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. For the latest nightly, see the nightly version of this page.

List of unstable features

Unstable-specific features
- -Z allow-features — Provides a way to restrict which unstable features are used.
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.
- direct-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.
- msrv-policy — MSRV-aware resolver and version selection
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.
- check-cfg — Compile-time validation of cfg expressions.
- host-config — Allows setting [target]-like configuration settings for host build targets.
- target-applies-to-host — Alters whether certain flags will be passed to host build targets.
rustdoc
- rustdoc-map — Provides mappings for documentation to link to external sites like docs.rs.
- scrape-examples — Shows examples within documentation.
Cargo.toml extensions
- Profile rustflags option — Passed directly to rustc.
- codegen-backend — Select the codegen backend used by rustc.
- per-package-target — Sets the --target to use for each individual package.
- artifact dependencies — Allow build artifacts to be included into other build artifacts and build them for different targets.
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
- publish-timeout — Controls the timeout between uploading the crate and being available in the index
- asymmetric-token — Adds support for authentication tokens using asymmetric cryptography (cargo:paseto provider).
Other
- gitoxide — Use gitoxide instead of git2 for a set of operations.
- script — Enable support for single-file .rs packages.

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

Original Issue: #3479
Tracking Issue: #7404

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

Original Issue: #4988
Tracking 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

Original Issue: #4100
Tracking Issue: #5657

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.

direct-minimal-versions

Original Issue: #4100
Tracking Issue: #5657

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

Indirect dependencies are resolved as normal so as not to be blocked on their minimal version validation.

out-dir

Original Issue: #4875
Tracking Issue: #6790

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

Tracking Issue: #7040
Tracking Rustc Issue: #64245

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

Build-plan

Tracking Issue: #5579

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

Tracking Issue: rust-lang/rust#49803
RFC: #2196

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

Tracking Issue: #44663

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

msrv-policy

#9930 (MSRV-aware resolver)
#10653 (MSRV-aware cargo-add)
#10903 (MSRV-aware cargo-install)

The msrv-policy feature enables experiments in MSRV-aware policy for cargo in preparation for an upcoming RFC.

build-std

Tracking Repository: https://github.com/rust-lang/wg-cargo-std-aware

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:

Cargo — https://github.com/rust-lang/cargo/issues/new — for implementation bugs
The tracking repository — https://github.com/rust-lang/wg-cargo-std-aware/issues/new — for larger design questions.

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

Tracking Repository: https://github.com/rust-lang/wg-cargo-std-aware

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

Tracking Issue: #67650
Original Pull Request: #7460

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!

config-include

Tracking Issue: #7723

This feature requires the -Zconfig-include command-line option.

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 an array of config file paths. Only path ending with .toml is accepted.

# a path ending with `.toml`
include = "path/to/mordor.toml"

# or an array of paths
include = ["frodo.toml", "samwise.toml"]

Unlike other config values, the merge behavior of the include key is different. When a config file contains an include key:

The config values are first loaded from the include path.
- If the value of the include key is an array of paths, the config values are loaded and merged from left to right for each path.
- Recurse this step if the config values from the include path also contain an include key.
Then, the config file’s own values are merged on top of the config from the include path.

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

Tracking Issue: #8002

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

Original Issue: rust-lang/cargo#7878
Tracking Issue: rust-lang/cargo#10271

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

Tracking Issue: #8296

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

Tracking Issue: #9406
Original Pull Request: #9030
Original Issue: #7004

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

Artifact dependencies 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.

artifact-dependencies: Dependency declarations

Artifact-dependencies adds the following keys to a dependency declaration in Cargo.toml:

artifact — This specifies the Cargo Target to build. Normally without this field, Cargo will only build the [lib] target from a dependency. This field allows specifying which target will be built, and made available as a binary at build time:
- "bin" — Compiled executable binaries, corresponding to all of the [[bin]] sections in the dependency’s manifest.
- "bin:<bin-name>" — Compiled executable binary, corresponding to a specific binary target specified by the given <bin-name>.
- "cdylib" — A C-compatible dynamic library, corresponding to a [lib] section with crate-type = ["cdylib"] in the dependency’s manifest.
- "staticlib" — A C-compatible static library, corresponding to a [lib] section with crate-type = ["staticlib"] in the dependency’s manifest.
The artifact value can be a string, or it can be an array of strings to specify multiple targets.

