rtk-tdd

Rust TDD Workflow

Three Laws of TDD

1. Do NOT write production code without a failing test
2. Write only enough test to fail (including compilation failure)
3. Write only enough production code to pass the failing test

Cycle: RED (test fails) -> GREEN (minimum to pass) -> REFACTOR (cleanup, cargo test)

Red-Green-Refactor Steps

1. Write test in #[cfg(test)] mod tests of the SAME file
2. cargo test MODULE::tests::test_name  -- must FAIL (red)
3. Implement the minimum in the function
4. cargo test MODULE::tests::test_name  -- must PASS (green)
5. Refactor if needed, re-run cargo test (still green)
6. cargo fmt && cargo clippy --all-targets && cargo test  (final gate)

Never skip step 2. If the test passes immediately, it tests nothing.

Idiomatic Rust Test Patterns

Pattern	Usage	When
Arrange-Act-Assert	Base structure for every test	Always
assert_eq! / assert!	Direct comparison / booleans	Deterministic values
assert!(result.is_err())	Error path testing	Invalid inputs
Result<()> return type	Tests with ? operator	Fallible functions
#[should_panic]	Expected panic	Invariants, preconditions
tempfile::NamedTempFile	File/I/O tests	Filesystem-dependent code

Patterns by Code Type

Code Type	Test Pattern	Example
Pure function (str -> str)	Input literal -> assert output	assert_eq!(truncate("hello", 3), "...")
Parsing/filtering	Raw string -> filter -> contains/not-contains	assert!(filter(raw).contains("expected"))
Validation/security	Boundary inputs -> assert bool	assert!(!is_valid("../etc/passwd"))
Error handling	Bad input -> is_err()	assert!(parse("garbage").is_err())
Struct/enum roundtrip	Construct -> serialize -> deserialize -> eq	assert_eq!(from_str(to_str(x)), x)

Naming Convention

test_{function}_{scenario}
test_{function}_{input_type}

Examples: test_truncate_edge_case, test_parse_invalid_input, test_filter_empty_string

When NOT to Use Pure TDD

• Functions calling Command::new() -> test the parser, not the execution
• std::process::exit() -> refactor to Result first, then test the Result
• Direct I/O (SQLite, network) -> use tempfile/mock or test the pure logic separately
• Main/CLI wiring -> covered by integration/smoke tests

Pre-Commit Gate

cargo fmt --all --check
cargo clippy --all-targets
cargo test

All 3 must pass. No exceptions. No #[allow(...)] without documented justification.