What are modules and crates in Rust and how does the module system work?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Pick a crate type

   A binary crate has main.rs and produces an executable; a library crate has lib.rs and is meant to be reused.

2. Step 2

   Declare modules

   Use `mod name` to create a module inline or in a separate file/folder, forming a tree under the crate root.

3. Step 3

   Set visibility

   Items are private by default; mark functions, structs, or modules `pub` to expose them beyond their parent.

4. Step 4

   Reference by path

   Address items with paths like crate::utils::helper or self/super for relative navigation.

5. Step 5

   Bring names into scope

   Use `use` to shorten paths so you can call helper() instead of the full crate::utils::helper().

6. Step 6

   Manage with Cargo

   Declare dependencies in Cargo.toml and group multiple crates in a workspace for larger projects.

Code Example

// lib.rs — the crate root
mod math {
    // private by default
    fn secret() -> i32 { 42 }

    // exposed to other modules
    pub fn add(a: i32, b: i32) -> i32 {
        a + b + secret()
    }
}

use math::add;

pub fn demo() -> i32 {
    add(1, 2) // calls math::add
}

Follow-up Questions

• What is the difference between a binary crate and a library crate?
• How does the pub keyword control visibility across modules?
• What do crate::, self::, and super:: mean in a path?
• What is a Cargo workspace and when would you use one?
• How does Cargo resolve dependencies from crates.io?
• How do you split a module across multiple files?