Dependencies

Most programs have dependencies on some libraries. If you have ever managed dependencies by hand, you know how much of a pain this can be. Luckily, the Rust ecosystem comes standard with cargo! cargo can manage dependencies for a project.

To create a new Rust project: 

# A binary
cargo new foo

# A library
cargo new --lib bar

For the rest of this chapter, let’s assume we are making a binary, rather than a library, but all of the concepts are the same.

After the above commands, you should see a file hierarchy like this: 

.
├── bar
│   ├── Cargo.toml
│   └── src
│       └── lib.rs
└── foo
    ├── Cargo.toml
    └── src
        └── main.rs
  1. The main.rs is the root source file for your new foo project – nothing new there.
  2. The Cargo.toml is the config file for cargo for this project.

If you look inside it, you should see something like this:

 
[package]
name = "foo"
version = "0.1.0"
authors = ["mark"]

[dependencies]

  1. The name field under [package] determines the name of the project.

  2. This is used by crates.io if you publish the crate (more later).

  3. It is also the name of the output binary when you compile.

  4. The version field is a crate version number using Semantic Versioning.

  5. The authors field is a list of authors used when publishing the crate.

  6. The [dependencies] section lets you add dependencies for your project.

For example, suppose that we want our program to have a great CLI:

  • You can find lots of great packages on crates.io (the official Rust package registry).
  • One popular choice is clap.
  • As of this writing, the most recent published version of clap is 2.27.1.
  • To add a dependency to our program, we can simply add the following to our Cargo.toml under [dependencies]: clap = "2.27.1".
  • And that’s it! You can start using clap in your program.

cargo also supports other types of dependencies.

Here is just a small sampling: 

[package]
name = "foo"
version = "0.1.0"
authors = ["mark"]

[dependencies]
clap = "2.27.1" # from crates.io
rand = { git = "https://github.com/rust-lang-nursery/rand" } # from online repo
bar = { path = "../bar" } # from a path in the local filesystem

cargo is more than a dependency manager. All of the available configuration options are listed in the format specification of Cargo.toml.

  1. cargo build: To build our project we can execute cargo build anywhere in the project directory (including subdirectories!).
  2. cargo run: We can also do cargo run to build and run.
  3. Notice that these commands will:
  • resolve all dependencies,
  • download crates if needed,
  • and build everything, including your crate.

(Note that it only rebuilds what it has not already built, similar to make).

Voila! That’s all there is to it!