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Unstable Features

Experimental Cargo features are only available on the nightly channel. You are encouraged to experiment with these features to see if they meet your needs, and if there are any issues or problems. Check the linked tracking issues listed below for more information on the feature, and click the GitHub subscribe button if you want future updates.

After some period of time, if the feature does not have any major concerns, it can be stabilized, which will make it available on stable once the current nightly release reaches the stable channel (anywhere from 6 to 12 weeks).

There are three different ways that unstable features can be enabled based on how the feature works:

  • New syntax in Cargo.toml requires a cargo-features key at the top of Cargo.toml, before any tables. For example:

    # This specifies which new Cargo.toml features are enabled.
cargo-features = ["test-dummy-unstable"]

[package]
name = "my-package"
version = "0.1.0"
im-a-teapot = true  # This is a new option enabled by test-dummy-unstable.

  • New command-line flags, options, and subcommands require the -Z unstable-options CLI option to also be included. For example, the new --out-dir option is only available on nightly:

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

  • -Z command-line flags are used to enable new functionality that may not have an interface, or the interface has not yet been designed, or for more complex features that affect multiple parts of Cargo. For example, the mtime-on-use feature can be enabled with:

    cargo +nightly build -Z mtime-on-use

    Run cargo -Z help to see a list of flags available.

    Anything which can be configured with a -Z flag can also be set in the cargo config file (.cargo/config.toml) in the unstable table. For example:

    [unstable]
mtime-on-use = true
build-std = ["core", "alloc"]

Each new feature described below should explain how to use it.

List of unstable features

  • Unstable-specific features
  • Build scripts and linking
    • Metabuild — Provides declarative build scripts.
  • Resolver and features
    • no-index-update — Prevents cargo from updating the index cache.
    • avoid-dev-deps — Prevents the resolver from including dev-dependencies during resolution.
    • minimal-versions — Forces the resolver to use the lowest compatible version instead of the highest.
    • public-dependency — Allows dependencies to be classified as either public or private.
  • Output behavior
    • out-dir — Adds a directory where artifacts are copied to.
    • Different binary name — Assign a name to the built binary that is separate from the crate name.
  • Compile behavior
    • mtime-on-use — Updates the last-modified timestamp on every dependency every time it is used, to provide a mechanism to delete unused artifacts.
    • doctest-xcompile — Supports running doctests with the --target flag.
    • build-std — Builds the standard library instead of using pre-built binaries.
    • build-std-features — Sets features to use with the standard library.
    • binary-dep-depinfo — Causes the dep-info file to track binary dependencies.
    • panic-abort-tests — Allows running tests with the "abort" panic strategy.
    • crate-type — Supports passing crate types to the compiler.
    • keep-going — Build as much as possible rather than aborting on the first error.
  • rustdoc
  • Cargo.toml extensions
  • Information and metadata
    • Build-plan — Emits JSON information on which commands will be run.
    • unit-graph — Emits JSON for Cargo's internal graph structure.
    • cargo rustc --print — Calls rustc with --print to display information from rustc.
  • Configuration
    • config-include — Adds the ability for config files to include other files.
    • cargo config — Adds a new subcommand for viewing config files.
  • Registries
    • credential-process — Adds support for fetching registry tokens from an external authentication program.
    • cargo logout — Adds the logout command to remove the currently saved registry token.
    • publish-timeout — Controls the timeout between uploading the crate and being available in the index
    • registry-auth — Adds support for authenticated registries, and generate registry authentication tokens using asymmetric cryptography.

allow-features

This permanently-unstable flag makes it so that only a listed set of unstable features can be used. Specifically, if you pass -Zallow-features=foo,bar, you'll continue to be able to pass -Zfoo and -Zbar to cargo, but you will be unable to pass -Zbaz. You can pass an empty string (-Zallow-features=) to disallow all unstable features.

-Zallow-features also restricts which unstable features can be passed to the cargo-features entry in Cargo.toml. If, for example, you want to allow

cargo-features = ["test-dummy-unstable"]

where test-dummy-unstable is unstable, that features would also be disallowed by -Zallow-features=, and allowed with -Zallow-features=test-dummy-unstable.

The list of features passed to cargo's -Zallow-features is also passed to any Rust tools that cargo ends up calling (like rustc or rustdoc). Thus, if you run cargo -Zallow-features=, no unstable Cargo or Rust features can be used.

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.

mtime-on-use

  • Original Issue: #6477
  • Cache usage meta tracking issue: #7150

The -Z mtime-on-use flag is an experiment to have Cargo update the mtime of used files to make it easier for tools like cargo-sweep to detect which files are stale. For many workflows this needs to be set on all invocations of cargo. To make this more practical setting the unstable.mtime_on_use flag in .cargo/config.toml or the corresponding ENV variable will apply the -Z mtime-on-use to all invocations of nightly cargo. (the config flag is ignored by stable)

avoid-dev-deps

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

Note: It is not recommended to use this feature. Because it enforces minimal versions for all transitive dependencies, its usefulness is limited since not all external dependencies declare proper lower version bounds. It is intended that it will be changed in the future to only enforce minimal versions for direct dependencies.

