Rust TDD Workflow Three Laws of TDD Do NOT write production code without a failing test Write only enough test to fail (including compilation failure) 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 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}_{input_type

[allow(...)]

without documented justification.