo mutants is now 30% faster. (In trees where most of the time is spent running tests the effect will be less.)

  • Fixed: Open log files in append mode to fix messages from other processes occasionally being partly overwritten.

  • Improved: cargo mutants should now give useful results in packages that use #![deny(unused)] or other mechanisms to reject warnings. Mutated functions often ignore some parameters, which would previously be rejected by this configuration without proving anything interesting about test coverage. Now, --cap-lints=allow is passed in RUSTFLAGS while building mutants, so that they're not falsely rejected and the tests can be exercised.

  • Improved: The build dir name includes the root package name.

  • Improved: The progress bar shows more information.

  • Improved: The final message shows how many mutants were tested and how long it took.


Released 2022-07-18

  • New: Summarize the overall number of mutants generated, caught, missed, etc, at the end.

  • Fixed: Works properly with crates that have relative path dependencies in Cargo.toml or .cargo/config.toml, by rewriting them to absolute paths in the scratch directory.


Released 2022-07-11

  • New: You can skip functions by adding #[cfg_attr(test, mutants::skip), in which case the mutants crate can be only a dev-dependency.

  • Improved: Don't generate pointless mutations of functions with an empty body (ignoring comments.)

  • Improved: Remove extra whitespace from the display of function names and return types: the new formatting is closer to the spacing used in idiomatic Rust.

  • Improved: Show the last line of compiler/test output while running builds, so that it's more clear where time is being spent.

  • Docs: Instructions on how to check for missed mutants from CI.


Released 2022-04-17

  • Improved: Find source files by looking at cargo metadata output, rather than assuming they're in src/**/*.rs. This makes cargo mutants work properly on trees where it previously failed to find the source.

  • New --version option.

  • New: Write a lock.json into the mutants.out directory including the start timestamp, cargo-mutants version, hostname and username. Take a lock on this file while cargo mutants is running, so that it doesn't crash or get confused if two tasks try to write to the same directory at the same time.

  • New: Restored a --list-files option.

  • Changed: Error if no mutants are generated, which probably indicates a bug or configuration error(?)


Released 2022-04-14

  • New --file command line option to mutate only functions in source files matching a glob.

  • Improved: Don't attempt to mutate functions called new or implementations of Default. cargo-mutants can not yet generate good mutations for these so they are generally false positives.

  • Improved: Better display of <impl Foo for Bar>::foo and similar type paths.

  • New: --output directory to write mutants.out somewhere other than the source directory.


Released 2022-03-26

  • Fix: Ignore errors setting file mtimes during copies, which can cause failures on Windows if some files are readonly.

  • Fix: Log file names now include only the source file relative path, the line number, and a counter, so they are shorter, and shouldn't cause problems on filesystems with length limits.

  • Change: version-control directories like .git are not copied with the source tree: they should have no effect on the build, so copying them is just a waste.

  • Changed/improved json logs in mutants.out:

    • Show durations as fractional seconds.

    • Outcomes include a "summary" field.


Released 2022-03-23

  • Switch from Indicatif to Nutmeg to draw progress bars and output. This fixes a bug where terminal output line-wraps badly, and adds a projection for the total estimated time to completion.

  • Change: Mutants are now tested in random order by default, so that repeated runs are more likely to surface interesting new findings early, rather than repeating previous results. The previous behavior of testing mutants in the deterministic order they're encountered in the tree can be restored with --no-shuffle.


Released 2022-02-16

  • The progress bar now shows which mutant is being tested out of how many total.

  • The automatic timeout is now set to the minimum of 20 seconds, or 5x the time of the tests in a baseline tree, to reduce the incidence of false timeouts on machines with variable throughput.

  • Ctrl-c (or SIGINT) interrupts the program during copying the tree. Previously it was not handled until the copy was complete.

  • New --no-copy-target option.


Released 2022-02-10


Released 2022-02-06

  • A new --timeout SECS option to limit the runtime of any cargo test invocation, so that mutations that cause tests to hang don't cause cargo mutants to hang.

    A default timeout is set based on the time to run tests in an unmutated tree. There is no timeout by default on the unmutated tree.

    On Unix, the cargo subprocesses run in a new process group. As a consequence ctrl-c is explicitly caught and propagated to the child processes.

  • Show a progress bar while looking for mutation opportunities, and show the total number found.

  • Show how many mutation opportunities were found, before testing begins.

  • New --shuffle option tests mutants in random order.

  • By default, the output now only lists mutants that were missed or that timed out. Mutants that were caught, and mutants that did not build, can be printed with --caught and --unviable respectively.


Released 2021-11-30

  • Logs and other information are written into mutants.out in the source directory, rather than target/mutants.

  • New --all-logs option prints all Cargo output to stdout, which is verbose but useful for example in CI, by making all the output directly available in captured stdout.

  • The output distinguishes check or build failures (probably due to an unviable mutant) from test failures (probably due to lacking coverage.)

  • A new file mutants.out/mutants.json lists all the generated mutants.

  • Show function return types in some places, to make it easier to understand whether the mutants were useful or viable.

  • Run cargo check --tests and cargo build --tests in the source directory to freshen the build and download any dependencies, before copying it to a scratch directory.

  • New --check option runs cargo check on generated mutants to see if they are viable, without actually running the tests. This is useful in tuning cargo-mutants to generate better mutants.

  • New --no-times output hides times (and tree sizes) from stdout, mostly to make the output deterministic and easier to match in tests.

  • Mutate methods too!


Released 2021-11-10

  • Fixed cargo install cargo-mutants (sometimes?) failing due to the derive feature not getting set on the serde dependency.

  • Show progress while copying the tree.

  • Respect the $CARGO environment variable so that the same toolchain is used to run tests as was used to invoke cargo mutants. Concretely, cargo +nightly mutants should work correctly.


Released 2021-11-06

  • Skip functions or modules marked #[test], #[cfg(test)] or #[mutants::skip].

  • Early steps towards type-guided mutations:

    • Generate mutations of true and false for functions that return bool
    • Empty and arbitrary strings for functions returning String.
    • Return Ok(Default::default()) for functions that return Result<_, _>.
  • Rename --list-mutants to just --list.

  • New --list --json.

  • Colored output makes test names and mutations easier to read (for me at least.)

  • Return distinct exit codes for different situations including that uncaught mutations were found.


  • Functions that should not be mutated can be marked with #[mutants::skip] from the mutants helper crate.


First release.