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

This can also be specified in .cargo/config.toml files.

[build]
out-dir = "out"

doctest-xcompile

This flag changes cargo test's behavior when handling doctests when a target is passed. Currently, if a target is passed that is different from the host cargo will simply skip testing doctests. If this flag is present, cargo will continue as normal, passing the tests to doctest, while also passing it a --target option, as well as enabling -Zunstable-features --enable-per-target-ignores and passing along information from .cargo/config.toml. See the rustc issue for more information.

cargo test --target foo -Zdoctest-xcompile

New dir-name attribute

Some of the paths generated under target/ have resulted in a de-facto "build protocol", where cargo is invoked as a part of a larger project build. So, to preserve the existing behavior, there is also a new attribute dir-name, which when left unspecified, defaults to the name of the profile. For example:

[profile.release-lto]
inherits = "release"
dir-name = "lto"  # Emits to target/lto instead of target/release-lto
lto = true

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

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 metabuild key in the package, list the dependencies in build-dependencies, and add any metadata that the metabuild packages require under package.metadata. 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.

public-dependency

The 'public-dependency' feature allows marking dependencies as 'public' or 'private'. When this feature is enabled, additional information is passed to rustc to allow the 'exported_private_dependencies' lint to function properly.

This requires the appropriate key to be set in cargo-features:

cargo-features = ["public-dependency"]

[dependencies]
my_dep = { version = "1.2.3", public = true }
private_dep = "2.0.0" # Will be 'private' by default

build-std

The build-std feature enables Cargo to compile the standard library itself as part of a crate graph compilation. This feature has also historically been known as "std-aware Cargo". This feature is still in very early stages of development, and is also a possible massive feature addition to Cargo. This is a very large feature to document, even in the minimal form that it exists in today, so if you're curious to stay up to date you'll want to follow the tracking repository and its set of issues.

The functionality implemented today is behind a flag called -Z build-std. This flag indicates that Cargo should compile the standard library from source code using the same profile as the main build itself. Note that for this to work you need to have the source code for the standard library available, and at this time the only supported method of doing so is to add the rust-src rust rustup component:

$ rustup component add rust-src --toolchain nightly

It is also required today that the -Z build-std flag is combined with the --target flag. Note that you're not forced to do a cross compilation, you're just forced to pass --target in one form or another.

Usage looks like:

$ cargo new foo
$ cd foo
$ cargo +nightly run -Z build-std --target x86_64-unknown-linux-gnu
   Compiling core v0.0.0 (...)
   ...
   Compiling foo v0.1.0 (...)
    Finished dev [unoptimized + debuginfo] target(s) in 21.00s
     Running `target/x86_64-unknown-linux-gnu/debug/foo`
Hello, world!

Here we recompiled the standard library in debug mode with debug assertions (like src/main.rs is compiled) and everything was linked together at the end.

Using -Z build-std will implicitly compile the stable crates core, std, alloc, and proc_macro. If you're using cargo test it will also compile the test crate. If you're working with an environment which does not support some of these crates, then you can pass an argument to -Zbuild-std as well:

$ cargo +nightly build -Z build-std=core,alloc

The value here is a comma-separated list of standard library crates to build.

Requirements

As a summary, a list of requirements today to use -Z build-std are:

  • You must install libstd's source code through rustup component add rust-src
  • You must pass --target
  • You must use both a nightly Cargo and a nightly rustc
  • The -Z build-std flag must be passed to all cargo invocations.

Reporting bugs and helping out

The -Z build-std feature is in the very early stages of development! This feature for Cargo has an extremely long history and is very large in scope, and this is just the beginning. If you'd like to report bugs please either report them to:

Also if you'd like to see a feature that's not yet implemented and/or if something doesn't quite work the way you'd like it to, feel free to check out the issue tracker of the tracking repository, and if it's not there please file a new issue!

build-std-features

This flag is a sibling to the -Zbuild-std feature flag. This will configure the features enabled for the standard library itself when building the standard library. The default enabled features, at this time, are backtrace and panic_unwind. This flag expects a comma-separated list and, if provided, will override the default list of features enabled.

binary-dep-depinfo

  • Tracking rustc issue: #63012

The -Z binary-dep-depinfo flag causes Cargo to forward the same flag to rustc which will then cause rustc to include the paths of all binary dependencies in the "dep info" file (with the .d extension). Cargo then uses that information for change-detection (if any binary dependency changes, then the crate will be rebuilt). The primary use case is for building the compiler itself, which has implicit dependencies on the standard library that would otherwise be untracked for change-detection.

panic-abort-tests

The -Z panic-abort-tests flag will enable nightly support to compile test harness crates with -Cpanic=abort. Without this flag Cargo will compile tests, and everything they depend on, with -Cpanic=unwind because it's the only way test-the-crate knows how to operate. As of rust-lang/rust#64158, however, the test crate supports -C panic=abort with a test-per-process, and can help avoid compiling crate graphs multiple times.

