Specifying Dependencies

Your crates can depend on other libraries from crates.io or other registries, 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.

Specifying dependencies from crates.io

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:

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

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 should update us to version 0.1.13 if it is the latest 0.1.z release, but would not update us to 0.2.0. If instead we had specified the version string as ^1.0, cargo should update to 1.1 if it is the latest 1.y release, 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
^0.2    :=  >=0.2.0, <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

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

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

Note: crates.io does not allow bare * versions.

Comparison requirements

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

Here are some examples of comparison requirements:

>= 1.2.0
> 1
< 2
= 1.2.3

Multiple requirements

As shown in the examples above, multiple version requirements can be separated with a comma, e.g., >= 1.2, < 1.5.

Specifying dependencies from other registries

To specify a dependency from a registry other than crates.io, first the registry must be configured in a .cargo/config.toml file. See the registries documentation for more information. In the dependency, set the registry key to the name of the registry to use.

some-crate = { version = "1.0", registry = "my-registry" }

Note: crates.io does not allow packages to be published with dependencies on other registries.

Specifying dependencies from git repositories

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:

regex = { git = "https://github.com/rust-lang/regex" }

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 - for example, specifying a member crate name of a workspace and setting git to the repository containing the workspace).

Since we haven’t specified any other information, Cargo assumes that we intend to use the latest commit on the main branch to build our package. 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:

regex = { git = "https://github.com/rust-lang/regex", branch = "next" }

Anything that is not a branch or tag falls under rev. This can be a commit hash like rev = "4c59b707", or a named reference exposed by the remote repository such as rev = "refs/pull/493/head". What references are available varies by where the repo is hosted; GitHub in particular exposes a reference to the most recent commit of every pull request as shown, but other git hosts often provide something equivalent, possibly under a different naming scheme.

Once a git dependency has been added, Cargo will lock that dependency to the latest commit at the time. New commits will not be pulled down automatically once the lock is in place. However, they can be pulled down manually with cargo update.

See Git Authentication for help with git authentication for private repos.

Note: crates.io does not allow packages to be published with git dependencies (git dev-dependencies are ignored). See the Multiple locations section for a fallback alternative.

Specifying path dependencies

Over time, our hello_world package 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 package:

# 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:

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:

hello_utils = { path = "hello_utils", version = "0.1.0" }

Note: crates.io does not allow packages to be published with path dependencies (path dev-dependencies are ignored). See the Multiple locations section for a fallback alternative.

Multiple locations

It is possible to specify both a registry version and a git or path location. The git or path dependency will be used locally (in which case the version is checked against the local copy), and when published to a registry like crates.io, it will use the registry version. Other combinations are not allowed. Examples:

# Uses `my-bitflags` when used locally, and uses
# version 1.0 from crates.io when published.
bitflags = { path = "my-bitflags", version = "1.0" }

# Uses the given git repo when used locally, and uses
# version 1.0 from crates.io when published.
smallvec = { git = "https://github.com/servo/rust-smallvec", version = "1.0" }

# N.B. that if a version doesn't match, Cargo will fail to compile!

One example where this can be useful is when you have split up a library into multiple packages within the same workspace. You can then use path dependencies to point to the local packages within the workspace to use the local version during development, and then use the crates.io version once it is published. This is similar to specifying an override, but only applies to this one dependency declaration.

Platform specific dependencies

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:

winhttp = "0.4.0"

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.

If you want to know which cfg targets are available on your platform, run rustc --print=cfg from the command line. If you want to know which cfg targets are available for another platform, such as 64-bit Windows, run rustc --print=cfg --target=x86_64-pc-windows-msvc.

Unlike in your Rust source code, you cannot use [target.'cfg(feature = "fancy-feature")'.dependencies] to add dependencies based on optional features. Use the [features] section instead:

foo = { version = "1.0", optional = true }
bar = { version = "1.0", optional = true }

fancy-feature = ["foo", "bar"]

The same applies to cfg(debug_assertions), cfg(test) and cfg(proc_macro). These values will not work as expected and will always have the default value returned by rustc --print=cfg. There is currently no way to add dependencies based on these configuration values.

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

winhttp = "0.4.0"

openssl = "1.0.1"

Custom target specifications

If you’re using a custom target specification (such as --target foo/bar.json), use the base filename without the .json extension:

winhttp = "0.4.0"

openssl = "1.0.1"
native = { path = "native/i686" }

Note: Custom target specifications are not usable on the stable channel.

Development dependencies

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

mio = "0.0.1"

Note: When a package is published, only dev-dependencies that specify a version will be included in the published crate. For most use cases, dev-dependencies are not needed when published, though some users (like OS packagers) may want to run tests within a crate, so providing a version if possible can still be beneficial.

Build dependencies

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:

cc = "1.0.3"

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

cc = "1.0.3"

In this case, the dependency will only be built when the host platform matches the specified target.

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.

Choosing features

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

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 features chapter.

Renaming dependencies in Cargo.toml

When writing a [dependencies] section in Cargo.toml the key you write for a dependency typically matches up to the name of the crate you import from in the code. For some projects, though, you may wish to reference the crate with a different name in the code regardless of how it's published on crates.io. For example you may wish to:

  • Avoid the need to use foo as bar in Rust source.
  • Depend on multiple versions of a crate.
  • Depend on crates with the same name from different registries.

To support this Cargo supports a package key in the [dependencies] section of which package should be depended on:

name = "mypackage"
version = "0.0.1"

foo = "0.1"
bar = { git = "https://github.com/example/project", package = "foo" }
baz = { version = "0.1", registry = "custom", package = "foo" }

In this example, three crates are now available in your Rust code:

extern crate foo; // crates.io
extern crate bar; // git repository
extern crate baz; // registry `custom`

All three of these crates have the package name of foo in their own Cargo.toml, so we're explicitly using the package key to inform Cargo that we want the foo package even though we're calling it something else locally. The package key, if not specified, defaults to the name of the dependency being requested.

Note that if you have an optional dependency like:

bar = { version = "0.1", package = 'foo', optional = true }

you're depending on the crate foo from crates.io, but your crate has a bar feature instead of a foo feature. That is, names of features take after the name of the dependency, not the package name, when renamed.

Enabling transitive dependencies works similarly, for example we could add the following to the above manifest:

log-debug = ['bar/log-debug'] # using 'foo/log-debug' would be an error!