Dependency Resolution - The Cargo Book

# Dependency Resolution

One of Cargo's primary tasks is to determine the versions of dependencies to use based on the version requirements specified in each package. This process is called "dependency resolution" and is performed by the "resolver". The result of the resolution is stored in the `Cargo.lock` file which "locks" the dependencies to specific versions, and keeps them fixed over time.

The resolver attempts to unify common dependencies while considering possibly conflicting requirements. The sections below provide some details on how these constraints are handled, and how to work with the resolver.

See the chapter Specifying Dependencies for more details about how dependency requirements are specified.

The `cargo tree` command can be used to visualize the result of the resolver.

## SemVer compatibility

Cargo uses SemVer for specifying version numbers. This establishes a common convention for what is compatible between different versions of a package. See the SemVer Compatibility chapter for guidance on what is considered a "compatible" change. This notion of "compatibility" is important because Cargo assumes it should be safe to update a dependency within a compatibility range without breaking the build.

Versions are considered compatible if their left-most non-zero major/minor/patch component is the same. For example, `1.0.3` and `1.1.0` are considered compatible, and thus it should be safe to update from the older release to the newer one. However, an update from `1.1.0` to `2.0.0` would not be allowed to be made automatically. This convention also applies to versions with leading zeros. For example, `0.1.0` and `0.1.2` are compatible, but `0.1.0` and `0.2.0` are not. Similarly, `0.0.1` and `0.0.2` are not compatible.

As a quick refresher, the version requirement syntax Cargo uses for dependencies is:

Requirement | Example | Equivalence | Description
--- | --- | --- | ---
Caret | `1.2.3` or `^1.2.3` | `>=1.2.3, <2.0.0` | Any SemVer-compatible version of at least the given value.
Tilde | `~1.2` | `>=1.2.0, <1.3.0` | Minimum version, with restricted compatibility range.
Wildcard | `1.*` | `>=1.0.0, <2.0.0` | Any version in the `*` position.
Equals | `=1.2.3` | `=1.2.3` | Exactly the specified version only.
Comparison | `>1.1` | `>=1.2.0` | Naive numeric comparison of specified digits.
Compound | `>=1.2, <1.5` | `>1.2.0, <1.5.0` | Multiple requirements that must be simultaneously satisfied.

When multiple packages specify a dependency for a common package, the resolver attempts to ensure that they use the same version of that common package, as long as they are within a SemVer compatibility range. It also attempts to use the greatest version currently available within that compatibility range. For example, if there are two packages in the resolve graph with the following requirements:

    # Package A
    [dependencies]
    bitflags = "1.0"

    # Package B
    [dependencies]
    bitflags = "1.1"

If at the time the `Cargo.lock` file is generated, the greatest version of `bitflags` is `1.2.1`, then both packages will use `1.2.1` because it is the greatest within the compatibility range. If `2.0.0` is published, it will still use `1.2.1` because `2.0.0` is considered incompatible.

If multiple packages have a common dependency with semver-incompatible versions, then Cargo will allow this, but will build two separate copies of the dependency. For example:

    # Package A
    [dependencies]
    rand = "0.7"

    # Package B
    [dependencies]
    rand = "0.6"

The above will result in Package A using the greatest `0.7` release (`0.7.3` at the time of this writing) and Package B will use the greatest `0.6` release (`0.6.5` for example). This can lead to potential problems, see the Version-incompatibility hazards section for more details.

Multiple versions within the same compatibility range are not allowed and will result in a resolver error if it is constrained to two different versions within a compatibility range. For example, if there are two packages in the resolve graph with the following requirements:

    # Package A
    [dependencies]
    log = "=0.4.11"

    # Package B
    [dependencies]
    log = "=0.4.8"

The above will fail because it is not allowed to have two separate copies of the `0.4` release of the `log` package.

### Version-incompatibility hazards

When multiple versions of a crate appear in the resolve graph, this can cause problems when types from those crates are exposed by the crates using them. This is because the types and items are considered different by the Rust compiler, even if they have the same name. Libraries should take care when publishing a SemVer-incompatible version (for example, publishing `2.0.0` after `1.0.0` has been in use), particularly for libraries that are widely used.

The "semver trick" is a workaround for this problem of publishing a breaking change while retaining compatibility with older versions. The linked page goes into detail about what the problem is and how to address it. In short, when a library wants to publish a SemVer-breaking release, publish the new release, and also publish a point release of the previous version that reexports the types from the newer version.

These incompatibilities usually manifest as a compile-time error, but sometimes they will only appear as a runtime misbehavior. For example, let's say there is a common library named `foo` that ends up appearing with both version `1.0.0` and `2.0.0` in the resolve graph. If `downcast_ref` is used on a object created by a library using version `1.0.0`, and the code calling `downcast_ref` is downcasting to a type from version `2.0.0`, the downcast will fail at runtime.

It is important to make sure that if you have multiple versions of a library that you are properly using them, especially if it is ever possible for the types from different versions to be used together. The `cargo tree -d` command can be used to identify duplicate versions and where they come from. Similarly, it is important to consider the impact on the ecosystem if you publish a SemVer-incompatible version of a popular library.

