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 timings feature can be enabled with:

    cargo +nightly build -Z timings

    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
multitarget = true
timings = ["html"]

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.
    • Namespaced features — Separates optional dependencies into a separate namespace from regular features, and allows feature names to be the same as some dependency name.
    • Weak dependency features — Allows setting features for dependencies without enabling optional dependencies.
  • Output behavior
    • out-dir — Adds a directory where artifacts are copied to.
    • terminal-width — Tells rustc the width of the terminal so that long diagnostic messages can be truncated to be more readable.
    • Different binary name — Assign a name to the built binary that is seperate 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.
    • multitarget — Supports building for multiple targets at the same time.
    • 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.
  • rustdoc
  • Cargo.toml extensions
  • Information and metadata
    • Build-plan — Emits JSON information on which commands will be run.
    • timings — Generates a report on how long individual dependencies took to run.
    • unit-graph — Emits JSON for Cargo's internal graph structure.
    • future incompat report — Displays a report for future incompatibilities that may error in the future.
    • cargo rustc --print — Calls rustc with --print to display information from rustc.
  • Configuration
    • config-cli — Adds the ability to pass configuration options on the command-line.
    • 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.

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

multitarget

This flag allows passing multiple --target flags to the cargo subcommand selected. When multiple --target flags are passed the selected build targets will be built for each of the selected architectures.

For example to compile a library for both 32 and 64-bit:

cargo build --target x86_64-unknown-linux-gnu --target i686-unknown-linux-gnu

or running tests for both targets:

cargo test --target x86_64-unknown-linux-gnu --target i686-unknown-linux-gnu

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

Namespaced features

The namespaced-features option makes two changes to how features can be specified:

  • Features may now be defined with the same name as a dependency.
  • Optional dependencies can be explicitly enabled in the [features] table with the dep: prefix, which enables the dependency without enabling a feature of the same name.

By default, an optional dependency foo will define a feature foo = ["dep:foo"] unless dep:foo is mentioned in any other feature, or the foo feature is already defined. This helps prevent unnecessary boilerplate of listing every optional dependency, but still allows you to override the implicit feature.

This allows two use cases that were previously not possible:

  • You can "hide" an optional dependency, so that external users cannot explicitly enable that optional dependency.
  • There is no longer a need to create "funky" feature names to work around the restriction that features cannot shadow dependency names.

To enable namespaced-features, use the -Z namespaced-features command-line flag.

An example of hiding an optional dependency:

[dependencies]
regex = { version = "1.4.1", optional = true }
lazy_static = { version = "1.4.0", optional = true }

[features]
regex = ["dep:regex", "dep:lazy_static"]

In this example, the "regex" feature enables both regex and lazy_static. The lazy_static feature does not exist, and a user cannot explicitly enable it. This helps hide internal details of how your package is implemented.

An example of avoiding "funky" names:

[dependencies]
bigdecimal = "0.1"
chrono = "0.4"
num-bigint = "0.2"
serde = {version = "1.0", optional = true }

[features]
serde = ["dep:serde", "bigdecimal/serde", "chrono/serde", "num-bigint/serde"]

In this case, serde is a natural name to use for a feature, because it is relevant to your exported API. However, previously you would need to use a name like serde1 to work around the naming limitation if you wanted to also enable other features.

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.

timings

The timings feature gives some information about how long each compilation takes, and tracks concurrency information over time.

cargo +nightly build -Z timings

The -Ztimings flag can optionally take a comma-separated list of the following values:

  • html — Saves a file called cargo-timing.html to the current directory with a report of the compilation. Files are also saved with a timestamp in the filename if you want to look at older runs.
  • info — Displays a message to stdout after each compilation finishes with how long it took.
  • json — Emits some JSON information about timing information.

The default if none are specified is html,info.

Reading the graphs

There are two graphs in the output. The "unit" graph shows the duration of each unit over time. A "unit" is a single compiler invocation. There are lines that show which additional units are "unlocked" when a unit finishes. That is, it shows the new units that are now allowed to run because their dependencies are all finished. Hover the mouse over a unit to highlight the lines. This can help visualize the critical path of dependencies. This may change between runs because the units may finish in different orders.

The "codegen" times are highlighted in a lavender color. In some cases, build pipelining allows units to start when their dependencies are performing code generation. This information is not always displayed (for example, binary units do not show when code generation starts).

The "custom build" units are build.rs scripts, which when run are highlighted in orange.

The second graph shows Cargo's concurrency over time. The three lines are:

  • "Waiting" (red) — This is the number of units waiting for a CPU slot to open.
  • "Inactive" (blue) — This is the number of units that are waiting for their dependencies to finish.
  • "Active" (green) — This is the number of units currently running.

