Unstable Features - The Cargo Book

## Unstable Features

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

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

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

### 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` 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
- Stabilization Issue: #5133

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

### minimal-versions

- 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`.

### 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` 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`. See the rustc issue for more information.

    cargo test --target foo -Zdoctest-xcompile

### Custom named profiles

- Tracking Issue: rust-lang/cargo#6988
- RFC: #2678

With this feature you can define custom profiles having new names. With the custom profile enabled, build artifacts can be emitted by default to directories other than `release` or `debug`, based on the custom profile's name.

For example:

    cargo-features = ["named-profiles"]

    [profile.release-lto]
    inherits = "release"
    lto = true

An `inherits` key is used in order to receive attributes from other profiles, so that a new custom profile can be based on the standard `dev` or `release` profile presets. Cargo emits errors in case `inherits` loops are detected. When considering inheritance hierarchy, all profiles directly or indirectly inherit from either from `release` or from `dev`.

Valid profile names are: must not be empty, use only alphanumeric characters or `-` or `_`.

Passing `--profile` with the profile's name to various Cargo commands, directs operations to use the profile's attributes. Overrides that are specified in the profiles from which the custom profile inherits are inherited too.

For example, using `cargo build` with `--profile` and the manifest from above:

    cargo +nightly build --profile release-lto -Z unstable-options

When a custom profile is used, build artifcats go to a different target by default. In the example above, you can expect to see the outputs under `target/release-lto`.

#### 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

- Original issue: #1286
- Tracking Issue: #5565

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

    [package]
    namespaced-features = true

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

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

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

### Build-plan

- 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

### 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!

### timings

- Tracking Issue: #7405

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

- Tracking Issue: #7722

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

- Tracking Issue: #7723

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.

### Features

- Tracking Issues:
  - itarget #7914
  - build_dep #7915
  - dev_dep #7916

The `-Zfeatures` option causes Cargo to use a new feature resolver that can resolve features differently from before. It takes a comma separated list of options to indicate which new behaviors to enable. With no options, it should behave the same as without the flag.

    cargo +nightly -Zfeatures=itarget,build_dep

The available options are:

- `itarget` — Ignores features for target-specific dependencies for targets that don't match the current compile target. For example:

      [dependency.common]
      version = "1.0"
      features = ["f1"]

      [target.'cfg(windows)'.dependencies.common]
      version = "1.0"
      features = ["f2"]

  When building this example for a non-Windows platform, the `f2` feature will not be enabled.

- `host_dep` — Prevents features enabled on build dependencies or proc-macros from being enabled for normal dependencies. For example:

      [dependencies]
      log = "0.4"

      [build-dependencies]
      log = {version = "0.4", features=['std']}

  When building the build script, the `log` crate will be built with the `std` feature. When building the library of your package, it will not enable the feature.

  Note that proc-macro decoupling requires changes to the registry, so it won't be decoupled until the registry is updated to support the new field.

- `dev_dep` — Prevents features enabled on dev dependencies from being enabled for normal dependencies. For example:

      [dependencies]
      serde = {version = "1.0", default-features = false}

      [dev-dependencies]
      serde = {version = "1.0", features = ["std"]}

  In this example, the library will normally link against `serde` without the `std` feature. However, when built as a test or example, it will include the `std` feature.

  This mode is ignored if you are building any test, bench, or example. That is, dev dependency features will still be unified if you run commands like `cargo test` or `cargo build --all-targets`.

- `all` — Enable all feature options (`itarget,build_dep,dev_dep`).

- `compare` — This option compares the resolved features to the old resolver, and will print any differences.

### package-features

- Tracking Issue: #5364

The `-Zpackage-features` flag changes the way features can be passed on the command-line for a workspace. The normal behavior can be confusing, as the features passed are always enabled on the package in the current directory, even if that package is not selected with a `-p` flag. Feature flags also do not work in the root of a virtual workspace. `-Zpackage-features` tries to make feature flags behave in a more intuitive manner.

- `cargo build -p other_member --features …` — This now only enables the given features as defined in `other_member` (ignores whatever is in the current directory).
- `cargo build -p a -p b --features …` — This now enables the given features on both `a` and `b`. Not all packages need to define every feature, it only enables matching features. It is still an error if none of the packages define a given feature.
- `--features` and `--no-default-features` are now allowed in the root of a virtual workspace.
- `member_name/feature_name` syntax may now be used on the command-line to enable features for a specific member.

The ability to set features for non-workspace members is no longer allowed, as the resolver fundamentally does not support that ability.

### crate-versions

- Tracking Issue: #7907

The `-Z crate-versions` flag will make `cargo doc` include appropriate crate versions for the current crate and all of its dependencies (unless `--no-deps` was provided) in the compiled documentation.

You can find an example screenshot for the cargo itself in the tracking issue.

### 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",
            "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],
    }