Coverage measurements tell you which lines of code (or other units) are reached while running a test. They don't tell you whether the test really checks anything about the behavior of the code.
For example, a function that writes a file and returns a
Result might be
covered by a test that checks the return value, but not by a test that checks
that the file was actually written. cargo-mutants will try mutating the function
to simply return
Ok(()) and report that this was not caught by any tests.
Historically, rust coverage measurements have required manual setup of several
OS and toolchain-dependent tools, although this is improving. Because
cargo-mutants just runs
cargo it has no OS-specific or tight toolchain
integrations, and so is simple to install and run on any Rust source tree.
cargo-mutants also needs no special tools to view or interpret the results.
Coverage tools also in some cases produce output that is hard to interpret, with lines sometimes shown as covered or not due to toolchain quirks that aren't easy to map to direct changes to the test suite. cargo-mutants produces a direct list of changes that are not caught by the test suite, which can be quickly reviewed and prioritized.
One drawback of mutation testing is that it runs the whole test suite once per generated mutant, so it can be slow on large trees with slow test suites. There are some techniques to speed up cargo-mutants, including running multiple tests in parallel.