Rust Testing Patterns

Comprehensive Rust testing patterns for writing reliable, maintainable tests following TDD methodology.

When to Use

• Writing new Rust functions, methods, or traits
• Adding test coverage to existing code
• Creating benchmarks for performance-critical code
• Implementing property-based tests for input validation
• Following TDD workflow in Rust projects

How It Works

1. Identify target code — Find the function, trait, or module to test
2. Write a test — Use #[test] in a #[cfg(test)] module, rstest for parameterized tests, or proptest for property-based tests
3. Mock dependencies — Use mockall to isolate the unit under test
4. Run tests (RED) — Verify the test fails with the expected error
5. Implement (GREEN) — Write minimal code to pass
6. Refactor — Improve while keeping tests green
7. Check coverage — Use cargo-llvm-cov, target 80%+

TDD Workflow for Rust

The RED-GREEN-REFACTOR Cycle

RED     → Write a failing test first
GREEN   → Write minimal code to pass the test
REFACTOR → Improve code while keeping tests green
REPEAT  → Continue with next requirement

Step-by-Step TDD in Rust

// RED: Write test first, use todo!() as placeholder
pub fn add(a: i32, b: i32) -> i32 { todo!() }

#[cfg(test)]
mod tests {
    use super::*;
    #[test]
    fn test_add() { assert_eq!(add(2, 3), 5); }
}
// cargo test → panics at 'not yet implemented'

// GREEN: Replace todo!() with minimal implementation
pub fn add(a: i32, b: i32) -> i32 { a + b }
// cargo test → PASS, then REFACTOR while keeping tests green

Unit Tests

Module-Level Test Organization

// src/user.rs
pub struct User {
    pub name: String,
    pub email: String,
}

impl User {
    pub fn new(name: impl Into<String>, email: impl Into<String>) -> Result<Self, String> {
        let email = email.into();
        if !email.contains('@') {
            return Err(format!("invalid email: {email}"));
        }
        Ok(Self { name: name.into(), email })
    }

    pub fn display_name(&self) -> &str {
        &self.name
    }
}

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

    #[test]
    fn creates_user_with_valid_email() {
        let user = User::new("Alice", "[email protected]").unwrap();
        assert_eq!(user.display_name(), "Alice");
        assert_eq!(user.email, "[email protected]");
    }

    #[test]
    fn rejects_invalid_email() {
        let result = User::new("Bob", "not-an-email");
        assert!(result.is_err());
        assert!(result.unwrap_err().contains("invalid email"));
    }
}

Assertion Macros

assert_eq!(2 + 2, 4);                                    // Equality
assert_ne!(2 + 2, 5);                                    // Inequality
assert!(vec![1, 2, 3].contains(&2));                     // Boolean
assert_eq!(value, 42, "expected 42 but got {value}");    // Custom message
assert!((0.1_f64 + 0.2 - 0.3).abs() < f64::EPSILON);   // Float comparison

Error and Panic Testing

Testing Result Returns

#[test]
fn parse_returns_error_for_invalid_input() {
    let result = parse_config("}{invalid");
    assert!(result.is_err());

    // Assert specific error variant
    let err = result.unwrap_err();
    assert!(matches!(err, ConfigError::ParseError(_)));
}

#[test]
fn parse_succeeds_for_valid_input() -> Result<(), Box<dyn std::error::Error>> {
    let config = parse_config(r#"{"port": 8080}"#)?;
    assert_eq!(config.port, 8080);
    Ok(()) // Test fails if any ? returns Err
}

Testing Panics

#[test]
#[should_panic]
fn panics_on_empty_input() {
    process(&[]);
}

#[test]
#[should_panic(expected = "index out of bounds")]
fn panics_with_specific_message() {
    let v: Vec<i32> = vec!