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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
bash
cargo fmt --all --check
cargo clippy --all-targets
cargo testAll 3 must pass. No exceptions. No #[allow(...)] without documented justification.
