rust-testing

name	rust-testing
description	Rust testing strategies and patterns. Activates for test writing, test organization, mocking, property-based testing, integration tests, benchmark setup, and test coverage questions. Invoke with /rust-testing.

Rust Testing Skill

Test Organization

Unit Tests (same file)

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_name_describes_behavior() {
        // Arrange
        let input = create_test_input();
        // Act
        let result = function_under_test(input);
        // Assert
        assert_eq!(result, expected);
    }
}

Integration Tests (tests/ directory)

tests/
├── common/
│   └── mod.rs          # Shared test helpers (NOT a test file)
├── api_tests.rs        # Test the public API
└── cli_tests.rs        # Test CLI behavior

Test Naming Convention

test_<unit>_<scenario>_<expected>:

• test_parser_empty_input_returns_error
• test_cache_expired_entry_evicted

Testing Patterns

Test Fixtures with Builder

struct TestUserBuilder {
    name: String,
    email: String,
}

impl TestUserBuilder {
    fn new() -> Self { Self { name: "Test".into(), email: "[e-mail]".into() } }
    fn name(mut self, name: &str) -> Self { self.name = name.into(); self }
    fn build(self) -> User { User { name: self.name, email: self.email } }
}

Async Tests (tokio)

#[tokio::test]
async fn test_async_operation() {
    let result = async_function().await;
    assert!(result.is_ok());
}

Snapshot Testing

Use insta for output-heavy tests:

#[test]
fn test_serialization() {
    let output = serialize(&my_struct);
    insta::assert_snapshot!(output);
}

Property-Based Testing

use proptest::prelude::*;

proptest! {
    #[test]
    fn roundtrip_serialization(value: i64) {
        let serialized = serialize(value);
        let deserialized = deserialize(&serialized);
        prop_assert_eq!(value, deserialized);
    }
}

Mocking Strategy

Approach	When	Crate
Trait-based (manual)	Simple interfaces	—
mockall	Complex trait mocking	mockall
Test doubles	Repository/service patterns	—
wiremock	HTTP API mocking	wiremock
testcontainers	Real databases in tests	testcontainers

Test Utilities

assert Variants

• assert_eq!(left, right) — equality with debug output
• assert_ne!(left, right) — inequality
• assert!(condition, "message {}", var) — boolean with format
• assert_matches!(value, Pattern) — pattern matching

Testing Errors

#[test]
fn test_returns_specific_error() {
    let result = risky_operation();
    assert!(matches!(result, Err(MyError::NotFound { .. })));
}

Test Coverage

# With cargo-llvm-cov
cargo llvm-cov --html --open

# With cargo-tarpaulin
cargo tarpaulin --out Html

Benchmarking with Criterion

use criterion::{criterion_group, criterion_main, Criterion};

fn bench_parse(c: &mut Criterion) {
    let input = include_str!("../testdata/large.json");
    c.bench_function("parse_large_json", |b| {
        b.iter(|| parse(criterion::black_box(input)))
    });
}

criterion_group!(benches, bench_parse);
criterion_main!(benches);