## Specifying Dependencies

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.

### 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:

    [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

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

### Inequality requirements

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 requirements

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

### 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:

    [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" }

### Specifying path dependencies

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" }

### Overriding dependencies

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.

### Testing a bugfix

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' }

### Working with an unpublished minor version

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 noting 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.

#### Overriding repository URL

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!

### Prepublishing a breaking change

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.

### Overriding with local dependencies

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

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" }

### Development dependencies

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"

### 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:

    [build-dependencies]
    cc = "1.0.3"

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.

### Choosing features

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.