It's currently unclear how this feature will be stabilized in Cargo, but we'd like to stabilize it somehow!

keep-going

cargo build --keep-going (and similarly for check, test etc) will build as many crates in the dependency graph as possible, rather than aborting the build at the first one that fails to build.

For example if the current package depends on dependencies fails and works, one of which fails to build, cargo check -j1 may or may not build the one that succeeds (depending on which one of the two builds Cargo picked to run first), whereas cargo check -j1 --keep-going would definitely run both builds, even if the one run first fails.

The -Z unstable-options command-line option must be used in order to use --keep-going while it is not yet stable:

cargo check --keep-going -Z unstable-options

config-include

The include key in a config file can be used to load another config file. It takes a string for a path to another file relative to the config file, or a list of strings. It requires the -Zconfig-include command-line option.

# .cargo/config
include = '../../some-common-config.toml'

The config values are first loaded from the include path, and then the config file's own values are merged on top of it.

This can be paired with config-cli to specify a file to load from the command-line. Pass a path to a config file as the argument to --config:

cargo +nightly -Zunstable-options -Zconfig-include --config somefile.toml build

CLI paths are relative to the current working directory.

target-applies-to-host

  • Original Pull Request: #9322
  • Tracking Issue: #9453

Historically, Cargo's behavior for whether the linker and rustflags configuration options from environment variables and [target] are respected for build scripts, plugins, and other artifacts that are always built for the host platform has been somewhat inconsistent. When --target is not passed, Cargo respects the same linker and rustflags for build scripts as for all other compile artifacts. When --target is passed, however, Cargo respects linker from [target.<host triple>], and does not pick up any rustflags configuration. This dual behavior is confusing, but also makes it difficult to correctly configure builds where the host triple and the target triple happen to be the same, but artifacts intended to run on the build host should still be configured differently.

-Ztarget-applies-to-host enables the top-level target-applies-to-host setting in Cargo configuration files which allows users to opt into different (and more consistent) behavior for these properties. When target-applies-to-host is unset, or set to true, in the configuration file, the existing Cargo behavior is preserved (though see -Zhost-config, which changes that default). When it is set to false, no options from [target.<host triple>], RUSTFLAGS, or [build] are respected for host artifacts regardless of whether --target is passed to Cargo. To customize artifacts intended to be run on the host, use [host] (host-config).

In the future, target-applies-to-host may end up defaulting to false to provide more sane and consistent default behavior.

# config.toml
target-applies-to-host = false

cargo +nightly -Ztarget-applies-to-host build --target x86_64-unknown-linux-gnu

host-config

  • Original Pull Request: #9322
  • Tracking Issue: #9452

The host key in a config file can be used pass flags to host build targets such as build scripts that must run on the host system instead of the target system when cross compiling. It supports both generic and host arch specific tables. Matching host arch tables take precedence over generic host tables.

It requires the -Zhost-config and -Ztarget-applies-to-host command-line options to be set, and that target-applies-to-host = false is set in the Cargo configuration file.

# config.toml
[host]
linker = "/path/to/host/linker"
[host.x86_64-unknown-linux-gnu]
linker = "/path/to/host/arch/linker"
rustflags = ["-Clink-arg=--verbose"]
[target.x86_64-unknown-linux-gnu]
linker = "/path/to/target/linker"

The generic host table above will be entirely ignored when building on a x86_64-unknown-linux-gnu host as the host.x86_64-unknown-linux-gnu table takes precedence.

Setting -Zhost-config changes the default for target-applies-to-host to false from true.

cargo +nightly -Ztarget-applies-to-host -Zhost-config build --target x86_64-unknown-linux-gnu

unit-graph

The --unit-graph flag can be passed to any build command (build, check, run, test, bench, doc, etc.) to emit a JSON object to stdout which represents Cargo's internal unit graph. Nothing is actually built, and the command returns immediately after printing. Each "unit" corresponds to an execution of the compiler. These objects also include which unit each unit depends on.

cargo +nightly build --unit-graph -Z unstable-options

This structure provides a more complete view of the dependency relationship as Cargo sees it. In particular, the "features" field supports the new feature resolver where a dependency can be built multiple times with different features. cargo metadata fundamentally cannot represent the relationship of features between different dependency kinds, and features now depend on which command is run and which packages and targets are selected. Additionally it can provide details about intra-package dependencies like build scripts or tests.

