pub struct Input {
    // fields
}

pub fn process(input: &Input) -> Result<Output, Error> {
    todo!()
}

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

    #[test]
    fn valid_input_succeeds() {
        let input = Input { /* ... */ };
        let result = process(&input);
        assert!(result.is_ok());
    }

    #[test]
    fn invalid_input_returns_error() {
        let input = Input { /* ... */ };
        let result = process(&input);
        assert!(result.is_err());
    }
}

cargo test

pub fn process(input: &Input) -> Result<Output, Error> {
    // Minimal implementation that handles both paths
    validate(input)?;
    Ok(Output { /* ... */ })
}

cargo llvm-cov
cargo llvm-cov --fail-under-lines 80

cargo test                        # Run all tests
cargo test -- --nocapture         # Show println output
cargo test test_name              # Run specific test
cargo test --no-fail-fast         # Don't stop on first failure
cargo test --lib                  # Unit tests only
cargo test --test integration     # Integration tests only
cargo test --doc                  # Doc tests only
cargo bench                       # Run benchmarks

src/
├── lib.rs             # Library root
├── service.rs         # Implementation
└── service/
    └── tests.rs       # Or inline #[cfg(test)] mod tests {}
tests/
└── integration.rs     # Integration tests
benches/
└── benchmark.rs       # Criterion benchmarks