Module cargo::core::features

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Support for nightly features in Cargo itself.

This file is the version of feature_gate.rs in upstream Rust for Cargo itself and is intended to be the avenue for which new features in Cargo are gated by default and then eventually stabilized. All known stable and unstable features are tracked in this file.

If you’re reading this then you’re likely interested in adding a feature to Cargo, and the good news is that it shouldn’t be too hard! First determine how the feature should be gated:

  • New syntax in Cargo.toml should use cargo-features.
  • New CLI options should use -Z unstable-options.
  • 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 should use a new -Z flag.

See below for more details.

When adding new tests for your feature, usually the tests should go into a new module of the testsuite. See https://doc.crates.io/contrib/tests/writing.html for more information on writing tests. Particularly, check out the “Testing Nightly Features” section for testing unstable features.

After you have added your feature, be sure to update the unstable documentation at src/doc/src/reference/unstable.md to include a short description of how to use your new feature.

And hopefully that’s it!

New Cargo.toml syntax

The steps for adding new Cargo.toml syntax are:

  1. Add the cargo-features unstable gate. Search below for “look here” to find the features! macro invocation and add your feature to the list.

  2. Update the Cargo.toml parsing code to handle your new feature.

  3. Wherever you added the new parsing code, call features.require(Feature::my_feature_name())? if the new syntax is used. This will return an error if the user hasn’t listed the feature in cargo-features or this is not the nightly channel.

-Z unstable-options

-Z unstable-options is intended to force the user to opt-in to new CLI flags, options, and new subcommands.

The steps to add a new command-line option are:

  1. Add the option to the CLI parsing code. In the help text, be sure to include (unstable) to note that this is an unstable option.
  2. Where the CLI option is loaded, be sure to call CliUnstable::fail_if_stable_opt. This will return an error if -Z unstable options was not passed.

-Z options

New -Z options cover all other functionality that isn’t covered with cargo-features or -Z unstable-options.

The steps to add a new -Z option are:

  1. Add the option to the CliUnstable struct below. Flags can take an optional value if you want.
  2. Update the CliUnstable::add function to parse the flag.
  3. Wherever the new functionality is implemented, call Config::cli_unstable to get an instance of CliUnstable and check if the option has been enabled on the CliUnstable instance. Nightly gating is already handled, so no need to worry about that.

-Z vs cargo-features

In some cases there might be some changes that cargo-features is unable to sufficiently encompass. An example would be a syntax change in Cargo.toml that also impacts the index or resolver. The resolver doesn’t know about cargo-features, so it needs a -Z flag to enable the experimental functionality.

In those cases, you usually should introduce both a -Z flag (to enable the changes outside of the manifest) and a cargo-features entry (to enable the new syntax in Cargo.toml). The cargo-features entry ensures that any experimental syntax that gets uploaded to crates.io is clearly intended for nightly-only builds. Otherwise, users accessing those crates may get confusing errors, particularly if the syntax changes during the development cycle, and the user tries to access it with a stable release.

-Z with external files

Some files, such as config.toml config files, or the config.json index file, are used in a global location which can make interaction with stable releases problematic. In general, before the feature is stabilized, stable Cargo should behave roughly similar to how it behaved before the unstable feature was introduced. If Cargo would normally have ignored or warned about the introduction of something, then it probably should continue to do so.

For example, Cargo generally ignores (or warns) about config.toml entries it doesn’t know about. This allows a limited degree of forwards-compatibility with future versions of Cargo that add new entries.

Whether or not to warn on stable may need to be decided on a case-by-case basis. For example, you may want to avoid generating a warning for options that are not critical to Cargo’s operation in order to reduce the annoyance of constant warnings. However, ignoring some options may prevent proper operation, so a warning may be valuable for a user trying to diagnose why it isn’t working correctly.

Stabilization

For the stabilization process, see https://doc.crates.io/contrib/process/unstable.html#stabilization.

The steps for stabilizing are roughly:

  1. Update the feature to be stable, based on the kind of feature:
  2. cargo-features: Change the feature to stable in the features! macro invocation below, and include the version and a URL for the documentation.
  3. -Z unstable-options: Find the call to fail_if_stable_opt and remove it. Be sure to update the man pages if necessary.
  4. -Z flag: Change the parsing code in CliUnstable::add to call stabilized_warn or stabilized_err and remove the field from CliUnstable. Remove the (unstable) note in the clap help text if necessary.
  5. Remove masquerade_as_nightly_cargo from any tests, and remove cargo-features from Cargo.toml test files if any. You can quickly find what needs to be removed by searching for the name of the feature, e.g. print_im_a_teapot
  6. Update the docs in unstable.md to move the section to the bottom and summarize it similar to the other entries. Update the rest of the documentation to add the new feature.

Structs

Enums

Constants

Functions

  • channel
    Returns the current release channel (“stable”, “beta”, “nightly”, “dev”).