#testing #cargo #mutation-testing #coverage #mutants

app cargo-mutants

Find inadequately-tested code that can be removed without any tests failing

20 releases (3 stable)

Uses new Rust 2021

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MIT license




Tests crates.io libs.rs

cargo-mutants is a mutation testing tool for Rust. It helps you improve your program's quality by finding functions whose body could be replaced without causing any tests to fail.

Coverage measurements can be helpful, but they really tell you what code is reached by a test, and not whether the test really checks anything about the behavior of the code. Mutation tests give different information, about whether the tests really check the code's behavior.


cargo install cargo-mutants

To install shell completions run for example:

cargo mutants --completions fish >~/.config/fish/completions/cargo-mutants.fish

## Using cargo-mutants

Just run `cargo mutants` in a Rust source directory, and it will point out
functions that may be inadequately tested:

; cargo mutants
Freshen source tree ... ok in 0.031s
Copy source and build products to scratch directory ... 192 MB in 0.116s
Unmutated baseline ... ok in 0.235s
Auto-set test timeout to 20.0s
Found 17 mutants to test
src/lib.rs:168: replace <??>::new -> CopyOptions < 'f > with Default::default() ... NOT CAUGHT in 0.736s
src/lib.rs:386: replace Error::source -> Option < & (dyn std :: error :: Error + 'static) > with Default::default() ... NOT CAUGHT in 0.643s
src/lib.rs:485: replace copy_symlink -> Result < () > with Ok(Default::default()) ... NOT CAUGHT in 0.767s

In v0.5.1 of the cp_r crate, the copy_symlink function was reached by a test but not adequately tested.

Command-line options

-d, --dir: Test the Rust tree in the given directory, rather than the default directory.

-f, --file FILE: Mutate only functions in files matching the given name or glob. If the glob contains / it matches against the path from the source tree root; otherwise it matches only against the file name. If used together with --exclude argument, then the files to be examined are matched before the files to be excluded.

-e, --exclude FILE: Exclude files from mutants generation, matching the given name or glob. If the glob contains / it matches against the path from the source tree root; otherwise it matches only against the file name. If used together with --file argument, then the files to be examined are matched before the files to be excluded.

--list: Show what mutants could be generated, without running them.

--diff: With --list, also include a diff of the source change for each mutant.

--json: With --list, show the list in json.

--check: Run cargo check on all generated mutants to find out which ones are viable, but don't actually run the tests.

--no-copy-target: Don't copy the /target directory from the source, and don't freshen the source directory before copying it. The first "baseline" build in the scratch directory will be a clean build with nothing in /target. This will typically be slower (which is why /target is copied by default) but it might help in debugging any issues with the build. (And, in niche cases where there is a very large volume of old unreferenced content in /target, it might conceivably be faster, but that's probably better dealt with by cargo clean in the source directory.)

--no-shuffle: Test mutants in the fixed order they're found in the source rather than the default behavior of running them in random order. (Shuffling is intended to surface new and different mutants earlier on repeated partial runs of cargo-mutants.)

-v, --caught: Also print mutants that were caught by tests.

-V, --unviable: Also print mutants that failed cargo build.

--no-times: Don't print elapsed times.

--timeout: Set a fixed timeout for each cargo test run, to catch mutations that cause a hang. By default a timeout is automatically determined.

--cargo-arg: Passes the option argument to cargo check, build, and test. For example, --cargo-arg --release.

Passing arguments to cargo test

Command-line options following a -- delimiter are passed through to cargo test, which can be used for example to exclude doctests (which tend to be slow to build and run):

cargo mutants -- --all-targets

You can use a second double-dash to pass options through to the test targets:

cargo mutants -- -- --test-threads 1 --nocapture

Understanding the results

If tests fail in a clean copy of the tree, there might be an (intermittent) failure in the source directory, or there might be some problem that stops them passing when run from a different location, such as a relative path in Cargo.toml. Fix this first.

Otherwise, cargo mutants generates every mutant it can. All mutants fall in to one of these categories:

  • caught — A test failed with this mutant applied. This is a good sign about test coverage. You can look in mutants.out/log to see which tests failed.

  • missed — No test failed with this mutation applied, which seems to indicate a gap in test coverage. Or, it may be that the mutant is undistinguishable from the correct code. You may wish to add a better test, or mark that the function should be skipped.

