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 crate can come in one of two forms: a binary crate or a library crate. 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 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.