#
cargo/core/
features.rs

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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:
    //!
    //! * Error when the feature is used without the gate
    //!   * Required if ignoring the feature violates the users intent in non-superficial ways
    //!   * A low-effort / safe way to protect the user from being broken if the format of the feature changes in
    //!     incompatible was (can be worked around)
    //!   * Good for: CLI (gate: `-Zunstable-options` or `-Z` if combined with other changes), `Cargo.toml` (gate: `cargo-features`)
    //! * Warn that the feature is ignored due to lack of the gate
    //!   * For if you could opt-in to the unimplemented feature on Cargo today and Cargo would
    //!     operate just fine
    //!   * If gate is not enabled, prefer to warn if the format of the feature is incompatible
    //!     (instead of error or ignore)
    //!   * Good for: `Cargo.toml`, `.cargo/config.toml`, `config.json` index file (gate: `-Z`)
    //! * Ignore the feature that is used without a gate
    //!   * For when ignoring the feature has so little impact that annoying the user is not worth it
    //!     (e.g. a config field that changes Cargo's terminal output)
    //!   * For behavior changes without an interface (e.g. the resolver)
    //!   * Good for: `.cargo/config.toml`, `config.json` index file (gate: `-Z`)
    //!
    //! For features that touch multiple parts of Cargo, multiple feature gating strategies (error,
    //! warn, ignore) and mechanisms (`-Z`, `cargo-features`) may be used.
    //!
    //! When adding new tests for your feature, usually the tests should go into a
    //! new module of the testsuite named after the feature. 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. Be sure to test the feature gate itself.
    //!
    //! 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!
    //!
    //! ## `cargo-features`
    //!
    //! The steps for adding new Cargo.toml syntax are:
    //!
    //! 1. Add the cargo-features unstable gate. Search the code 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 in the macro invocation of
    //!    [`unstable_cli_options!`]. 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
    //!    [`GlobalContext::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.
    //!    If warning when feature is used without the gate, be sure to gracefully degrade (with a
    //!    warning) when the `Cargo.toml` / `.cargo/config.toml` field usage doesn't match the
    //!    schema.
    //! 4. For any `Cargo.toml` fields, strip them in [`prepare_for_publish`] if the gate isn't set
    //!
    //! ## 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:
    //!   1. `cargo-features`: Change the feature to `stable` in the [`features!`]
    //!      macro invocation below, and include the version and a URL for the
    //!      documentation.
    //!   2. `-Z unstable-options`: Find the call to [`fail_if_stable_opt`] and
    //!      remove it. Be sure to update the man pages if necessary.
    //!   3. `-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.
    //! 2. 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`
    //! 3. 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.
    //!
    //! [`GlobalContext::cli_unstable`]: crate::util::context::GlobalContext::cli_unstable
    //! [`fail_if_stable_opt`]: CliUnstable::fail_if_stable_opt
    //! [`features!`]: macro.features.html
    //! [`unstable_cli_options!`]: macro.unstable_cli_options.html
    //! [`prepare_for_publish`]: crate::util::toml::prepare_for_publish

    use std::collections::BTreeSet;
    use std::env;
    use std::fmt::{self, Write};
    use std::path::PathBuf;
    use std::str::FromStr;

    use anyhow::{bail, Error};
    use cargo_util::ProcessBuilder;
    use serde::{Deserialize, Serialize};

    use crate::core::resolver::ResolveBehavior;
    use crate::util::errors::CargoResult;
    use crate::util::indented_lines;
    use crate::GlobalContext;

    pub const SEE_CHANNELS: &str =
        "See https://doc.rust-lang.org/book/appendix-07-nightly-rust.html for more information \
         about Rust release channels.";

    /// Value of [`allow-features`](CliUnstable::allow_features)
    pub type AllowFeatures = BTreeSet<String>;