  • unviable — The attempted mutation doesn't compile. This is inconclusive about test coverage and no action is needed, but indicates an opportunity for cargo-mutants to either generate better mutants, or at least not generate unviable mutants.

  • timeout — The mutation caused the test suite to run for a long time, until it was eventually killed. You might want to investigate the cause and potentially mark the function to be skipped.

By default only missed mutants and timeouts are printed because they're the most actionable. Others can be shown with the -v and -V options.

Skipping functions

To mark functions so they are not mutated:

  1. Add a Cargo dependency on the mutants crate, version "0.0.3" or later. (This must be a regular dependency not a dev-dependency, because the annotation will be on non-test code.)

  2. Mark functions with #[mutants::skip] or other attributes containing mutants::skip (e.g. #[cfg_attr(test, mutants::skip)).

See testdata/tree/hang_avoided_by_attr/ for an example.

The crate is tiny and the attribute has no effect on the compiled code. It only flags the function for cargo-mutants.

Note: Currently, cargo-mutants does not (yet) evaluate attributes like cfg_attr, it only looks for the sequence mutants::skip in the attribute.

Note: Rust's "inner macro attributes" feature is currently unstable, so #![mutants::skip] can't be used at the top of a file in stable Rust.

Exit codes

  • 0: Success. No mutants were found that weren't caught by tests.

  • 1: Usage error: bad command-line arguments etc.

  • 2: Found some mutants that were not covered by tests.

  • 3: Some tests timed out: possibly the mutatations caused an infinite loop, or the timeout is too low.

  • 4: The tests are already failing or hanging before any mutations are applied, so no mutations were tested.


A mutants.out directory is created in the source directory, or whichever directory you specify with --output. It contains:

  • A logs/ directory, with one log file for each mutation plus the baseline unmutated case. The log contains the diff of the mutation plus the output from cargo.

  • A lock.json, on which an fs2 lock is held while cargo-mutants is running, to avoid two tasks trying to write to the same directory at the same time. The lock contains the start time, cargo-mutants version, username, and hostname. lock.json is left in mutants.out when the run completes, but the lock on it is released.

  • caught.txt, missed.txt, timeout.txt, unviable.txt, each listing mutants with the corresponding outcome.

  • A mutants.json file describing all the generated mutants.

  • An outcomes.json file describing the results of all tests.

Hangs and timeouts

Some mutations to the tree can cause the test suite to hang. For example, in this code, cargo-mutants might try changing should_stop to always return false:

    while !should_stop() {
      // something

cargo mutants automatically sets a timeout when running tests with mutations applied, and reports mutations that hit a timeout. The automatic timeout is the greater of 20 seconds, or 5x the time to run tests with no mutations.

The CARGO_MUTANTS_MINIMUM_TEST_TIMEOUT environment variable, measured in seconds, overrides the minimum time.

You can also set an explicit timeout with the --timeout option. In this case the timeout is also applied to tests run with no mutation.

The timeout does not apply to cargo check or cargo build, only cargo test.

When a test times out, you can mark it with #[mutants::skip] so that future cargo mutants runs go faster.


Most of the runtime for cargo-mutants is spent in running the program test suite and in running incremental builds: both are done once per viable mutant.

So, anything you can do to make the cargo build and cargo test suite faster will have a multiplicative effect on cargo mutants run time, and of course will also make normal development more pleasant.

There's lots of good advice on the web, including https://matklad.github.io/2021/09/04/fast-rust-builds.html.

In particular, on Linux, using the Mold linker can improve build times significantly: because cargo-mutants does many incremental builds, link time is important.

Rust doctests are pretty slow, so if you're using them only as testable documentation and not to assert correctness of the code, you can skip them with cargo mutants -- --all-targets.

On some but not all projects, cargo-mutants can be faster if you use -C --release, which will make the build slower but may make the tests faster. Typically this will help on projects with very long CPU-intensive test suites. Cargo-mutants now shows the breakdown of build versus test time which may help you work out if this will help. (On projects like this you might also choose just to turn up optimization for all debug builds in .cargo/config.toml.

By default cargo-mutants copies the target/ directory from the source tree. Rust target directories can accumulate excessive volumes of old build products.

