Rust Crate and Package

A crate can contain one or more Rust modules, which in turn can contain code, such as functions, types, and constants.

A crate is of two types:

• Binary crate
• Library crate

A binary crate is a Rust program that compiles to an executable or multiple executables and has a main() function for each executable.

A library crate doesn't compile to an executable and doesn't have a main() function. A library crate generally defines a shared functionality that can be used in multiple projects.

Crates can be bundled together into a package.

Creating a Package in Rust

Packages can be created using the Cargo package manager, which is built into Rust. Cargo comes pre-installed with Rust.

We can use cargo to create a package. A package contains one or more crates that provides a set of functionality.

Note: A package can contain many binary crates, but at most only one library crate.

Creating a Binary Package in Rust

To create a binary package, we can use the cargo command in the terminal.

$ cargo new hello_world --bin

Output

Created binary (application) `hello_world` package

We create a binary package hello_world using cargo and the --bin option. It is the default cargo behavior.

Let's look at the contents of the hello_world package.

hello_world
├── Cargo.toml
└── src
    └── main.rs

Here,

• hello_world is the package directory
• Cargo.toml is a file that contains metadata about the crate, such as its name, version, and dependencies
• src/main.rs is the crate root and contains the source code of the binary package

Creating a Library Package in Rust

Similarly, we can create a library package in Rust using cargo.

$ cargo new hello_world_lib --lib

Output

Created library `hello_world_lib` package

We create a library package hello_world_lib using cargo and the --lib option.

Let's look at the contents of the hello_world_lib package.

hello_world_lib
├── Cargo.toml
└── src
    └── lib.rs

Here,

• hello_world_lib is the package directory
• Cargo.toml is a file that contains metadata about the crate, such as its name, version, and dependencies
• src/lib.rs is the crate root and contains the source code of the library package

A package can contain src/main.rs and src/lib.rs. In this case, it has two crates: a binary and a library, both with the same name as the package. For example,

hello_world
├── Cargo.toml
└── src
    └── lib.rs
    └── main.rs

Note: Cargo by convention passes the crate root files to the Rust compiler to build the library or binary.