The following is a description of the JSON structure:

{
  /* Version of the JSON output structure. If any backwards incompatible
     changes are made, this value will be increased.
  */
  "version": 1,
  /* Array of all build units. */
  "units": [
    {
      /* An opaque string which indicates the package.
         Information about the package can be obtained from `cargo metadata`.
      */
      "pkg_id": "my-package 0.1.0 (path+file:///path/to/my-package)",
      /* The Cargo target. See the `cargo metadata` documentation for more
         information about these fields.
         https://doc.rust-lang.org/cargo/commands/cargo-metadata.html
      */
      "target": {
        "kind": ["lib"],
        "crate_types": ["lib"],
        "name": "my-package",
        "src_path": "/path/to/my-package/src/lib.rs",
        "edition": "2018",
        "test": true,
        "doctest": true
      },
      /* The profile settings for this unit.
         These values may not match the profile defined in the manifest.
         Units can use modified profile settings. For example, the "panic"
         setting can be overridden for tests to force it to "unwind".
      */
      "profile": {
        /* The profile name these settings are derived from. */
        "name": "dev",
        /* The optimization level as a string. */
        "opt_level": "0",
        /* The LTO setting as a string. */
        "lto": "false",
        /* The codegen units as an integer.
           `null` if it should use the compiler's default.
        */
        "codegen_units": null,
        /* The debug information level as an integer.
           `null` if it should use the compiler's default (0).
        */
        "debuginfo": 2,
        /* Whether or not debug-assertions are enabled. */
        "debug_assertions": true,
        /* Whether or not overflow-checks are enabled. */
        "overflow_checks": true,
        /* Whether or not rpath is enabled. */
        "rpath": false,
        /* Whether or not incremental is enabled. */
        "incremental": true,
        /* The panic strategy, "unwind" or "abort". */
        "panic": "unwind"
      },
      /* Which platform this target is being built for.
         A value of `null` indicates it is for the host.
         Otherwise it is a string of the target triple (such as
         "x86_64-unknown-linux-gnu").
      */
      "platform": null,
      /* The "mode" for this unit. Valid values:

         * "test" — Build using `rustc` as a test.
         * "build" — Build using `rustc`.
         * "check" — Build using `rustc` in "check" mode.
         * "doc" — Build using `rustdoc`.
         * "doctest" — Test using `rustdoc`.
         * "run-custom-build" — Represents the execution of a build script.
      */
      "mode": "build",
      /* Array of features enabled on this unit as strings. */
      "features": ["somefeat"],
      /* Whether or not this is a standard-library unit,
         part of the unstable build-std feature.
         If not set, treat as `false`.
      */
      "is_std": false,
      /* Array of dependencies of this unit. */
      "dependencies": [
        {
          /* Index in the "units" array for the dependency. */
          "index": 1,
          /* The name that this dependency will be referred as. */
          "extern_crate_name": "unicode_xid",
          /* Whether or not this dependency is "public",
             part of the unstable public-dependency feature.
             If not set, the public-dependency feature is not enabled.
          */
          "public": false,
          /* Whether or not this dependency is injected into the prelude,
             currently used by the build-std feature.
             If not set, treat as `false`.
          */
          "noprelude": false
        }
      ]
    },
    // ...
  ],
  /* Array of indices in the "units" array that are the "roots" of the
     dependency graph.
  */
  "roots": [0],
}

Profile rustflags option

This feature provides a new option in the [profile] section to specify flags that are passed directly to rustc. This can be enabled like so:

cargo-features = ["profile-rustflags"]

[package]
# ...

[profile.release]
rustflags = [ "-C", "..." ]

To set this in a profile in Cargo configuration, you need to use either -Z profile-rustflags or [unstable] table to enable it. For example,

# .cargo/config.toml
[unstable]
profile-rustflags = true

[profile.release]
rustflags = [ "-C", "..." ]

rustdoc-map

This feature adds configuration settings that are passed to rustdoc so that it can generate links to dependencies whose documentation is hosted elsewhere when the dependency is not documented. First, add this to .cargo/config:

[doc.extern-map.registries]
crates-io = "https://docs.rs/"

Then, when building documentation, use the following flags to cause links to dependencies to link to docs.rs:

cargo +nightly doc --no-deps -Zrustdoc-map

The registries table contains a mapping of registry name to the URL to link to. The URL may have the markers {pkg_name} and {version} which will get replaced with the corresponding values. If neither are specified, then Cargo defaults to appending {pkg_name}/{version}/ to the end of the URL.

Another config setting is available to redirect standard library links. By default, rustdoc creates links to https://doc.rust-lang.org/nightly/. To change this behavior, use the doc.extern-map.std setting:

[doc.extern-map]
std = "local"

A value of "local" means to link to the documentation found in the rustc sysroot. If you are using rustup, this documentation can be installed with rustup component add rust-docs.

The default value is "remote".

