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"]

rename-dependency

The rename-dependency feature allows you to import a dependency with a different name from the source. This can be useful in a few scenarios:

  • Depending on crates with the same name from different registries.
  • Depending on multiple versions of a crate.
  • Avoid needing extern crate foo as bar in Rust source.

Just include the package key to specify the actual name of the dependency. You must include cargo-features at the top of your Cargo.toml.

cargo-features = ["rename-dependency"]

[package]
name = "mypackage"
version = "0.0.1"

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

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


# #![allow(unused_variables)]
#fn main() {
extern crate foo;  // crates.io
extern crate bar;  // registry `custom`
extern crate baz;  // git repository
#}

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"]

[project]
...
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

Edition

You can opt in to a specific Rust Edition for your package with the edition key in Cargo.toml. If you don't specify the edition, it will default to

  1. You need to include the appropriate cargo-features:
cargo-features = ["edition"]

[package]
...
edition = "2018"

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.

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:

[project]
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"].