Several rules determine what a package can contain. A package can contain at most one library crate. It can contain as many binary crates as you’d like, but it must contain at least one crate (either library or binary).

When we entered the command, Cargo created a Cargo.toml file, giving us a package. Looking at the contents of Cargo.toml, there’s no mention of src/main.rs because Cargo follows a convention that src/main.rs is the crate root of a binary crate with the same name as the package. Likewise, Cargo knows that if the package directory contains src/lib.rs, the package contains a library crate with the same name as the package, and src/lib.rs is its crate root. Cargo passes the crate root files to rustc to build the library or binary.

A crate will group related functionality together in a scope so the functionality is easy to share between multiple projects. For example, the rand crate we used in provides functionality that generates random numbers. We can use that functionality in our own projects by bringing the rand crate into our project’s scope. All the functionality provided by the rand crate is accessible through the crate’s name, .

Let’s move on and talk about the module system!