cargo-mutants

https://github.com/sourcefrog/cargo-mutants

cargo-mutants is a mutation testing tool for Rust. It guides you to missing test coverage by finding functions whose implementation could be replaced by something trivial and the tests would all still pass.

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

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.

NOTE: cargo-mutants is still pretty new! It can find some interesting results, but because it has a very basic idea of which functions to mutate and how, it generates significant false positives and false negatives. The proof-of-concept is successful, though, and I think the results can be iteratively improved.

Install

cargo install cargo-mutants

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 --dir ~/src/unix_mode/
baseline test with no mutations ... ok
replace type_bits with Default::default() in src/lib.rs:42:32 ... caught
replace is_file with Default::default() in src/lib.rs:52:35 ... caught
replace is_dir with Default::default() in src/lib.rs:62:34 ... caught
replace is_symlink with Default::default() in src/lib.rs:72:38 ... caught
replace is_fifo with Default::default() in src/lib.rs:77:35 ... caught
replace is_char_device with Default::default() in src/lib.rs:82:42 ... caught
replace is_block_device with Default::default() in src/lib.rs:87:43 ... NOT CAUGHT!
...

In this version of the unix_mode crate, the is_block_device function was indeed untested. The gap was fixed by adding a doctest.

The Cargo output is logged into target/mutants/ within the original source directory, so you can see why individual tests failed.

To see what mutants could be generated without running them, use --list. --list also supports either a --json or --diff option.

Skipping functions

To mark functions so they are not mutated:

1. Add a Cargo dependency on the mutants crate.

2. Mark functions with #[mutants::skip].

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

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 in a copy of the clean tree, so no mutations were tested.

Tips

• Trees that deny style lints such as unused parameters are likely to fail to build when mutated, without really saying much about the value of the tests. I suggest you don't statically deny warnings in your source code, but rather set RUSTFLAGS when you do want to check this.

Goals

• Draw attention to code that is not tested or only "pseudo-tested": reached by tests but the tests don't actually depend on the behavior of the function.

• Be fast enough to plausibly run on thousand-file trees, although not necessarily instant. It's OK if it takes some minutes to run.

• Easy start: no annotations need to be added to the source tree to use cargo-mutants, you just run it and it should say something useful about any Rust tree.
  I'd like it to be useful on a tree you haven't seen before, where you're not sure of the quality of the tests.

• Understandable output.

• Reproducible results, at least assuming the test suite is deterministic and generates reproducible failures.

• Signal to noise: most warnings should indicate something that really should be tested more; false positives should be easy to durably dismiss. It ought to be feasible to run it from CI to produce a reliable warning that inadequately-tested code is being added, without being too annoying.

• Opt-in annotations to disable mutation of some methods or to explain how to mutate them usefully.

• Cause no changes to the release build. (Concretely, cargo-mutants doesn't change the original source tree, and doesn't require that the user make any changes, so the chance of it causing any breakage should be low.)

• Rust only, Cargo only.

Limitations, caveats, known bugs, and future enhancements

• In this version, the only mutation it applies is to return Default::default(). For many functions, and in particular for the common case of returning a Result, this will fail to build, which is not an interesting result.

  cargo-mutants can see the return type in the tree, so it's possible to do much better by returning Ok(Default::default()) and so on for other return types. I expect to fix this soon.

  cargo-mutants sees the AST of the tree but doesn't fully "understand" the types, so it may always have some limitations in generating good mutants.

• It might be helpful to distinguish whether the build failed, or the mutation was caught by tests. Ideally, cargo-mutants would never generate code that just won't build, firstly because it's a waste of time, and secondly because it may indicate a missed opportunity to generate more interesting mutants that would build.

• Some mutations will cause the program to hang or spin, for example if the mutation causes the condition of a while loop to always be true. For now, you'll need to notice and interrupt cargo mutants yourself, but I plan to add a timeout. (On Unix we need to run the build in a process group so that the actual test process is terminated.)

• Copying the tree to build it doesn't work well if the Cargo.toml points to dependencies by a relative path (other than in subdirectories). This could be handled by an option to mutate in-place (maybe into a copy made by the user) or possibly an option to copy a larger containing directory. You can work around this by editing Cargo.toml to make the paths absolute, before running cargo mutants.

• Copying a Rust tree and its target/ directory seems to cause a full build the first time cargo test runs there, even if mtimes are preserved. (Perhaps the path is part of the calculation whether files need to be rebuilt?) Later incremental builds are faster. sccache might help with this but I have not yet tested it. However, copying target/ is still generally faster than not copying it.

• It should skip functions with #[cfg(...)] attributes that don't match the current platform, but it does not yet.

• It should also probably skip unsafe functions, and maybe functions containing unsafe {} blocks.

• Some mutations will cause the program to hang or spin, for example if called as the condition of a while loop. Enucleate currently detects this but does not kill the test process, so you'll need to find and kill it yourself. (On Unix we might need to use setpgrp.)

• 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.

• It currently assumes all the source is in src/ of the directory, but Cargo doesn't require that, and some crates have their source in a different directory. This could be fixed by reading cargo metadata.

• It could discard all parameters to avoid strict warnings about them being unused. (I haven't seen any crates yet that enforce this.)

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.

How it works

The basic approach is:

• Make a copy of the whole tree into a scratch directory. The same directory is reused across all the mutations to benefit from incremental builds.
• Build a list of possible mutations and for each one:
  • Apply the mutation to the scratch tree.
  • 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. (Doing so is a shortcoming in this tool.)

The list of possible mutations is generated by:

• Walk all source files and enumerate all item (top-level) and method functions. For each one:
  • Filter functions that should not be mutated:
    • Already-trivial functions whose result would not be changed by mutation.
    • Functions marked with an #[mutants::skip] attribute.
    • Functions marked #[test].
    • Functions marked #[cfg(test)] or inside a mod so marked.
  • Apply every supported mutation:
    • Replace the body with Default::default().
      • If the function returns a Result, instead replace the body with Ok(Default::default()).
    • (maybe more in future)

The nice thing about Default is that it's defined on many commonly-used types including (), so cargo-mutants does not need to really understand the function return type at this early stage. Some functions will fail to build because they return a type that does not implement Default, and that's OK.

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.

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 (at least at moderate-size trees) still making it feasible to exhaustively generate every mutant.

Stability

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

Mutagen

There's an existing Rust mutation testing tool called Mutagen.

Some differences are:

• Mutagen seems more mature.

• Mutagen requires changes to the source tree, and for functions to be mutated to be marked with an attribute. cargo-mutants can work with any unmodified tree.

• Mutagen builds the tree only once; cargo-mutants does an incremental build for each mutation.

  This is slower, although for some trees the incremental build may be relatively cheap compared to running the test suite.

  On the up side building for each mutation gives cargo-mutants the freedom to try mutations it's not sure will compile.

• Mutagen has a neat system to use coverage information to run only the tests that could possibly be affected. This could potentially be ported.

• Mutagen needs a nightly compiler; cargo-mutants should work with any reasonably-recent compiler and is tested on stable.

• (Probably there are more. Please let me know.)