Note: This does not show the concurrency in the compiler itself. rustc coordinates with Cargo via the "job server" to stay within the concurrency limit. This currently mostly applies to the code generation phase.

Tips for addressing compile times:

  • Look for slow dependencies.
    • Check if they have features that you may wish to consider disabling.
    • Consider trying to remove the dependency completely.
  • Look for a crate being built multiple times with different versions. Try to remove the older versions from the dependency graph.
  • Split large crates into smaller pieces.
  • If there are a large number of crates bottlenecked on a single crate, focus your attention on improving that one crate to improve parallelism.

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!

config-cli

The --config CLI option allows arbitrary config values to be passed in via the command-line. The argument should be in TOML syntax of KEY=VALUE:

cargo +nightly -Zunstable-options --config net.git-fetch-with-cli=true fetch

The --config option may be specified multiple times, in which case the values are merged in left-to-right order, using the same merging logic that multiple config files use. CLI values take precedence over environment variables, which take precedence over config files.

Some examples of what it looks like using Bourne shell syntax:

# Most shells will require escaping.
cargo --config http.proxy=\"http://example.com\" …

# Spaces may be used.
cargo --config "net.git-fetch-with-cli = true" …

# TOML array example. Single quotes make it easier to read and write.
cargo --config 'build.rustdocflags = ["--html-in-header", "header.html"]' …

# Example of a complex TOML key.
cargo --config "target.'cfg(all(target_arch = \"arm\", target_os = \"none\"))'.runner = 'my-runner'" …

# Example of overriding a profile setting.
cargo --config profile.dev.package.image.opt-level=3 …

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

The target-applies-to-host key in a config file can be used set the desired behavior for passing target config flags to build scripts.

It requires the -Ztarget-applies-to-host command-line option.

The current default for target-applies-to-host is true, which will be changed to false in the future, if -Zhost-config is used the new false default will be set for target-applies-to-host.

# 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.

# config.toml
[host]
linker = "/path/to/host/linker"
[host.x86_64-unknown-linux-gnu]
linker = "/path/to/host/arch/linker"
[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 strip option

This feature provides a new option in the [profile] section to strip either symbols or debuginfo from a binary. This can be enabled like so:

cargo-features = ["strip"]

[package]
# ...

[profile.release]
strip = "debuginfo"

Other possible string values of strip are none, symbols, and off. The default is none.

You can also configure this option with the two absolute boolean values true and false. The former enables strip at its higher level, symbols, while the latter disables strip completely.

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.

terminal-width

This feature provides a new flag, -Z terminal-width, which is used to pass a terminal width to rustc so that error messages containing long lines can be intelligently truncated.

For example, passing -Z terminal-width=20 (an arbitrarily low value) might produce the following error:

error[E0308]: mismatched types
  --> src/main.rs:2:17
  |
2 | ..._: () = 42;
  |       --   ^^ expected `()`, found integer
  |       |
  |       expected due to this

error: aborting due to previous error

In contrast, without -Z terminal-width, the error would look as shown below:

error[E0308]: mismatched types
 --> src/main.rs:2:17
  |
2 |     let _: () = 42;
  |            --   ^^ expected `()`, found integer
  |            |
  |            expected due to this

error: aborting due to previous error

Weak dependency features

The -Z weak-dep-features command-line options enables the ability to use dep_name?/feat_name syntax in the [features] table. The ? indicates that the optional dependency dep_name will not be automatically enabled. The feature feat_name will only be added if something else enables the dep_name dependency.

Example:

[dependencies]
serde = { version = "1.0.117", optional = true, default-features = false }

[features]
std = ["serde?/std"]

In this example, the std feature enables the std feature on the serde dependency. However, unlike the normal serde/std syntax, it will not enable the optional dependency serde unless something else has included it.

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.

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 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_NAME — Name of the registry the authentication token is for.
  • CARGO_REGISTRY_API_URL — The URL of the registry API.

cargo logout

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

When used with credentials 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

future incompat report

The -Z future-incompat-report flag causes Cargo to check for future-incompatible warnings in all dependencies. These are warnings for changes that may become hard errors in the future, causing the dependency to stop building in a future version of rustc. If any warnings are found, a small notice is displayed indicating that the warnings were found, and provides instructions on how to display a full report.

A full report can be displayed with the cargo report future-incompatibilities -Z future-incompat-report --id ID command, or by running the build again with the --future-incompat-report flag. The developer should then update their dependencies to a version where the issue is fixed, or work with the developers of the dependencies to help resolve the issue.

This feature can be configured through a [future-incompat-report] section in .cargo/config. Currently, the supported options are:

[future-incompat-report]
frequency = FREQUENCY

The supported values for FREQUENCY are 'alwaysand 'never', which control whether or not a message is printed out at the end ofcargo build/cargo check`.

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"]

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

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

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.

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"

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.