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## Packages and Crates

The first parts of the module system we’ll cover are packages and crates.

A crate is the smallest amount of code that the Rust compiler considers at a time. Even if you run `rustc` rather than `cargo` and pass a single source code file (as we did all the way back in “Rust Program Basics” in Chapter 1), the compiler considers that file to be a crate. Crates can contain modules, and the modules may be defined in other files that get compiled with the crate, as we’ll see in the coming sections.

A crate can come in one of two forms: a binary crate or a library crate. Binary crates are programs you can compile to an executable that you can run, such as a command line program or a server. Each must have a function called `main` that defines what happens when the executable runs. All the crates we’ve created so far have been binary crates.

Library crates don’t have a `main` function, and they don’t compile to an executable. Instead, they define functionality intended to be shared with multiple projects. For example, the `rand` crate we used in Chapter 2 provides functionality that generates random numbers. Most of the time when Rustaceans say “crate,” they mean library crate, and they use “crate” interchangeably with the general programming concept of a “library.”

The crate root is a source file that the Rust compiler starts from and makes up the root module of your crate (we’ll explain modules in depth in “Control Scope and Privacy with Modules”).

A package is a bundle of one or more crates that provides a set of functionality. A package contains a Cargo.toml file that describes how to build those crates. Cargo is actually a package that contains the binary crate for the command line tool you’ve been using to build your code. The Cargo package also contains a library crate that the binary crate depends on. Other projects can depend on the Cargo library crate to use the same logic the Cargo command line tool uses.

A package can contain as many binary crates as you like, but at most only one library crate. A package must contain at least one crate, whether that’s a library or binary crate.

Let’s walk through what happens when we create a package. First, we enter the command `cargo new my-project`:

    $ cargo new my-project
         Created binary (application) `my-project` package
    $ ls my-project
    Cargo.toml
    src
    $ ls my-project/src
    main.rs

After we run `cargo new my-project`, we use `ls` to see what Cargo creates. In the my-project directory, there’s a Cargo.toml file, giving us a package. There’s also a src directory that contains main.rs. Open Cargo.toml in your text editor and note that there’s no mention of src/main.rs. 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.

Here, we have a package that only contains src/main.rs, meaning it only contains a binary crate named `my-project`. If a package contains src/main.rs and src/lib.rs, it has two crates: a binary and a library, both with the same name as the package. A package can have multiple binary crates by placing files in the src/bin directory: Each file will be a separate binary crate.