Workspace and package support

cargo-mutants now supports testing Cargo workspaces that contain multiple packages.

All source files in all packages in the workspace are tested. For each mutant, only the containing packages tests are run.

Hard-to-test cases

Some functions don't cause a test suite failure if emptied, but also cannot be removed. For example, functions to do with managing caches or that have other performance side effects.

Ideally, these should be tested, but doing so in a way that's not flaky can be difficult. cargo-mutants can help in a few ways:

  • It helps to at least highlight to the developer that the function is not covered by tests, and so should perhaps be treated with extra care, or tested manually.
  • A #[mutants::skip] annotation can be added to suppress warnings and explain the decision.
  • Sometimes these effects can be tested by making the side-effect observable with, for example, a counter of the number of memory allocations or cache misses/hits.

Continuous integration

Here is an example of a GitHub Actions workflow that runs mutation tests and uploads the results as an artifact. This will fail if it finds any uncaught mutants.

name: cargo-mutants

on: [pull_request, push]

    runs-on: ubuntu-latest
      - uses: actions/checkout@v2
      - uses: actions-rs/toolchain@v1
          toolchain: stable
      - name: Install cargo-mutants
        run: cargo install cargo-mutants
      - name: Run mutant tests
        run: cargo mutants -- --all-features
      - name: Archive results
        uses: actions/upload-artifact@v3
        if: failure()
          name: mutation-report
          path: mutants.out

How to help

Experience reports in GitHub Discussions or issues are very welcome:

  • Did it find a bug or important coverage gap?
  • Did it fail to build and test your tree? (Some cases that aren't supported yet are already listed in this doc or the bug tracker.)

It's especially helpful if you can either point to an open source tree that will reproduce the problem (or success) or at least describe how to reproduce it.

If you are interested in contributing a patch, please read CONTRIBUTING.md.


The goal of cargo-mutants is to be easy to run on any Rust source tree, and to tell you something interesting about areas where bugs might be lurking or the tests might be insufficient.

Being easy to use means:

  • cargo-mutants requires no changes to the source tree or other setup: just install and run. So, if it does not find anything interesting to say about a well-tested tree, it didn't cost you much. (This worked out really well: cargo install cargo-mutants && cargo mutants will do it.)

  • There is no chance that running cargo-mutants will change the released behavior of your program (other than by helping you to fix bugs!), because you don't need to change the source to use it.

  • cargo-mutants should be reasonably fast even on large Rust trees. The overall run time is, roughly, the product of the number of viable mutations multiplied by the time to run the test suite for each mutation. Typically, one cargo mutants run will give you all the information it can find about missing test coverage in the tree, and you don't need to run it again as you iterate on tests, so it's relatively OK if it takes a while. (There is currently very little overhead beyond the cost to do an incremental build and run the tests for each mutant, but that can still be significant for large trees. There's room to improve by testing multiple mutants in parallel.)

  • cargo-mutants should run correctly on any Rust source trees that are built and tested by Cargo, that will build and run their tests in a copy of the tree, and that have hermetic tests. (It's not all the way there yet; in particular it assumes the source is in src/.)

  • cargo-mutants shouldn't crash or hang, even if it generates mutants that cause the software under test to crash or hang. (This is generally met today: cargo-mutants runs tests with an automatically set and configurable timeout.)

  • The results should be reproducible, assuming the build and test suite is deterministic. (This should be true today; please file a bug if it's not. Mutants are run in random order unless --no-shuffle is specified, but this should not affect the results.)

  • cargo-mutants should avoid generating unviable mutants that don't compile, because that wastes time. However, when it's uncertain whether the mutant will build, it's worth trying things that might find interesting results even if they might fail to build. (It does currently generate some unviable mutants, but typically not too many, and they don't have a large effect on runtime in most trees.)

  • Realistically, cargo-mutants may generate some mutants that aren't caught by tests but also aren't interesting, or aren't feasible to test. In those cases it should be easy to permanently dismiss them (e.g. by adding a #[mutants::skip] attribute or a config file.) (The attribute exists but there is no config file yet.)

Showing interesting results mean:

  • cargo-mutants should tell you about places where the code could be wrong and the test suite wouldn't catch it. If it doesn't find any interesting results on typical trees, there's no point. Aspirationally, it will even find useful results in code with high line coverage, when there is code that is reached by a test, but no test depends on its behavior.

