Mutation Testing
Mutation testing is an approach to randomized testing code that uses a different approach to property testing and fuzzing. Instead of randomly generating inputs to the program or functions, it works by randomly mutating the code and running the existing tests. The goal is to find mutations that do not break the tests: this usually means that that section of code is not covered by tests, or that the tests are not sufficient to explore all possible paths through the code. On a high level, mutation testing frameworks try to inject bugs into your code and see if your existing tests will catch them.
In some ways, you could say that mutation tests are testing your tests. If you have good tests, then changing anything about your code should result in at least one failing test. If that is not the case, then your tests do not cover all properties (or branches, or edge cases) of your code.
cargo-mutants
Reading
Mutation Testing in Rust by Nicolas Fränkel
Nicolas explains how to use cargo-mutants in Rust. He does this by setting up
an example Rust project, and running it against it. In the process, he
discovers a missed mutation test in cargo-mutants and creates a pull request
for it. He shows how to fix his (intentionally buggy) code with the discovered
mutation.