The value may also take a URL for a custom location.

per-package-target

The per-package-target feature adds two keys to the manifest: package.default-target and package.forced-target. The first makes the package be compiled by default (ie. when no --target argument is passed) for some target. The second one makes the package always be compiled for the target.

Example:

[package]
forced-target = "wasm32-unknown-unknown"

In this example, the crate is always built for wasm32-unknown-unknown, for instance because it is going to be used as a plugin for a main program that runs on the host (or provided on the command line) target.

artifact-dependencies

  • Tracking Issue: #9096
  • Original Pull Request: #9992

Allow Cargo packages to depend on bin, cdylib, and staticlib crates, and use the artifacts built by those crates at compile time.

Run cargo with -Z bindeps to enable this functionality.

Example: use cdylib artifact in build script

The Cargo.toml in the consuming package, building the bar library as cdylib for a specific build target…

[build-dependencies]
bar = { artifact = "cdylib", version = "1.0", target = "wasm32-unknown-unknown" }

…along with the build script in build.rs.

fn main() {
  wasm::run_file(std::env::var("CARGO_CDYLIB_FILE_BAR").unwrap());
}

Example: use binary artifact and its library in a binary

The Cargo.toml in the consuming package, building the bar binary for inclusion as artifact while making it available as library as well…

[dependencies]
bar = { artifact = "bin", version = "1.0", lib = true }

…along with the executable using main.rs.

fn main() {
  bar::init();
  command::run(env!("CARGO_BIN_FILE_BAR"));
}

publish-timeout

The publish.timeout key in a config file can be used to control how long cargo publish waits between posting a package to the registry and it being available in the local index.

A timeout of 0 prevents any checks from occurring.

It requires the -Zpublish-timeout command-line options to be set.

# config.toml
[publish]
timeout = 300  # in seconds

registry-auth

Enables Cargo to include the authorization token for API requests, crate downloads and sparse index updates by adding a configuration option to config.json in the registry index.

To use this feature, the registry server must include "auth-required": true in config.json, and you must pass the -Z registry-auth flag on the Cargo command line.

When using the sparse protocol, Cargo will attempt to fetch the config.json file before fetching any other files. If the server responds with an HTTP 401, then Cargo will assume that the registry requires authentication and re-attempt the request for config.json with the authentication token included.

On authentication failure (or missing authentication token) the server MAY include a WWW-Authenticate header with a Cargo login_url challenge to indicate where the user can go to get a token.

WWW-Authenticate: Cargo login_url="https://test-registry-login/me

This same flag is also used to enable asymmetric authentication tokens.

Add support for Cargo to authenticate the user to registries without sending secrets over the network.

In config.toml and credentials.toml files there is a field called private-key, which is a private key formatted in the secret subset of PASERK and is used to sign asymmetric tokens

A keypair can be generated with cargo login --generate-keypair which will:

  • generate a public/private keypair in the currently recommended fashion.
  • save the private key in credentials.toml.
  • print the public key in PASERK public format.

It is recommended that the private-key be saved in credentials.toml. It is also supported in config.toml, primarily so that it can be set using the associated environment variable, which is the recommended way to provide it in CI contexts. This setup is what we have for the token field for setting a secret token.

There is also an optional field called private-key-subject which is a string chosen by the registry. This string will be included as part of an asymmetric token and should not be secret. It is intended for the rare use cases like "cryptographic proof that the central CA server authorized this action". Cargo requires it to be non-whitespace printable ASCII. Registries that need non-ASCII data should base64 encode it.

Both fields can be set with cargo login --registry=name --private-key --private-key-subject="subject" which will prompt you to put in the key value.

A registry can have at most one of private-key, token, or credential-process set.

All PASETOs will include iat, the current time in ISO 8601 format. Cargo will include the following where appropriate:

  • sub an optional, non-secret string chosen by the registry that is expected to be claimed with every request. The value will be the private-key-subject from the config.toml file.
  • mutation if present, indicates that this request is a mutating operation (or a read-only operation if not present), must be one of the strings publish, yank, or unyank.
    • name name of the crate related to this request.
    • vers version string of the crate related to this request.
    • cksum the SHA256 hash of the crate contents, as a string of 64 lowercase hexadecimal digits, must be present only when mutation is equal to publish
  • challenge the challenge string received from a 401/403 from this server this session. Registries that issue challenges must track which challenges have been issued/used and never accept a given challenge more than once within the same validity period (avoiding the need to track every challenge ever issued).

The "footer" (which is part of the signature) will be a JSON string in UTF-8 and include:

  • url the RFC 3986 compliant URL where cargo got the config.json file,
    • If this is a registry with an HTTP index, then this is the base URL that all index queries are relative to.
    • If this is a registry with a GIT index, it is the URL Cargo used to clone the index.
  • kid the identifier of the private key used to sign the request, using the PASERK IDs standard.

