24.1 What Is Testing?
24.1.1 Why Testing, and What Can Tests Prove?
A test verifies that a piece of code produces the intended result under specific conditions. In practice:
- Tests confirm that functions handle various inputs and edge cases as expected.
- Tests cannot guarantee the absence of all bugs; they only show that specific scenarios pass.
Nevertheless, comprehensive testing reduces the chance of regressions and helps maintain a reliable codebase as it evolves.
24.1.2 Rust Is Safe—So Are Tests Necessary?
Rust’s powerful type system and borrow checker eliminate many issues at compile time, particularly memory-related errors. Additionally, out-of-bounds array access or invalid pointer usage is prevented at runtime. However, the compiler does not know your business rules or intended domain logic. For example:
- Logic Errors: A function can be perfectly memory-safe yet still produce the wrong output if its algorithm is incorrect. For instance, using the wrong formula to compute a value.
- Behavioral Requirements: The code might never panic, but it could still break higher-level domain constraints. For example, a function might accept or return data that your specification deems invalid (such as negative numbers in a context that forbids them).
By writing tests, you go beyond compiler-guaranteed memory safety to ensure your program meets its intended domain requirements and produces correct results.
24.1.3 Benefits of Tests
A well-structured test suite offers several advantages:
- Confidence: Tests confirm that functionality remains correct when you refactor or add new features.
- Maintainability: Tests act as living documentation, illustrating your code's expected behavior.
- Collaboration: In a team setting, tests help reveal if someone else's changes break existing functionality.
24.1.4 Test-Driven Development (TDD)
TDD is an iterative process where tests are written before the implementation:
- Write a test for a new feature or behavior.
- Implement just enough code to make the test pass.
- Refactor while ensuring the test still passes.
This approach encourages cleaner software designs and continuous verification of correctness.