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 toResultfirst, 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 testAll 3 must pass. No exceptions. No #[allow(...)] without documented justification.