### Pre-releases

SemVer has the concept of "pre-releases" with a dash in the version, such as `1.0.0-alpha`, or `1.0.0-beta`. Cargo will avoid automatically using pre-releases unless explicitly asked. For example, if `1.0.0-alpha` of package `foo` is published, then a requirement of `foo = "1.0"` will not match, and will return an error. The pre-release must be specified, such as `foo = "1.0.0-alpha"`. Similarly `cargo install` will avoid pre-releases unless explicitly asked to install one.

Cargo allows "newer" pre-releases to be used automatically. For example, if `1.0.0-beta` is published, then a requirement `foo = "1.0.0-alpha"` will allow updating to the `beta` version. Beware that pre-release versions can be unstable, and as such care should be taken when using them. Some projects may choose to publish breaking changes between pre-release versions. It is recommended to not use pre-release dependencies in a library if your library is not also a pre-release. Care should also be taken when updating your `Cargo.lock`, and be prepared if a pre-release update causes issues.

The pre-release tag may be separated with periods to distinguish separate components. Numeric components will use numeric comparison. For example, `1.0.0-alpha.4` will use numeric comparison for the `4` component. That means that if `1.0.0-alpha.11` is published, that will be chosen as the greatest release. Non-numeric components are compared lexicographically.

### Version metadata

SemVer has the concept of "version metadata" with a plus in the version, such as `1.0.0+21AF26D3`. This metadata is usually ignored, and should not be used in a version requirement. You should never publish multiple versions that differ only in the metadata tag (note, this is a known issue with crates.io that currently permits this).

## Other constraints

Version requirements aren't the only constraint that the resolver considers when selecting and unifying dependencies. The following sections cover some of the other constraints that can affect resolution.

### Features

The resolver resolves the graph as-if the features of all workspace members are enabled. This ensures that any optional dependencies are available and properly resolved with the rest of the graph when features are added or removed with the `--features` command-line flag. The actual features used when compiling a crate will depend on the features enabled on the command-line.

Dependencies are resolved with the union of all features enabled on them. For example, if one package depends on the `im` package with the `serde` dependency enabled and another package depends on it with the `rayon` dependency enabled, then `im` will be built with both features enabled, and the `serde` and `rayon` crates will be included in the resolve graph. If no packages depend on `im` with those features, then those optional dependencies will be ignored, and they will not affect resolution.

The resolver will skip over versions of packages that are missing required features. For example, if a package depends on version `^1` of `regex` with the `perf` feature, then the oldest version it can select is `1.3.0`, because versions prior to that did not contain the `perf` feature. Similarly, if a feature is removed from a new release, then packages that require that feature will be stuck on the older releases that contain that feature. It is discouraged to remove features in a SemVer-compatible release. Beware that optional dependencies also define an implicit feature, so removing an optional dependency or making it non-optional can cause problems, see removing an optional dependency.

### `links`

The `links` field is used to ensure only one copy of a native library is linked into a binary. The resolver will attempt to find a graph where there is only one instance of each `links` name. If it is unable to find a graph that satisfies that constraint, it will return an error.

For example, it is an error if one package depends on `libgit2-sys` version `0.11` and another depends on `0.12`, because Cargo is unable to unify those, but they both link to the `git2` native library. Due to this requirement, it is encouraged to be very careful when making SemVer-incompatible releases with the `links` field if your library is in common use.

### Yanked versions

Yanked releases are those that are marked that they should not be used. When the resolver is building the graph, it will ignore all yanked releases unless they already exist in the `Cargo.lock` file.

## Dependency updates

Dependency resolution is automatically performed by all Cargo commands that need to know about the dependency graph. For example, `cargo build` will run the resolver to discover all the dependencies to build. After the first time it runs, the result is stored in the `Cargo.lock` file. Subsequent commands will run the resolver, keeping dependencies locked to the versions in `Cargo.lock` if it can.

If the dependency list in `Cargo.toml` has been modified, for example changing the version of a dependency from `1.0` to `2.0`, then the resolver will select a new version for that dependency that matches the new requirements. If that new dependency introduces new requirements, those new requirements may also trigger additional updates. The `Cargo.lock` file will be updated with the new result. The `--locked` or `--frozen` flags can be used to change this behavior to prevent automatic updates when requirements change, and return an error instead.

`cargo update` can be used to update the entries in `Cargo.lock` when new versions are published. Without any options, it will attempt to update all packages in the lock file. The `-p` flag can be used to target the update for a specific package, and other flags such as `--aggressive` or `--precise` can be used to control how versions are selected.

## Overrides

Cargo has several mechanisms to override dependencies within the graph. The Overriding Dependencies chapter goes into detail on how to use overrides. The overrides appear as an overlay to a registry, replacing the patched version with the new entry. Otherwise, resolution is performed like normal.

## Dependency kinds

There are three kinds of dependencies in a package: normal, build, and dev. For the most part these are all treated the same from the perspective of the resolver. One difference is that dev-dependencies for non-workspace members are always ignored, and do not influence resolution.

