Rust Testing Best Practices

Comprehensive testing guide for Rust applications, covering CLI testing, library testing, async patterns, and CI integration. Contains 42 rules across 8 categories, prioritized by impact to guide test design, mocking strategies, and CI optimization.

When to Apply

Reference these guidelines when:

• Writing unit tests for Rust libraries or modules
• Creating integration tests for CLI applications
• Setting up mocking with mockall or trait-based design
• Testing async code with Tokio
• Configuring CI pipelines for Rust projects

Rule Categories by Priority

Priority	Category	Impact	Prefix
1	Test Organization	CRITICAL	org-
2	Mocking and Test Doubles	CRITICAL	mock-
3	Async Testing	HIGH	async-
4	Property-Based Testing	HIGH	prop-
5	Test Fixtures and Setup	MEDIUM	fix-
6	Assertions and Error Testing	MEDIUM	assert-
7	CI Integration	MEDIUM	ci-
8	Test Performance	LOW-MEDIUM	perf-

Quick Reference

1. Test Organization (CRITICAL)

• org-unit-test-modules - Use cfg(test) modules for unit tests
• org-integration-tests-directory - Place integration tests in tests directory
• org-shared-test-utilities - Use tests/common/mod.rs for shared utilities
• org-binary-crate-pattern - Extract logic from main.rs into lib.rs
• org-test-naming - Name tests after behavior not implementation
• org-test-cli-with-assert-cmd - Use assert_cmd for CLI testing

2. Mocking and Test Doubles (CRITICAL)

• mock-trait-based-design - Design for testability with traits
• mock-automock-attribute - Use mockall automock for complex mocking
• mock-avoid-mocking-owned-types - Avoid mocking types you own
• mock-expect-call-counts - Verify mock call counts explicitly
• mock-predicate-arguments - Use predicates to verify mock arguments
• mock-returning-sequences - Use sequences for multiple return values
• mock-static-methods - Use mock! macro for static methods

3. Async Testing (HIGH)

• async-tokio-test-macro - Use tokio::test for async test functions
• async-time-control - Use paused time for timeout testing
• async-mock-io - Use tokio_test for mocking async IO
• async-spawn-blocking - Test spawn_blocking with multi-threaded runtime
• async-test-channels - Use channels for testing async communication

4. Property-Based Testing (HIGH)

• prop-proptest-basics - Use proptest for property-based testing
• prop-custom-strategies - Create custom strategies for domain types
• prop-shrinking - Use shrinking to find minimal failing cases
• prop-invariant-testing - Test invariants instead of specific values

5. Test Fixtures and Setup (MEDIUM)

• fix-rstest-fixtures - Use rstest fixtures for test setup
• fix-rstest-parametrized - Use rstest case for parameterized tests
• fix-temp-directories - Use TempDir for file system tests
• fix-test-context - Use test-context for setup and teardown
• fix-once-cell-shared-state - Use OnceCell for expensive shared setup

6. Assertions and Error Testing (MEDIUM)

• assert-specific-errors - Assert specific error types not just is_err
• assert-should-panic - Use should_panic for panic testing
• assert-debug-display - Implement Debug for clear failure messages
• assert-custom-messages - Add context to assertions with custom messages
• assert-floating-point - Use approximate comparison for floating point
• assert-collection-contents - Assert collection contents not just length

7. CI Integration (MEDIUM)

• ci-cargo-nextest - Use cargo-nextest for faster CI
• ci-caching - Cache Cargo dependencies in CI
• ci-test-isolation - Ensure test isolation in parallel CI
• ci-coverage - Generate coverage reports in CI

8. Test Performance (LOW-MEDIUM)

• perf-compile-time - Reduce test compilation time
• perf-test-filtering - Filter tests for faster feedback loops
• perf-avoid-io-in-unit-tests - Avoid real IO in unit tests
• perf-parallel-test-execution - Configure parallel test threads
• perf-benchmark-critical-paths - Benchmark critical paths with Criterion

How to Use

Read individual reference files for detailed explanations and code examples:

• Section definitions - Category structure and impact levels
• Rule template - Template for adding new rules

Reference Files

File	Description
references/_sections.md	Category definitions and ordering
assets/templates/_template.md	Template for new rules
metadata.json	Version and reference information