PASETO includes the message that was signed, so the server does not have to reconstruct the exact string from the request in order to check the signature. The server does need to check that the signature is valid for the string in the PASETO and that the contents of that string matches the request. If a claim should be expected for the request but is missing in the PASETO then the request must be rejected.

credential-process

The credential-process feature adds a config setting to fetch registry authentication tokens by calling an external process.

Token authentication is used by the cargo login, cargo publish, cargo owner, and cargo yank commands. Additionally, this feature adds a new cargo logout command.

To use this feature, you must pass the -Z credential-process flag on the command-line. Additionally, you must remove any current tokens currently saved in the credentials.toml file (which can be done with the new logout command).

credential-process Configuration

To configure which process to run to fetch the token, specify the process in the registry table in a config file:

[registry]
credential-process = "/usr/bin/cargo-creds"

If you want to use a different process for a specific registry, it can be specified in the registries table:

[registries.my-registry]
credential-process = "/usr/bin/cargo-creds"

The value can be a string with spaces separating arguments or it can be a TOML array of strings.

Command-line arguments allow special placeholders which will be replaced with the corresponding value:

  • {name} — The name of the registry.
  • {api_url} — The base URL of the registry API endpoints.
  • {action} — The authentication action (described below).

Process names with the prefix cargo: are loaded from the libexec directory next to cargo. Several experimental credential wrappers are included with Cargo, and this provides convenient access to them:

[registry]
credential-process = "cargo:macos-keychain"

The current wrappers are:

  • cargo:macos-keychain: Uses the macOS Keychain to store the token.
  • cargo:wincred: Uses the Windows Credential Manager to store the token.
  • cargo:1password: Uses the 1password op CLI to store the token. You must install the op CLI from the 1password website. You must run op signin at least once with the appropriate arguments (such as op signin my.1password.com user@example.com), unless you provide the sign-in-address and email arguments. The master password will be required on each request unless the appropriate OP_SESSION environment variable is set. It supports the following command-line arguments:
    • --account: The account shorthand name to use.
    • --vault: The vault name to use.
    • --sign-in-address: The sign-in-address, which is a web address such as my.1password.com.
    • --email: The email address to sign in with.

A wrapper is available for GNOME libsecret to store tokens on Linux systems. Due to build limitations, this wrapper is not available as a pre-compiled binary. This can be built and installed manually. First, install libsecret using your system package manager (for example, sudo apt install libsecret-1-dev). Then build and install the wrapper with cargo install cargo-credential-gnome-secret. In the config, use a path to the binary like this:

[registry]
credential-process = "cargo-credential-gnome-secret {action}"

credential-process Interface

There are two different kinds of token processes that Cargo supports. The simple "basic" kind will only be called by Cargo when it needs a token. This is intended for simple and easy integration with password managers, that can often use pre-existing tooling. The more advanced "Cargo" kind supports different actions passed as a command-line argument. This is intended for more pleasant integration experience, at the expense of requiring a Cargo-specific process to glue to the password manager. Cargo will determine which kind is supported by the credential-process definition. If it contains the {action} argument, then it uses the advanced style, otherwise it assumes it only supports the "basic" kind.

Basic authenticator

A basic authenticator is a process that returns a token on stdout. Newlines will be trimmed. The process inherits the user's stdin and stderr. It should exit 0 on success, and nonzero on error.

With this form, cargo login and cargo logout are not supported and return an error if used.

Cargo authenticator

The protocol between the Cargo and the process is very basic, intended to ensure the credential process is kept as simple as possible. Cargo will execute the process with the {action} argument indicating which action to perform:

  • store — Store the given token in secure storage.
  • get — Get a token from storage.
  • erase — Remove a token from storage.

The cargo login command uses store to save a token. Commands that require authentication, like cargo publish, uses get to retrieve a token. cargo logout uses the erase command to remove a token.

The process inherits the user's stderr, so the process can display messages. Some values are passed in via environment variables (see below). The expected interactions are:

  • store — The token is sent to the process's stdin, terminated by a newline. The process should store the token keyed off the registry name. If the process fails, it should exit with a nonzero exit status.

  • get — The process should send the token to its stdout (trailing newline will be trimmed). The process inherits the user's stdin, should it need to receive input.

    If the process is unable to fulfill the request, it should exit with a nonzero exit code.

  • erase — The process should remove the token associated with the registry name. If the token is not found, the process should exit with a 0 exit status.

Environment

The following environment variables will be provided to the executed command:

  • CARGO — Path to the cargo binary executing the command.
  • CARGO_REGISTRY_INDEX_URL — The URL of the registry index.
  • CARGO_REGISTRY_NAME_OPT — Optional name of the registry. Should not be used as a storage key. Not always available.

cargo logout

A new cargo logout command has been added to make it easier to remove a token from storage. This supports both credentials.toml file tokens and credential-process tokens.

