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 the “Writing and Running a Rust Program” section of 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 the “Defining Modules to Control Scope and Privacy” section).

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:

$ 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, we use ls to see what Cargo creates. In the 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 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.