4.9 Modules and crates

Explanation

Cargo projects usually separate reusable code from command-line entry points. src/lib.rs exposes reusable functions and modules. src/main.rs is for a binary program that reads arguments, calls library code, and prints or saves results.

Tests can live in #[cfg(test)] modules next to the code or in the tests/ directory as integration tests. Keep runtime code and test code separated so the library can be reused by small programs and experiments.

Minimal library layout:

// src/lib.rs
pub fn square(x: f64) -> f64 {
    x * x
}

Minimal integration test:

// tests/basic.rs
use my_crate::square;

#[test]
fn square_of_three_is_nine() {
    assert_eq!(square(3.0), 9.0);
}

Things to look up

Cargo package
Crate
src/lib.rs
src/main.rs
Integration test
#[cfg(test)]

Exercise

Ask an AI agent to create a tiny Cargo library with one reusable function and one integration test in tests/. Run cargo test and inspect which files are used.

Notes for the exercise

Keep runtime code and test code separated.
Put reusable numerical functions in the library when possible.
Use src/main.rs only when a command-line entry point is needed.