    /// The edition of the compiler ([RFC 2052])
    ///
    /// The following sections will guide you how to add and stabilize an edition.
    ///
    /// ## Adding a new edition
    ///
    /// - Add the next edition to the enum.
    /// - Update every match expression that now fails to compile.
    /// - Update the [`FromStr`] impl.
    /// - Update [`CLI_VALUES`] to include the new edition.
    /// - Set [`LATEST_UNSTABLE`] to Some with the new edition.
    /// - Add an unstable feature to the [`features!`] macro invocation below for the new edition.
    /// - Gate on that new feature in [`toml`].
    /// - Update the shell completion files.
    /// - Update any failing tests (hopefully there are very few).
    /// - Update unstable.md to add a new section for this new edition (see [this example]).
    ///
    /// ## Stabilization instructions
    ///
    /// - Set [`LATEST_UNSTABLE`] to None.
    /// - Set [`LATEST_STABLE`] to the new version.
    /// - Update [`is_stable`] to `true`.
    /// - Set [`first_version`] to the version it will be released.
    /// - Set the editionNNNN feature to stable in the [`features!`] macro invocation below.
    /// - Update any tests that are affected.
    /// - Update the man page for the `--edition` flag.
    /// - Update unstable.md to move the edition section to the bottom.
    /// - Update the documentation:
    ///   - Update any features impacted by the edition.
    ///   - Update manifest.md#the-edition-field.
    ///   - Update the `--edition` flag (options-new.md).
    ///   - Rebuild man pages.
    ///
    /// [RFC 2052]: https://rust-lang.github.io/rfcs/2052-epochs.html
    /// [`FromStr`]: Edition::from_str
    /// [`CLI_VALUES`]: Edition::CLI_VALUES
    /// [`LATEST_UNSTABLE`]: Edition::LATEST_UNSTABLE
    /// [`LATEST_STABLE`]: Edition::LATEST_STABLE
    /// [this example]: https://github.com/rust-lang/cargo/blob/3ebb5f15a940810f250b68821149387af583a79e/src/doc/src/reference/unstable.md?plain=1#L1238-L1264
    /// [`first_version`]: Edition::first_version
    /// [`is_stable`]: Edition::is_stable
    /// [`toml`]: crate::util::toml
    /// [`features!`]: macro.features.html
    #[derive(
        Default, Clone, Copy, Debug, Hash, PartialOrd, Ord, Eq, PartialEq, Serialize, Deserialize,
    )]
    pub enum Edition {
        /// The 2015 edition
        #[default]
        Edition2015,
        /// The 2018 edition
        Edition2018,
        /// The 2021 edition
        Edition2021,
        /// The 2024 edition
        Edition2024,
    }

    impl Edition {
        /// The latest edition that is unstable.
        ///
        /// This is `None` if there is no next unstable edition.
        pub const LATEST_UNSTABLE: Option<Edition> = None;
        /// The latest stable edition.
        pub const LATEST_STABLE: Edition = Edition::Edition2024;
        pub const ALL: &'static [Edition] = &[
            Self::Edition2015,
            Self::Edition2018,
            Self::Edition2021,
            Self::Edition2024,
        ];
        /// Possible values allowed for the `--edition` CLI flag.
        ///
        /// This requires a static value due to the way clap works, otherwise I
        /// would have built this dynamically.
        pub const CLI_VALUES: [&'static str; 4] = ["2015", "2018", "2021", "2024"];

        /// Returns the first version that a particular edition was released on
        /// stable.
        pub(crate) fn first_version(&self) -> Option<semver::Version> {
            use Edition::*;
            match self {
                Edition2015 => None,
                Edition2018 => Some(semver::Version::new(1, 31, 0)),
                Edition2021 => Some(semver::Version::new(1, 56, 0)),
                Edition2024 => Some(semver::Version::new(1, 85, 0)),
            }
        }

        /// Returns `true` if this edition is stable in this release.
        pub fn is_stable(&self) -> bool {
            use Edition::*;
            match self {
                Edition2015 => true,
                Edition2018 => true,
                Edition2021 => true,
                Edition2024 => true,
            }
        }

        /// Returns the previous edition from this edition.
        ///
        /// Returns `None` for 2015.
        pub fn previous(&self) -> Option<Edition> {
            use Edition::*;
            match self {
                Edition2015 => None,
                Edition2018 => Some(Edition2015),
                Edition2021 => Some(Edition2018),
                Edition2024 => Some(Edition2021),
            }
        }

        /// Returns the next edition from this edition, returning the last edition
        /// if this is already the last one.
        pub fn saturating_next(&self) -> Edition {
            use Edition::*;
            match self {
                Edition2015 => Edition2018,
                Edition2018 => Edition2021,
                Edition2021 => Edition2024,
                Edition2024 => Edition2024,
            }
        }

        /// Updates the given [`ProcessBuilder`] to include the appropriate flags
        /// for setting the edition.
        pub(crate) fn cmd_edition_arg(&self, cmd: &mut ProcessBuilder) {
            cmd.arg(format!("--edition={}", self));
            if !self.is_stable() {
                cmd.arg("-Z").arg("unstable-options");
            }
        }

