Dependencies

A package can depend on other packages by listing them in the [dependencies] section of its Cargo.toml file.
The most common way to specify a dependency is by providing its name and version:

[dependencies]
thiserror = "1"

This will add thiserror as a dependency to your package, with a minimum version of 1.0.0. thiserror will be pulled from crates.io, Rust's official package registry. When you run cargo build, cargo will go through a few stages:

  • Dependency resolution
  • Downloading the dependencies
  • Compiling your project (your own code and the dependencies)

Dependency resolution is skipped if your project has a Cargo.lock file and your manifest files are unchanged. A lockfile is automatically generated by cargo after a successful round of dependency resolution: it contains the exact versions of all dependencies used in your project, and is used to ensure that the same versions are consistently used across different builds (e.g. in CI). If you're working on a project with multiple developers, you should commit the Cargo.lock file to your version control system.

You can use cargo update to update the Cargo.lock file with the latest (compatible) versions of all your dependencies.

Path dependencies

You can also specify a dependency using a path. This is useful when you're working on multiple local packages.

[dependencies]
my-library = { path = "../my-library" }

The path is relative to the Cargo.toml file of the package that's declaring the dependency.

Other sources

Check out the Cargo documentation for more details on where you can get dependencies from and how to specify them in your Cargo.toml file.

Dev dependencies

You can also specify dependencies that are only needed for development—i.e. they only get pulled in when you're running cargo test.
They go in the [dev-dependencies] section of your Cargo.toml file:

[dev-dependencies]
static_assertions = "1.1.0"

We've been using a few of these throughout the book to shorten our tests.

Exercise

The exercise for this section is located in 05_ticket_v2/11_dependencies