  • In superbly-tested projects cargo-mutants may find nothing to say, but hey, at least it was easy to run, and hopefully the assurance that the tests really do seem to be good is useful data.

  • Most, ideally all, findings should indicate something that really should be tested more, or that may already be buggy, or that's at least worth looking at.

  • It should be easy to understand what the output is telling you about a potential bug that wouldn't be caught. (This seems true today.) It might take some thought to work out why the existing tests don't cover it, or how to check it, but at least you know where to begin.

  • As much as possible cargo-mutants should avoid generating trivial mutants, where the mutated code is effectively equivalent to the original code, and so it's not interesting that the test suite doesn't catch the change. (Not much has been done here yet.)

  • For trees that are thoroughly tested, you can use cargo mutants in CI to check that they remain so.

How it works

The basic approach is:

  • Make a copy of the source tree into a scratch directory, excluding version-control directories like .git and the /target directory. The same directory is reused across all the mutations to benefit from incremental builds.

    • After copying the tree, cargo-mutants scans the top-level Cargo.toml and any .cargo/config.toml for relative dependencies. If there are any, the paths are rewritten to be absolute, so that they still work when cargo is run in the scratch directory.

    • Before applying any mutations, check that cargo test succeeds in the scratch directory: perhaps a test is already broken, or perhaps the tree doesn't build when copied because it relies on relative paths to find dependencies, etc.

  • Build a list of mutations:

    • Run cargo metadata to find directories containing Rust source files.
    • Walk all source files and parse each one looking for functions.
    • Skip functions that should not be mutated for any of several reasons: because they're tests, because they have a #[mutants::skip] attribute, etc.
    • For each function, depending on its return type, generate every mutation pattern that produces a result of that type.
  • For each mutation:

    • Apply the mutation to the scratch tree by patching the affected file.
    • Run cargo test in the tree, saving output to a log file.
    • If the build fails or the tests fail, that's good: the mutation was somehow caught.
    • If the build and tests succeed, that might mean test coverage was inadequate, or it might mean we accidentally generated a no-op mutation.
    • Revert the mutation to return the tree to its clean state.

The file is parsed using the syn crate, but mutations are applied textually, rather than to the token stream, so that unmutated code retains its prior formatting, comments, line numbers, etc. This makes it possible to show a text diff of the mutation and should make it easier to understand any error messages from the build of the mutated code.

For more details, see DESIGN.md.

Related work

cargo-mutants was inspired by reading about the Descartes mutation-testing tool for Java described in Increment magazine's testing issue.

It's an interesting insight that mutation at the level of a whole function is a practical sweet-spot to discover missing tests, while still making it feasible to exhaustively generate every mutant, at least for moderate-sized trees.

See also: more information on how cargo-mutants compares to other techniques and tools.

Supported Rust versions

cargo-mutants is tested to build on any Rust version from the last 6 months.

Currently it is tested with Rust 1.59.

After installing cargo-mutants, you should be able to use it to run tests under any toolchain, even toolchains that are far too old to build cargo-mutants, using the standard + option to cargo:

cargo +1.48 mutants

Limitations, caveats, known bugs, and future enhancements

CAUTION: This tool builds and runs code with machine-generated modifications. If the code under test, or the test suite, has side effects such as writing or deleting files, running it with mutations may be dangerous. Think first about what side effects the test suite could possibly have, and/or run it in a restricted or disposable environment.

cargo-mutants behavior, output formats, command-line syntax, json output formats, etc, may change from one release to the next.

cargo-mutants sees the AST of the tree but doesn't fully "understand" the types. Possibly it could learn to get type information from the compiler (or rust-analyzer?), which would help it generate more interesting viable mutants, and fewer unviable mutants.

To make this faster on large trees, we could keep several scratch trees and test them in parallel, which is likely to exploit CPU resources more thoroughly than Cargo's own parallelism: in particular Cargo tends to fall down to a single task during linking, and often comes down to running a single straggler test at a time. https://github.com/sourcefrog/cargo-mutants/issues/39

Code of Conduct

Interaction with or participation in this project is governed by the Rust Code of Conduct.


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