When used with credentials.toml file tokens, it needs the -Z unstable-options command-line option:

cargo logout -Z unstable-options

When used with the credential-process config, use the -Z credential-process command-line option:

cargo logout -Z credential-process

cargo config

The cargo config subcommand provides a way to display the configuration files that cargo loads. It currently includes the get subcommand which can take an optional config value to display.

cargo +nightly -Zunstable-options config get build.rustflags

If no config value is included, it will display all config values. See the --help output for more options available.

doctest-in-workspace

The -Z doctest-in-workspace flag changes the behavior of the current working directory used when running doctests. Historically, Cargo has run rustdoc --test relative to the root of the package, with paths relative from that root. However, this is inconsistent with how rustc and rustdoc are normally run in a workspace, where they are run relative to the workspace root. This inconsistency causes problems in various ways, such as when passing RUSTDOCFLAGS with relative paths, or dealing with diagnostic output.

The -Z doctest-in-workspace flag causes cargo to switch to running rustdoc from the root of the workspace. It also passes the --test-run-directory to rustdoc so that when running the tests, they are run from the root of the package. This preserves backwards compatibility and is consistent with how normal unittests are run.

rustc --print

cargo rustc --print=VAL forwards the --print flag to rustc in order to extract information from rustc. This runs rustc with the corresponding --print flag, and then immediately exits without compiling. Exposing this as a cargo flag allows cargo to inject the correct target and RUSTFLAGS based on the current configuration.

The primary use case is to run cargo rustc --print=cfg to get config values for the appropriate target and influenced by any other RUSTFLAGS.

Different binary name

The different-binary-name feature allows setting the filename of the binary without having to obey the restrictions placed on crate names. For example, the crate name must use only alphanumeric characters or - or _, and cannot be empty.

The filename parameter should not include the binary extension, cargo will figure out the appropriate extension and use that for the binary on its own.

The filename parameter is only available in the [[bin]] section of the manifest.

cargo-features = ["different-binary-name"]

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

[[bin]]
name = "foo"
filename = "007bar"
path = "src/main.rs"

scrape-examples

The -Z rustdoc-scrape-examples flag tells Rustdoc to search crates in the current workspace for calls to functions. Those call-sites are then included as documentation. You can use the flag like this:

cargo doc -Z unstable-options -Z rustdoc-scrape-examples

By default, Cargo will scrape examples from the example targets of packages being documented. You can individually enable or disable targets from being scraped with the doc-scrape-examples flag, such as:

# Enable scraping examples from a library
[lib]
doc-scrape-examples = true

# Disable scraping examples from an example target
[[example]]
name = "my-example"
doc-scrape-examples = false

Note on tests: enabling doc-scrape-examples on test targets will not currently have any effect. Scraping examples from tests is a work-in-progress.

Note on dev-dependencies: documenting a library does not normally require the crate's dev-dependencies. However, example targets require dev-deps. For backwards compatibility, -Z rustdoc-scrape-examples will not introduce a dev-deps requirement for cargo doc. Therefore examples will not be scraped from example targets under the following conditions:

  1. No target being documented requires dev-deps, AND
  2. At least one crate with targets being documented has dev-deps, AND
  3. The doc-scrape-examples parameter is unset or false for all [[example]] targets.

If you want examples to be scraped from example targets, then you must not satisfy one of the above conditions. For example, you can set doc-scrape-examples to true for one example target, and that signals to Cargo that you are ok with dev-deps being build for cargo doc.

check-cfg

-Z check-cfg command line enables compile time checking of name and values in #[cfg], cfg!, #[link] and #[cfg_attr] with the rustc and rustdoc unstable --check-cfg command line.

It's values are:

  • features: enables features checking via --check-cfg=values(feature, ...). Note than this command line options will probably become the default when stabilizing.
  • names: enables well known names checking via --check-cfg=names().
  • values: enables well known values checking via --check-cfg=values().
  • output: enable the use of rustc-check-cfg in build script.

For instance:

cargo check -Z unstable-options -Z check-cfg=features
cargo check -Z unstable-options -Z check-cfg=names
cargo check -Z unstable-options -Z check-cfg=values
cargo check -Z unstable-options -Z check-cfg=features,names,values

Or for output:

// build.rs
println!("cargo:rustc-check-cfg=names(foo, bar)");

cargo check -Z unstable-options -Z check-cfg=output

cargo:rustc-check-cfg=CHECK_CFG

The rustc-check-cfg instruction tells Cargo to pass the given value to the --check-cfg flag to the compiler. This may be used for compile-time detection of unexpected conditional compilation name and/or values.

This can only be used in combination with -Zcheck-cfg=output otherwise it is ignored with a warning.

If you want to integrate with Cargo features, use -Zcheck-cfg=features instead of trying to do it manually with this option.

Stabilized and removed features

Compile progress

The compile-progress feature has been stabilized in the 1.30 release. Progress bars are now enabled by default. See term.progress for more information about controlling this feature.

