The Cargo Book

Your crates can depend on other libraries from crates.io, git repositories, or subdirectories on your local file system. You can also temporarily override the location of a dependency— for example, to be able to test out a bug fix in the dependency that you are working on locally. You can have different dependencies for different platforms, and dependencies that are only used during development. Let's take a look at how to do each of these.

Cargo is configured to look for dependencies on crates.io by default. Only the name and a version string are required in this case. In the cargo guide, we specified a dependency on the time crate:

[dependencies]
time = "0.1.12"

The string "0.1.12" is a semver version requirement. Since this string does not have any operators in it, it is interpreted the same way as if we had specified "^0.1.12", which is called a caret requirement.

Caret requirements allow SemVer compatible updates to a specified version. An update is allowed if the new version number does not modify the left-most non-zero digit in the major, minor, patch grouping. In this case, if we ran cargo update -p time, cargo would update us to version 0.1.13 if it was available, but would not update us to 0.2.0. If instead we had specified the version string as ^1.0, cargo would update to 1.1 but not 2.0. The version 0.0.x is not considered compatible with any other version.

Here are some more examples of caret requirements and the versions that would be allowed with them:

^1.2.3 := >=1.2.3 <2.0.0
^1.2 := >=1.2.0 <2.0.0
^1 := >=1.0.0 <2.0.0
^0.2.3 := >=0.2.3 <0.3.0
^0.0.3 := >=0.0.3 <0.0.4
^0.0 := >=0.0.0 <0.1.0
^0 := >=0.0.0 <1.0.0

This compatibility convention is different from SemVer in the way it treats versions before 1.0.0. While SemVer says there is no compatibility before 1.0.0, Cargo considers 0.x.y to be compatible with 0.x.z, where y ≥ z and x > 0.

Tilde requirements specify a minimal version with some ability to update. If you specify a major, minor, and patch version or only a major and minor version, only patch-level changes are allowed. If you only specify a major version, then minor- and patch-level changes are allowed.

~1.2.3 is an example of a tilde requirement.

~1.2.3 := >=1.2.3 <1.3.0
~1.2 := >=1.2.0 <1.3.0
~1 := >=1.0.0 <2.0.0

Wildcard requirements allow for any version where the wildcard is positioned.

*, 1.* and 1.2.* are examples of wildcard requirements.

* := >=0.0.0
1.* := >=1.0.0 <2.0.0
1.2.* := >=1.2.0 <1.3.0

Inequality requirements allow manually specifying a version range or an exact version to depend on.

Here are some examples of inequality requirements:

>= 1.2.0
> 1
< 2
= 1.2.3

Multiple version requirements can also be separated with a comma, e.g. >= 1.2, < 1.5.

To depend on a library located in a git repository, the minimum information you need to specify is the location of the repository with the git key:

[dependencies]
rand = { git = "https://github.com/rust-lang-nursery/rand" }

Cargo will fetch the git repository at this location then look for a Cargo.toml for the requested crate anywhere inside the git repository (not necessarily at the root).

Since we haven’t specified any other information, Cargo assumes that we intend to use the latest commit on the master branch to build our project. You can combine the git key with the rev, tag, or branch keys to specify something else. Here's an example of specifying that you want to use the latest commit on a branch named next:

[dependencies]
rand = { git = "https://github.com/rust-lang-nursery/rand", branch = "next" }

Over time, our hello_world project from the guide has grown significantly in size! It’s gotten to the point that we probably want to split out a separate crate for others to use. To do this Cargo supports path dependencies which are typically sub-crates that live within one repository. Let’s start off by making a new crate inside of our hello_world project:

# inside of hello_world/
$ cargo new hello_utils

This will create a new folder hello_utils inside of which a Cargo.toml and src folder are ready to be configured. In order to tell Cargo about this, open up hello_world/Cargo.toml and add hello_utils to your dependencies:

[dependencies]
hello_utils = { path = "hello_utils" }

This tells Cargo that we depend on a crate called hello_utils which is found in the hello_utils folder (relative to the Cargo.toml it’s written in).

And that’s it! The next cargo build will automatically build hello_utils and all of its own dependencies, and others can also start using the crate as well. However, crates that use dependencies specified with only a path are not permitted on crates.io. If we wanted to publish our hello_world crate, we would need to publish a version of hello_utils to crates.io and specify its version in the dependencies line as well:

[dependencies]
hello_utils = { path = "hello_utils", version = "0.1.0" }

There are a number of methods in Cargo to support overriding dependencies and otherwise controlling the dependency graph. These options are typically, though, only available at the workspace level and aren't propagated through dependencies. In other words, "applications" have the ability to override dependencies but "libraries" do not.

