Unstable Features

Experimental Cargo features are only available on the nightly channel. You typically use one of the -Z flags to enable them. Run cargo -Z help to see a list of flags available.

-Z unstable-options is a generic flag for enabling other unstable command-line flags. Options requiring this will be called out below.

Some unstable features will require you to specify the cargo-features key in Cargo.toml.

Alternate Registries

Alternate registries allow you to use registries other than crates.io.

The name of a registry is defined in .cargo/config under the registries table:

[registries]
my-registry = { index = "https://my-intranet:8080/index" }

Authentication information for alternate registries can be added to .cargo/credentials:

[my-registry]
token = "api-token"

Inside Cargo.toml you can specify which registry a dependency comes from using the registry key. First you need to include the appropriate cargo-features at the top of the file:

cargo-features = ["alternative-registries"]

[package]
...

[dependencies]
other-create = { version = "1.0", registry = "my-registry"}

A --registry flag has been added to commands that interact with registries such as publish, login, etc. Example:

cargo +nightly publish -Z unstable-options --registry my-registry

The publish field in Cargo.toml has been extended to accept a list of registries that will restrict publishing only to those registries.

[package]
...
publish = ["my-registry"]

publish-lockfile

When creating a .crate file for distribution, Cargo has historically not included the Cargo.lock file. This can cause problems with using cargo install with a binary. You can specify that your package should include the Cargo.lock file when using cargo package or cargo publish by specifying the publish-lockfile key in Cargo.toml. This also requires the appropriate cargo-features:

cargo-features = ["publish-lockfile"]

[package]
...
publish-lockfile = true

Offline Mode

The -Z offline flag prevents Cargo from attempting to access the network for any reason. Typically Cargo will stop with an error if it wants to access the network and it is not available.

Beware that this may result in different dependency resolution than online mode. Cargo will restrict itself to crates that are available locally, even if there might be a newer version as indicated in the local copy of the index.

no-index-update

The -Z no-index-update flag ensures that Cargo does not attempt to update the registry index. This is intended for tools such as Crater that issue many Cargo commands, and you want to avoid the network latency for updating the index each time.

avoid-dev-deps

  • Original Issue: #4988
  • Stabilization Issue: #5133

When running commands such as cargo install or cargo build, Cargo currently requires dev-dependencies to be downloaded, even if they are not used. The -Z avoid-dev-deps flag allows Cargo to avoid downloading dev-dependencies if they are not needed. The Cargo.lock file will not be generated if dev-dependencies are skipped.

minimal-versions

When a Cargo.lock file is generated, the -Z minimal-versions flag will resolve the dependencies to the minimum semver version that will satisfy the requirements (instead of the greatest version).

The intended use-case of this flag is to check, during continuous integration, that the versions specified in Cargo.toml are a correct reflection of the minimum versions that you are actually using. That is, if Cargo.toml says foo = "1.0.0" that you don't accidentally depend on features added only in foo 1.5.0.

out-dir

This feature allows you to specify the directory where artifacts will be copied to after they are built. Typically artifacts are only written to the target/release or target/debug directories. However, determining the exact filename can be tricky since you need to parse JSON output. The --out-dir flag makes it easier to predictably access the artifacts. Note that the artifacts are copied, so the originals are still in the target directory. Example:

cargo +nightly build --out-dir=out -Z unstable-options

Profile Overrides

Profiles can be overridden for specific packages and custom build scripts. The general format looks like this:

cargo-features = ["profile-overrides"]

[package]
...

[profile.dev]
opt-level = 0
debug = true

# the `image` crate will be compiled with -Copt-level=3
[profile.dev.overrides.image]
opt-level = 3

# All dependencies (but not this crate itself or any workspace member)
# will be compiled with -Copt-level=2 . This includes build dependencies.
[profile.dev.overrides."*"]
opt-level = 2

# Build scripts and their dependencies will be compiled with -Copt-level=3
# By default, build scripts use the same rules as the rest of the profile
[profile.dev.build-override]
opt-level = 3

Overrides can only be specified for dev and release profiles.

Config Profiles

Profiles can be specified in .cargo/config files. The -Z config-profile command-line flag is required to use this feature. The format is the same as in a Cargo.toml manifest. If found in multiple config files, settings will be merged using the regular config hierarchy. Config settings take precedence over manifest settings.

[profile.dev]
opt-level = 3
cargo +nightly build -Z config-profile

Namespaced features

Currently, it is not possible to have a feature and a dependency with the same name in the manifest. If you set namespaced-features to true, the namespaces for features and dependencies are separated. The effect of this is that, in the feature requirements, dependencies have to be prefixed with crate:. Like this:

[package]
namespaced-features = true

[features]
bar = ["crate:baz", "foo"]
foo = []

[dependencies]
baz = { version = "0.1", optional = true }

To prevent unnecessary boilerplate from having to explicitly declare features for each optional dependency, implicit features get created for any optional dependencies where a feature of the same name is not defined. However, if a feature of the same name as a dependency is defined, that feature must include the dependency as a requirement, as foo = ["crate:foo"].

Build-plan

The --build-plan argument for the build command will output JSON with information about which commands would be run without actually executing anything. This can be useful when integrating with another build tool. Example:

cargo +nightly build --build-plan -Z unstable-options

default-run

The default-run option in the [package] section of the manifest can be used to specify a default binary picked by cargo run. For example, when there is both src/bin/a.rs and src/bin/b.rs:

[package]
default-run = "a"

Metabuild

Metabuild is a feature to have declarative build scripts. Instead of writing a build.rs script, you specify a list of build dependencies in the metabuild key in Cargo.toml. A build script is automatically generated that runs each build dependency in order. Metabuild packages can then read metadata from Cargo.toml to specify their behavior.

Include cargo-features at the top of Cargo.toml, a metadata key in the package, list the dependencies in build-dependencies, and add any metadata that the metabuild packages require. Example:

cargo-features = ["metabuild"]

[package]
name = "mypackage"
version = "0.0.1"
metabuild = ["foo", "bar"]

[build-dependencies]
foo = "1.0"
bar = "1.0"

[package.metadata.foo]
extra-info = "qwerty"

Metabuild packages should have a public function called metabuild that performs the same actions as a regular build.rs script would perform.