Example:
```
[dependencies]
bar = { version = "1.0", artifact = "staticlib" }
zoo = { version = "1.0", artifact = ["bin:cat", "bin:dog"]}
```
lib — This is a Boolean value which indicates whether or not to also build the dependency’s library as a normal Rust lib dependency. This field can only be specified when artifact is specified.

The default for this field is false when artifact is specified. If this is set to true, then the dependency’s [lib] target will also be built for the platform target the declaring package is being built for. This allows the package to use the dependency from Rust code like a normal dependency in addition to an artifact dependency.

Example:
```
[dependencies]
bar = { version = "1.0", artifact = "bin", lib = true }
```
target — The platform target to build the dependency for. This field can only be specified when artifact is specified.

The default if this is not specified depends on the dependency kind. For build dependencies, it will be built for the host target. For all other dependencies, it will be built for the same targets the declaring package is built for.

For a build dependency, this can also take the special value of "target" which means to build the dependency for the same targets that the package is being built for.
```
[build-dependencies]
bar = { version = "1.0", artifact = "cdylib", target = "wasm32-unknown-unknown"}
same-target = { version = "1.0", artifact = "bin", target = "target" }
```

artifact-dependencies: Environment variables

After building an artifact dependency, Cargo provides the following environment variables that you can use to access the artifact:

CARGO_<ARTIFACT-TYPE>_DIR_<DEP> — This is the directory containing all the artifacts from the dependency.

<ARTIFACT-TYPE> is the artifact specified for the dependency (uppercased as in CDYLIB, STATICLIB, or BIN) and <DEP> is the name of the dependency. As with other Cargo environment variables, dependency names are converted to uppercase, with dashes replaced by underscores.

If your manifest renames the dependency, <DEP> corresponds to the name you specify, not the original package name.
CARGO_<ARTIFACT-TYPE>_FILE_<DEP>_<NAME> — This is the full path to the artifact.

<ARTIFACT-TYPE> is the artifact specified for the dependency (uppercased as above), <DEP> is the name of the dependency (transformed as above), and <NAME> is the name of the artifact from the dependency.

Note that <NAME> is not modified in any way from the name specified in the crate supplying the artifact, or the crate name if not specified; for instance, it may be in lowercase, or contain dashes.

For convenience, if the artifact name matches the original package name, cargo additionally supplies a copy of this variable with the _<NAME> suffix omitted. For instance, if the cmake crate supplies a binary named cmake, Cargo supplies both CARGO_BIN_FILE_CMAKE and CARGO_BIN_FILE_CMAKE_cmake.

For each kind of dependency, these variables are supplied to the same part of the build process that has access to that kind of dependency:

For build-dependencies, these variables are supplied to the build.rs script, and can be accessed using std::env::var_os. (As with any OS file path, these may or may not be valid UTF-8.)
For normal dependencies, these variables are supplied during the compilation of the crate, and can be accessed using the env! macro.
For dev-dependencies, these variables are supplied during the compilation of examples, tests, and benchmarks, and can be accessed using the env! macro.

artifact-dependencies: Examples

Example: use a binary executable from a build script

In the Cargo.toml file, you can specify a dependency on a binary to make available for a build script:

[build-dependencies]
some-build-tool = { version = "1.0", artifact = "bin" }

Then inside the build script, the binary can be executed at build time:

fn main() {
    let build_tool = std::env::var_os("CARGO_BIN_FILE_SOME_BUILD_TOOL").unwrap();
    let status = std::process::Command::new(build_tool)
        .arg("do-stuff")
        .status()
        .unwrap();
    if !status.success() {
        eprintln!("failed!");
        std::process::exit(1);
    }
}

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

Tracking Issue: 11222

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. The current default is 60 seconds.

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

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

asymmetric-token

Tracking Issue: 10519
RFC: #3231

The -Z asymmetric-token flag enables the cargo:paseto credential provider which allows Cargo to authenticate 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 or token 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.

`cargo config`

Original Issue: #2362
Tracking Issue: #9301

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.

rustc `--print`

Tracking Issue: #9357

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

Tracking Issue: #9778
PR: #9627

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

RFC: #3123
Tracking Issue: #9910

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:

No target being documented requires dev-deps, AND
At least one crate with targets being documented has dev-deps, AND
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

RFC: #3013
Tracking Issue: #10554

-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:

#![allow(unused)]
fn main() {
// 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.

