rtk-tdd

SKILL.md

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.