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:

  1. Write a test for a new feature or behavior.
  2. Implement just enough code to make the test pass.
  3. Refactor while ensuring the test still passes.

This approach encourages cleaner software designs and continuous verification of correctness.