The desire to override a dependency or otherwise alter some dependencies can arise through a number of scenarios. Most of them, however, boil down to the ability to work with a crate before it's been published to crates.io. For example:

• A crate you're working on is also used in a much larger application you're working on, and you'd like to test a bug fix to the library inside of the larger application.
• An upstream crate you don't work on has a new feature or a bug fix on the master branch of its git repository which you'd like to test out.
• You're about to publish a new major version of your crate, but you'd like to do integration testing across an entire project to ensure the new major version works.
• You've submitted a fix to an upstream crate for a bug you found, but you'd like to immediately have your application start depending on the fixed version of the crate to avoid blocking on the bug fix getting merged.

These scenarios are currently all solved with the [patch] manifest section. Historically some of these scenarios have been solved with the [replace] section, but we'll document the [patch] section here.

Let's say you're working with the [uuid] crate but while you're working on it you discover a bug. You are, however, quite enterprising so you decide to also try out to fix the bug! Originally your manifest will look like:

uuid

[package]
name = "my-library"
version = "0.1.0"
authors = ["..."]

[dependencies]
uuid = "1.0"

First thing we'll do is to clone the uuid repository locally via:

$ git clone https://github.com/rust-lang-nursery/uuid

Next we'll edit the manifest of my-library to contain:

[patch.crates-io]
uuid = { path = "../path/to/uuid" }

Here we declare that we're patching the source crates-io with a new dependency. This will effectively add the local checked out version of uuid to the crates.io registry for our local project.

Next up we need to ensure that our lock file is updated to use this new version of uuid so our project uses the locally checked out copy instead of one from crates.io. The way [patch] works is that it'll load the dependency at ../path/to/uuid and then whenever crates.io is queried for versions of uuid it'll also return the local version.

This means that the version number of the local checkout is significant and will affect whether the patch is used. Our manifest declared uuid = "1.0" which means we'll only resolve to >= 1.0.0, < 2.0.0, and Cargo's greedy resolution algorithm also means that we'll resolve to the maximum version within that range. Typically this doesn't matter as the version of the git repository will already be greater or match the maximum version published on crates.io, but it's important to keep this in mind!

In any case, typically all you need to do now is:

$ cargo build
   Compiling uuid v1.0.0 (file://.../uuid)
   Compiling my-library v0.1.0 (file://.../my-library)
    Finished dev [unoptimized + debuginfo] target(s) in 0.32 secs

And that's it! You're now building with the local version of uuid (note the file:// in the build output). If you don't see the file:// version getting built then you may need to run cargo update -p uuid --precise $version where $version is the version of the locally checked out copy of uuid.

Once you've fixed the bug you originally found the next thing you'll want to do is to likely submit that as a pull request to the uuid crate itself. Once you've done this then you can also update the [patch] section. The listing inside of [patch] is just like the [dependencies] section, so once your pull request is merged you could change your path dependency to:

[patch.crates-io]
uuid = { git = 'https://github.com/rust-lang-nursery/uuid' }

Let's now shift gears a bit from bug fixes to adding features. While working on my-library you discover that a whole new feature is needed in the uuid crate. You've implemented this feature, tested it locally above with [patch], and submitted a pull request. Let's go over how you continue to use and test it before it's actually published.

Let's also say that the current version of uuid on crates.io is 1.0.0, but since then the master branch of the git repository has updated to 1.0.1. This branch includes your new feature you submitted previously. To use this repository we'll edit our Cargo.toml to look like

[package]
name = "my-library"
version = "0.1.0"
authors = ["..."]

[dependencies]
uuid = "1.0.1"

[patch.crates-io]
uuid = { git = 'https://github.com/rust-lang-nursery/uuid' }

Note that our local dependency on uuid has been updated to 1.0.1 as it's what we'll actually require once the crate is published. This version doesn't exist on crates.io, though, so we provide it with the [patch] section of the manifest.

Now when our library is built it'll fetch uuid from the git repository and resolve to 1.0.1 inside the repository instead of trying to download a version from crates.io. Once 1.0.1 is published on crates.io the [patch] section can be deleted.

It's also worth nothing that [patch] applies transitively. Let's say you use my-library in a larger project, such as:

[package]
name = "my-binary"
version = "0.1.0"
authors = ["..."]

[dependencies]
my-library = { git = 'https://example.com/git/my-library' }
uuid = "1.0"

[patch.crates-io]
uuid = { git = 'https://github.com/rust-lang-nursery/uuid' }

Remember that [patch] is applicable transitively but can only be defined at the top level so we consumers of my-library have to repeat the [patch] section if necessary. Here, though, the new uuid crate applies to both our dependency on uuid and the my-library -> uuid dependency. The uuid crate will be resolved to one version for this entire crate graph, 1.0.1, and it'll be pulled from the git repository.

In case the dependency you want to override isn't loaded from crates.io, you'll have to change a bit how you use [patch]:

[patch."https://github.com/your/repository"]
my-library = { path = "../my-library/path" }

And that's it!