        /// Whether or not this edition supports the `rust_*_compatibility` lint.
        ///
        /// Ideally this would not be necessary, but editions may not have any
        /// lints, and thus `rustc` doesn't recognize it. Perhaps `rustc` could
        /// create an empty group instead?
        pub(crate) fn supports_compat_lint(&self) -> bool {
            use Edition::*;
            match self {
                Edition2015 => false,
                Edition2018 => true,
                Edition2021 => true,
                Edition2024 => true,
            }
        }

        /// Whether or not this edition supports the `rust_*_idioms` lint.
        ///
        /// Ideally this would not be necessary...
        pub(crate) fn supports_idiom_lint(&self) -> bool {
            use Edition::*;
            match self {
                Edition2015 => false,
                Edition2018 => true,
                Edition2021 => false,
                Edition2024 => false,
            }
        }

        pub(crate) fn default_resolve_behavior(&self) -> ResolveBehavior {
            if *self >= Edition::Edition2024 {
                ResolveBehavior::V3
            } else if *self >= Edition::Edition2021 {
                ResolveBehavior::V2
            } else {
                ResolveBehavior::V1
            }
        }
    }

    impl fmt::Display for Edition {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            match *self {
                Edition::Edition2015 => f.write_str("2015"),
                Edition::Edition2018 => f.write_str("2018"),
                Edition::Edition2021 => f.write_str("2021"),
                Edition::Edition2024 => f.write_str("2024"),
            }
        }
    }

    impl FromStr for Edition {
        type Err = Error;
        fn from_str(s: &str) -> Result<Self, Error> {
            match s {
                "2015" => Ok(Edition::Edition2015),
                "2018" => Ok(Edition::Edition2018),
                "2021" => Ok(Edition::Edition2021),
                "2024" => Ok(Edition::Edition2024),
                s if s.parse().map_or(false, |y: u16| y > 2024 && y < 2050) => bail!(
                    "this version of Cargo is older than the `{}` edition, \
                     and only supports `2015`, `2018`, `2021`, and `2024` editions.",
                    s
                ),
                s => bail!(
                    "supported edition values are `2015`, `2018`, `2021`, or `2024`, \
                     but `{}` is unknown",
                    s
                ),
            }
        }
    }

    #[derive(Debug, PartialEq)]
    enum Status {
        Stable,
        Unstable,
        Removed,
    }

    /// A listing of stable and unstable new syntax in Cargo.toml.
    ///
    /// This generates definitions and impls for [`Features`] and [`Feature`]
    /// for each new syntax.
    ///
    /// Note that all feature names in the macro invocation are valid Rust
    /// identifiers, but the `_` character is translated to `-` when specified in
    /// the `cargo-features` manifest entry in `Cargo.toml`.
    ///
    /// See the [module-level documentation](self#new-cargotoml-syntax)
    /// for the process of adding a new syntax.
    macro_rules! features {
        (
            $(
                $(#[$attr:meta])*
                ($stab:ident, $feature:ident, $version:expr, $docs:expr),
            )*
        ) => (
            /// Unstable feature context for querying if a new Cargo.toml syntax
            /// is allowed to use.
            ///
            /// See the [module-level documentation](self#new-cargotoml-syntax) for the usage.
            #[derive(Default, Clone, Debug)]
            pub struct Features {
                $($feature: bool,)*
                /// The current activated features.
                activated: Vec<String>,
                /// Whether is allowed to use any unstable features.
                nightly_features_allowed: bool,
                /// Whether the source manifest is from a local package.
                is_local: bool,
            }

            impl Feature {
                $(
                    $(#[$attr])*
                    #[doc = concat!("\n\n\nSee <https://doc.rust-lang.org/nightly/cargo/", $docs, ">.")]
                    pub const fn $feature() -> &'static Feature {
                        fn get(features: &Features) -> bool {
                            stab!($stab) == Status::Stable || features.$feature
                        }
                        const FEAT: Feature = Feature {
                            name: stringify!($feature),
                            stability: stab!($stab),
                            version: $version,
                            docs: $docs,
                            get,
                        };
                        &FEAT
                    }
                )*

