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.
cargo-mutants
Reading
This book explains how cargo-mutants works, and how it can be deployed in Rust projects to find areas where bugs might be lurking.