Rust tests

Testing Strategy

Core Question

What is the right test boundary for this behavior, and how should the test be structured for maintainability?

Unit tests verify internal logic in isolation. Integration tests verify that public API contracts hold when components work together. Choosing wrong leads to either brittle tests that break on refactoring (too-low boundary) or slow, opaque tests that don't pinpoint failures (too-high boundary).

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Quick Decisions

Situation	Reach For	Why
Pure function with clear inputs/outputs	Unit test	Fast, precise, no setup needed
Private helper with tricky logic	Unit test via use super::*	Test the logic directly, not through layers of indirection
Public API contract	Integration test in tests/	Proves the API works as consumers will use it
Error path or edge case	Unit test per error variant	Exhaustive, fast, catches regressions early
Complex setup shared across tests	Test builder or helper function	DRY without sacrificing clarity
Multiple inputs, same logic	Macro-generated parameterized tests	Covers variants without copy-paste
Binary crate behavior	Integration test with Command	Tests the actual CLI interface end-to-end
Database or file system interaction	Integration test with isolation	Real dependencies need real cleanup strategies
Documenting usage for library consumers	Doctest	Tested documentation stays accurate
Roundtrip or invariant properties	Property test (see rust-quality)	Finds edge cases humans miss
Mock-heavy test getting complex	Reconsider the design	Too many mocks signals a wrong abstraction boundary

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Key Principles

1. Test behavior, not implementation. Tests that mirror internal structure break on every refactor. Test what the function promises, not how it delivers.

2. Unit tests are fast and precise. If a unit test needs a database, a network, or five seconds to run, it is an integration test in disguise. Move it.

3. Integration tests prove contracts. They exercise the public API as a consumer would. Failures here mean you shipped a breaking change.

4. Error paths need tests too. Every Result::Err variant and every None return should have at least one test. Error paths are where bugs hide because they are where developers look last.

5. Test names are documentation. parse_rejects_empty_input tells you what broke. test_parse_3 does not. Pattern: function_condition_expected.

6. Shared setup belongs in helpers, not copy-pasted. Extract builders and factory functions into the test module. Repeated arrange blocks drift apart and obscure the actual test logic.

7. One assertion per concept. A test can have multiple assert! calls if they verify one logical thing. Unrelated assertions belong in separate tests so failures are specific.

8. Tests own their data. Never depend on test execution order. Never share mutable state between tests without explicit isolation. Cargo runs tests in parallel by default.

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Usage Scenarios

Scenario 1: "I'm adding tests to existing code with no coverage" -> Start with integration tests for the public API to establish a safety net. Then add unit tests for complex internal logic. Integration tests catch the most breakage with the least knowledge of internals. Unit tests follow once you understand the code well enough to identify tricky logic worth testing directly.

Scenario 2: "My test suite is slow and I don't know why" -> Profile with cargo test --lib (unit only) vs cargo test --test '*' (integration only). If unit tests are slow, they are likely touching the filesystem or network — move those to integration tests. If integration tests are slow, look for missing isolation (shared databases, serial locks).

Scenario 3: "I have 50 test functions in one module and it's hard to navigate" -> Nest submodules inside mod tests: group by behavior (mod validation, mod parsing, mod error_cases). Each submodule gets use super::* from the parent test module. Tests become module::tests::validation::rejects_empty_input.

Scenario 4: "I need to test a CLI binary that reads stdin and writes files" -> Use std::process::Command or assert_cmd in integration tests. Create TempDir for file output. Assert on exit code, stdout, and stderr. Never test CLI behavior through library unit tests — test the actual binary.

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Reference Index

Reference	Read When
references/unit-tests.md	Writing unit tests: module organization, private function testing, error paths, test builders, parameterized tests, feature flags, test filtering
references/integration-tests.md	Writing integration tests: tests/ directory structure, shared utilities, binary testing, real dependencies, isolation, snapshots, workspaces

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Cross-References

When	Check
Property-based testing with proptest	rust-quality → Quick Decisions
Mocking with mockall, trait extraction	rust-quality → Quick Decisions
Benchmarking with criterion	rust-quality → Quick Decisions
Async test patterns (#[tokio::test])	rust-async → Quick Decisions
Error type design affecting test assertions	rust-errors → Quick Decisions
Project structure and test file placement	rust-architecture → Quick Decisions