Edition

Specifying the edition in Cargo.toml has been stabilized in the 1.31 release. See the edition field for more information about specifying this field.

rename-dependency

Specifying renamed dependencies in Cargo.toml has been stabilized in the 1.31 release. See renaming dependencies for more information about renaming dependencies.

Alternate Registries

Support for alternate registries has been stabilized in the 1.34 release. See the Registries chapter for more information about alternate registries.

Offline Mode

The offline feature has been stabilized in the 1.36 release. See the --offline flag for more information on using the offline mode.

publish-lockfile

The publish-lockfile feature has been removed in the 1.37 release. The Cargo.lock file is always included when a package is published if the package contains a binary target. cargo install requires the --locked flag to use the Cargo.lock file. See cargo package and cargo install for more information.

default-run

The default-run feature has been stabilized in the 1.37 release. See the default-run field for more information about specifying the default target to run.

cache-messages

Compiler message caching has been stabilized in the 1.40 release. Compiler warnings are now cached by default and will be replayed automatically when re-running Cargo.

install-upgrade

The install-upgrade feature has been stabilized in the 1.41 release. cargo install will now automatically upgrade packages if they appear to be out-of-date. See the cargo install documentation for more information.

Profile Overrides

Profile overrides have been stabilized in the 1.41 release. See Profile Overrides for more information on using overrides.

Config Profiles

Specifying profiles in Cargo config files and environment variables has been stabilized in the 1.43 release. See the config [profile] table for more information about specifying profiles in config files.

crate-versions

The -Z crate-versions flag has been stabilized in the 1.47 release. The crate version is now automatically included in the cargo doc documentation sidebar.

Features

The -Z features flag has been stabilized in the 1.51 release. See feature resolver version 2 for more information on using the new feature resolver.

package-features

The -Z package-features flag has been stabilized in the 1.51 release. See the resolver version 2 command-line flags for more information on using the features CLI options.

Resolver

The resolver feature in Cargo.toml has been stabilized in the 1.51 release. See the resolver versions for more information about specifying resolvers.

extra-link-arg

The extra-link-arg feature to specify additional linker arguments in build scripts has been stabilized in the 1.56 release. See the build script documentation for more information on specifying extra linker arguments.

configurable-env

The configurable-env feature to specify environment variables in Cargo configuration has been stabilized in the 1.56 release. See the config documentation for more information about configuring environment variables.

rust-version

The rust-version field in Cargo.toml has been stabilized in the 1.56 release. See the rust-version field for more information on using the rust-version field and the --ignore-rust-version option.

codegen-backend

The codegen-backend feature makes it possible to select the codegen backend used by rustc using a profile.

Example:

[package]
name = "foo"

[dependencies]
serde = "1.0.117"

[profile.dev.package.foo]
codegen-backend = "cranelift"

To set this in a profile in Cargo configuration, you need to use either -Z codegen-backend or [unstable] table to enable it. For example,

# .cargo/config.toml
[unstable]
codegen-backend = true

[profile.dev.package.foo]
codegen-backend = "cranelift"

patch-in-config

The -Z patch-in-config flag, and the corresponding support for [patch] section in Cargo configuration files has been stabilized in the 1.56 release. See the patch field for more information.

edition 2021

The 2021 edition has been stabilized in the 1.56 release. See the edition field for more information on setting the edition. See cargo fix --edition and The Edition Guide for more information on migrating existing projects.

Custom named profiles

Custom named profiles have been stabilized in the 1.57 release. See the profiles chapter for more information.

Profile strip option

The profile strip option has been stabilized in the 1.59 release. See the profiles chapter for more information.

Future incompat report

Support for generating a future-incompat report has been stabilized in the 1.59 release. See the future incompat report chapter for more information.

Namespaced features

Namespaced features has been stabilized in the 1.60 release. See the Features chapter for more information.

Weak dependency features

Weak dependency features has been stabilized in the 1.60 release. See the Features chapter for more information.

timings

The -Ztimings option has been stabilized as --timings in the 1.60 release. (--timings=html and the machine-readable --timings=json output remain unstable and require -Zunstable-options.)

config-cli

The --config CLI option has been stabilized in the 1.63 release. See the config documentation for more information.

multitarget

The -Z multitarget option has been stabilized in the 1.64 release. See build.target for more information about setting the default target platform triples.

crate-type

The --crate-type flag for cargo rustc has been stabilized in the 1.64 release. See the cargo rustc documentation for more information.

Workspace Inheritance

Workspace Inheritance has been stabilized in the 1.64 release. See workspace.package, workspace.dependencies, and inheriting-a-dependency-from-a-workspace for more information.

terminal-width

The -Z terminal-width option has been stabilized in the 1.68 release. The terminal width is always passed to the compiler when running from a terminal where Cargo can automatically detect the width.

sparse-registry

Sparse registry support has been stabilized in the 1.68 release. See Registry Protocols for more information.