How to use lib.rs vs main.rs

Use src/main.rs for the executable entry point and src/lib.rs for reusable library code. Create a library crate by adding src/lib.rs to your project directory.

// src/lib.rs
pub fn add(a: i32, b: i32) -> i32 {
    a + b
}

Use src/main.rs to call library functions or define a standalone binary.

// src/main.rs
use your_crate_name::add;

fn main() {
    println!("Result: {}", add(2, 3));
}

Think of main.rs as the engine that starts your car, while lib.rs is the toolbox of parts you can use in other cars. You use main.rs when you want to run a program immediately. You use lib.rs when you want to write code that other programs can use later.