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_{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.

rtk-tdd