As a final scenario, let's take a look at working with a new major version of a crate, typically accompanied with breaking changes. Sticking with our previous crates, this means that we're going to be creating version 2.0.0 of the uuid crate. After we've submitted all changes upstream we can update our manifest for my-library to look like:

[dependencies]
uuid = "2.0"

[patch.crates-io]
uuid = { git = "https://github.com/rust-lang-nursery/uuid", branch = "2.0.0" }

And that's it! Like with the previous example the 2.0.0 version doesn't actually exist on crates.io but we can still put it in through a git dependency through the usage of the [patch] section. As a thought exercise let's take another look at the my-binary manifest from above again as well:

[package]
name = "my-binary"
version = "0.1.0"
authors = ["..."]

[dependencies]
my-library = { git = 'https://example.com/git/my-library' }
uuid = "1.0"

[patch.crates-io]
uuid = { git = 'https://github.com/rust-lang-nursery/uuid', version = '2.0.0' }

Note that this will actually resolve to two versions of the uuid crate. The my-binary crate will continue to use the 1.x.y series of the uuid crate but the my-library crate will use the 2.0.0 version of uuid. This will allow you to gradually roll out breaking changes to a crate through a dependency graph without being force to update everything all at once.

Sometimes you're only temporarily working on a crate and you don't want to have to modify Cargo.toml like with the [patch] section above. For this use case Cargo offers a much more limited version of overrides called path overrides.

Path overrides are specified through .cargo/config instead of Cargo.toml, and you can find more documentation about this configuration. Inside of .cargo/config you'll specify a key called paths:

paths = ["/path/to/uuid"]

This array should be filled with directories that contain a Cargo.toml. In this instance, we’re just adding uuid, so it will be the only one that’s overridden. This path can be either absolute or relative to the directory that contains the .cargo folder.

Path overrides are more restricted than the [patch] section, however, in that they cannot change the structure of the dependency graph. When a path replacement is used then the previous set of dependencies must all match exactly to the new Cargo.toml specification. For example this means that path overrides cannot be used to test out adding a dependency to a crate, instead [patch] must be used in that situation. As a result usage of a path override is typically isolated to quick bug fixes rather than larger changes.

Note: using a local configuration to override paths will only work for crates that have been published to crates.io. You cannot use this feature to tell Cargo how to find local unpublished crates.

Platform-specific dependencies take the same format, but are listed under a target section. Normally Rust-like #[cfg] syntax will be used to define these sections:

[target.'cfg(windows)'.dependencies]
winhttp = "0.4.0"

[target.'cfg(unix)'.dependencies]
openssl = "1.0.1"

[target.'cfg(target_arch = "x86")'.dependencies]
native = { path = "native/i686" }

[target.'cfg(target_arch = "x86_64")'.dependencies]
native = { path = "native/x86_64" }

Like with Rust, the syntax here supports the not, any, and all operators to combine various cfg name/value pairs. Note that the cfg syntax has only been available since Cargo 0.9.0 (Rust 1.8.0).

In addition to #[cfg] syntax, Cargo also supports listing out the full target the dependencies would apply to:

[target.x86_64-pc-windows-gnu.dependencies]
winhttp = "0.4.0"

[target.i686-unknown-linux-gnu.dependencies]
openssl = "1.0.1"

If you’re using a custom target specification, quote the full path and file name:

[target."x86_64/windows.json".dependencies]
winhttp = "0.4.0"

[target."i686/linux.json".dependencies]
openssl = "1.0.1"
native = { path = "native/i686" }

[target."x86_64/linux.json".dependencies]
openssl = "1.0.1"
native = { path = "native/x86_64" }

You can add a [dev-dependencies] section to your Cargo.toml whose format is equivalent to [dependencies]:

[dev-dependencies]
tempdir = "0.3"

Dev-dependencies are not used when compiling a package for building, but are used for compiling tests, examples, and benchmarks.

These dependencies are not propagated to other packages which depend on this package.

You can also have target-specific development dependencies by using dev-dependencies in the target section header instead of dependencies. For example:

[target.'cfg(unix)'.dev-dependencies]
mio = "0.0.1"

You can depend on other Cargo-based crates for use in your build scripts. Dependencies are declared through the build-dependencies section of the manifest:

[build-dependencies]
gcc = "0.3"

The build script does not have access to the dependencies listed in the dependencies or dev-dependencies section. Build dependencies will likewise not be available to the package itself unless listed under the dependencies section as well. A package itself and its build script are built separately, so their dependencies need not coincide. Cargo is kept simpler and cleaner by using independent dependencies for independent purposes.

If a package you depend on offers conditional features, you can specify which to use:

[dependencies.awesome]
version = "1.3.5"
default-features = false # do not include the default features, and optionally
                         # cherry-pick individual features
features = ["secure-password", "civet"]

More information about features can be found in the manifest documentation.