Packages and Crates
The first parts of the module system we’ll cover are packages and crates.
A package is one or more crates that provide a set of functionality. A package contains a Cargo.toml file that describes how to build those crates.
A crate can be 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. They 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. 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.
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).
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).
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
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.
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.