Platform-specific dependencies with the `[target]` table are resolved as-if all platforms are enabled. In other words, the resolver ignores the platform or `cfg` expression.

### dev-dependency cycles

Usually the resolver does not allow cycles in the graph, but it does allow them for dev-dependencies. For example, project "foo" has a dev-dependency on "bar", which has a normal dependency on "foo" (usually as a "path" dependency). This is allowed because there isn't really a cycle from the perspective of the build artifacts. In this example, the "foo" library is built (which does not need "bar" because "bar" is only used for tests), and then "bar" can be built depending on "foo", then the "foo" tests can be built linking to "bar".

Beware that this can lead to confusing errors. In the case of building library unit tests, there are actually two copies of the library linked into the final test binary: the one that was linked with "bar", and the one built that contains the unit tests. Similar to the issues highlighted in the Version-incompatibility hazards section, the types between the two are not compatible. Be careful when exposing types of "foo" from "bar" in this situation, since the "foo" unit tests won't treat them the same as the local types.

If possible, try to split your package into multiple packages and restructure it so that it remains strictly acyclic.

## Recommendations

The following are some recommendations for setting the version within your package, and for specifying dependency requirements. These are general guidelines that should apply to common situations, but of course some situations may require specifying unusual requirements.

- Follow the SemVer guidelines when deciding how to update your version number, and whether or not you will need to make a SemVer-incompatible version change.

- Use caret requirements for dependencies, such as `"1.2.3"`, for most situations. This ensures that the resolver can be maximally flexible in choosing a version while maintaining build compatibility.

  - Specify all three components with the version you are currently using. This helps set the minimum version that will be used, and ensures that other users won't end up with an older version of the dependency that might be missing something that your package requires.
  - Avoid `*` requirements, as they are not allowed on crates.io, and they can pull in SemVer-breaking changes during a normal `cargo update`.
  - Avoid overly broad version requirements. For example, `>=2.0.0` can pull in any SemVer-incompatible version, like version `5.0.0`, which can result in broken builds in the future.
  - Avoid overly narrow version requirements if possible. For example, if you specify a tilde requirement like `bar="~1.3"`, and another package specifies a requirement of `bar="1.4"`, this will fail to resolve, even though minor releases should be compatible.

- Try to keep the dependency versions up-to-date with the actual minimum versions that your library requires. For example, if you have a requirement of `bar="1.0.12"`, and then in a future release you start using new features added in the `1.1.0` release of "bar", update your dependency requirement to `bar="1.1.0"`.

  If you fail to do this, it may not be immediately obvious because Cargo can opportunistically choose the newest version when you run a blanket `cargo update`. However, if another user depends on your library, and runs `cargo update -p your-library`, it will not automatically update "bar" if it is locked in their `Cargo.lock`. It will only update "bar" in that situation if the dependency declaration is also updated. Failure to do so can cause confusing build errors for the user using `cargo update -p`.

- If two packages are tightly coupled, then an `=` dependency requirement may help ensure that they stay in sync. For example, a library with a companion proc-macro library will sometimes make assumptions between the two libraries that won't work well if the two are out of sync (and it is never expected to use the two libraries independently). The parent library can use an `=` requirement on the proc-macro, and re-export the macros for easy access.

- `0.0.x` versions can be used for packages that are permanently unstable.

In general, the stricter you make the dependency requirements, the more likely it will be for the resolver to fail. Conversely, if you use requirements that are too loose, it may be possible for new versions to be published that will break the build.

## Troubleshooting

The following illustrates some problems you may experience, and some possible solutions.

### SemVer-breaking patch release breaks the build

Sometimes a project may inadvertently publish a point release with a SemVer-breaking change. When users update with `cargo update`, they will pick up this new release, and then their build may break. In this situation, it is recommended that the project should yank the release, and either remove the SemVer-breaking change, or publish it as a new SemVer-major version increase.

If the change happened in a third-party project, if possible try to (politely!) work with the project to resolve the issue.

While waiting for the release to be yanked, some workarounds depend on the circumstances:

- If your project is the end product (such as a binary executable), just avoid updating the offending package in `Cargo.lock`. This can be done with the `--precise` flag in `cargo update`.
- If you publish a binary on crates.io, then you can temporarily add an `=` requirement to force the dependency to a specific good version.
  - Binary projects can alternatively recommend users to use the `--locked` flag with `cargo install` to use the original `Cargo.lock` that contains the known good version.
- Libraries may also consider publishing a temporary new release with stricter requirements that avoid the troublesome dependency. You may want to consider using range requirements (instead of `=`) to avoid overly-strict requirements that may conflict with other packages using the same dependency. Once the problem has been resolved, you can publish another point release that relaxes the dependency back to a caret requirement.
- If it looks like the third-party project is unable or unwilling to yank the release, then one option is to update your code to be compatible with the changes, and update the dependency requirement to set the minimum version to the new release. You will also need to consider if this is a SemVer-breaking change of your own library, for example if it exposes types from the dependency.