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"

gitoxide

Tracking Issue: #11813

With the ‘gitoxide’ unstable feature, all or the specified git operations will be performed by the gitoxide crate instead of git2.

While -Zgitoxide enables all currently implemented features, one can individually select git operations to run with gitoxide with the -Zgitoxide=operation[,operationN] syntax.

Valid operations are the following:

fetch - All fetches are done with gitoxide, which includes git dependencies as well as the crates index.
shallow-index - perform a shallow clone of the index.
shallow-deps - perform a shallow clone of git dependencies.
checkout (planned) - checkout the worktree, with support for filters and submodules.

Details on shallow clones

To enable shallow clones, add -Zgitoxide=fetch,shallow_deps for fetching git dependencies or -Zgitoxide=fetch,shallow_index for fetching registry index.
Shallow-cloned and shallow-checked-out git repositories reside at their own -shallow suffixed directories, i.e,
- ~/.cargo/registry/index/*-shallow
- ~/.cargo/git/db/*-shallow
- ~/.cargo/git/checkouts/*-shallow
When the unstable feature is on, fetching/cloning a git repository is always a shallow fetch. This roughly equals to git fetch --depth 1 everywhere.
Even with the presence of Cargo.lock or specifying a commit { rev = "…" }, gitoxide is still smart enough to shallow fetch without unshallowing the existing repository.

script

Tracking Issue: #12207

Cargo can directly run .rs files as:

$ cargo +nightly -Zscript file.rs

where file.rs can be as simple as:

fn main() {}

A user may optionally specify a manifest in a cargo code fence in a module-level comment, like:

#!/usr/bin/env -S cargo +nightly -Zscript
```cargo
[dependencies]
clap = { version = "4.2", features = ["derive"] }
```

use clap::Parser;

#[derive(Parser, Debug)]
#[clap(version)]
struct Args {
    #[clap(short, long, help = "Path to config")]
    config: Option<std::path::PathBuf>,
}

fn main() {
    let args = Args::parse();
    println!("{:?}", args);
}

Single-file packages

In addition to today’s multi-file packages (Cargo.toml file with other .rs files), we are adding the concept of single-file packages which may contain an embedded manifest. There is no required distinguishment for a single-file .rs package from any other .rs file.

Single-file packages may be selected via --manifest-path, like cargo test --manifest-path foo.rs. Unlike Cargo.toml, these files cannot be auto-discovered.

A single-file package may contain an embedded manifest. An embedded manifest is stored using TOML in rust “frontmatter”, a markdown code-fence with cargo at the start of the infostring at the top of the file.

Inferred / defaulted manifest fields:

package.name = <slugified file stem>
package.version = "0.0.0" to call attention to this crate being used in unexpected places
package.publish = false to avoid accidental publishes, particularly if we later add support for including them in a workspace.
package.edition = <current> to avoid always having to add an embedded manifest at the cost of potentially breaking scripts on rust upgrades
- Warn when edition is unspecified to raise awareness of this

Disallowed manifest fields:

[workspace], [lib], [[bin]], [[example]], [[test]], [[bench]]
package.workspace, package.build, package.links, package.autobins, package.autoexamples, package.autotests, package.autobenches

The default CARGO_TARGET_DIR for single-file packages is at $CARGO_HOME/target/<hash>:

Avoid conflicts from multiple single-file packages being in the same directory
Avoid problems with the single-file package’s parent directory being read-only
Avoid cluttering the user’s directory

The lockfile for single-file packages will be placed in CARGO_TARGET_DIR. In the future, when workspaces are supported, that will allow a user to have a persistent lockfile.

Manifest-commands

You may pass a manifest directly to the cargo command, without a subcommand, like foo/Cargo.toml or a single-file package like foo.rs. This is mostly intended for being put in #! lines.

The precedence for how to interpret cargo <subcommand> is

Built-in xor single-file packages
Aliases
External subcommands

A parameter is identified as a manifest-command if it has one of:

Path separators
A .rs extension
The file name is Cargo.toml

Differences between cargo run --manifest-path <path> and cargo <path>

cargo <path> runs with the config for <path> and not the current dir, more like cargo install --path <path>
cargo <path> is at a verbosity level below the normal default. Pass -v to get normal output.

Documentation Updates

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.

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