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Unit Testing in Rust
Unit testing is the practice of testing small, isolated pieces of code, usually individual functions, to ensure they behave correctly.
In Rust, unit testing is not an afterthought. It is built into the language and the tooling from day one.
Why unit tests are important
Rust emphasizes correctness, safety, and refactoring without fear. Unit tests directly support these goals.
- Catch logic errors early
- Validate behavior at function boundaries
- Enable safe refactoring
- Document intended behavior
- Prevent regressions as the codebase grows
In Rust, if the compiler guarantees memory safety, tests guarantee behavioral safety.
Rust-first advantages of unit testing
Rust makes unit testing easier and more reliable than many languages.
Built-in test framework
- No external libraries required
- Tests live alongside production code
- Run using a single command:
cargo test
Strong type system + tests = fewer bugs
- Many bugs never compile
- Tests focus on logic, not defensive checks
- Less mocking, more real code execution
Fearless refactoring
- Borrow checker guarantees safety
- Tests guarantee correctness
- You can change internals without changing behavior
How Rust structures unit tests
Unit tests are typically placed in the same file as the code being tested.
- Test internal logic
- Live next to the code
- Can access private functions
Integration tests are placed under tests/ folder.
We will discuss about integration tests when we learn about Library & Modules.