                /// Whether this feature is allowed to use in the given [`Features`] context.
                fn is_enabled(&self, features: &Features) -> bool {
                    (self.get)(features)
                }

                pub(crate) fn name(&self) -> &str {
                    self.name
                }
            }

            impl Features {
                fn status(&mut self, feature: &str) -> Option<(&mut bool, &'static Feature)> {
                    if feature.contains("_") {
                        return None;
                    }
                    let feature = feature.replace("-", "_");
                    $(
                        if feature == stringify!($feature) {
                            return Some((&mut self.$feature, Feature::$feature()));
                        }
                    )*
                    None
                }
            }
        )
    }

    macro_rules! stab {
        (stable) => {
            Status::Stable
        };
        (unstable) => {
            Status::Unstable
        };
        (removed) => {
            Status::Removed
        };
    }

    // "look here"
    features! {
        /// A dummy feature that doesn't actually gate anything, but it's used in
        /// testing to ensure that we can enable stable features.
        (stable, test_dummy_stable, "1.0", ""),

        /// A dummy feature that gates the usage of the `im-a-teapot` manifest
        /// entry. This is basically just intended for tests.
        (unstable, test_dummy_unstable, "", "reference/unstable.html"),

        /// Downloading packages from alternative registry indexes.
        (stable, alternative_registries, "1.34", "reference/registries.html"),

        /// Using editions
        (stable, edition, "1.31", "reference/manifest.html#the-edition-field"),

        /// Renaming a package in the manifest via the `package` key.
        (stable, rename_dependency, "1.31", "reference/specifying-dependencies.html#renaming-dependencies-in-cargotoml"),

        /// Whether a lock file is published with this crate.
        (removed, publish_lockfile, "1.37", "reference/unstable.html#publish-lockfile"),

        /// Overriding profiles for dependencies.
        (stable, profile_overrides, "1.41", "reference/profiles.html#overrides"),

        /// "default-run" manifest option.
        (stable, default_run, "1.37", "reference/manifest.html#the-default-run-field"),

        /// Declarative build scripts.
        (unstable, metabuild, "", "reference/unstable.html#metabuild"),

        /// Specifying the 'public' attribute on dependencies.
        (unstable, public_dependency, "", "reference/unstable.html#public-dependency"),

        /// Allow to specify profiles other than 'dev', 'release', 'test', etc.
        (stable, named_profiles, "1.57", "reference/profiles.html#custom-profiles"),

        /// Opt-in new-resolver behavior.
        (stable, resolver, "1.51", "reference/resolver.html#resolver-versions"),

        /// Allow to specify whether binaries should be stripped.
        (stable, strip, "1.58", "reference/profiles.html#strip-option"),

        /// Specifying a minimal 'rust-version' attribute for crates.
        (stable, rust_version, "1.56", "reference/manifest.html#the-rust-version-field"),

        /// Support for 2021 edition.
        (stable, edition2021, "1.56", "reference/manifest.html#the-edition-field"),

        /// Allow to specify per-package targets (compile kinds).
        (unstable, per_package_target, "", "reference/unstable.html#per-package-target"),

        /// Allow to specify which codegen backend should be used.
        (unstable, codegen_backend, "", "reference/unstable.html#codegen-backend"),

        /// Allow specifying different binary name apart from the crate name.
        (unstable, different_binary_name, "", "reference/unstable.html#different-binary-name"),

        /// Allow specifying rustflags directly in a profile.
        (unstable, profile_rustflags, "", "reference/unstable.html#profile-rustflags-option"),

        /// Allow workspace members to inherit fields and dependencies from a workspace.
        (stable, workspace_inheritance, "1.64", "reference/unstable.html#workspace-inheritance"),

        /// Support for 2024 edition.
        (stable, edition2024, "1.85", "reference/manifest.html#the-edition-field"),

        /// Allow setting trim-paths in a profile to control the sanitisation of file paths in build outputs.
        (unstable, trim_paths, "", "reference/unstable.html#profile-trim-paths-option"),

        /// Allow multiple packages to participate in the same API namespace
        (unstable, open_namespaces, "", "reference/unstable.html#open-namespaces"),

        /// Allow paths that resolve relatively to a base specified in the config.
        (unstable, path_bases, "", "reference/unstable.html#path-bases"),
    }

    /// Status and metadata for a single unstable feature.
    #[derive(Debug)]
    pub struct Feature {
        /// Feature name. This is valid Rust identifier so no dash only underscore.
        name: &'static str,
        stability: Status,
        /// Version that this feature was stabilized or removed.
        version: &'static str,
        /// Link to the unstable documentation.
        docs: &'static str,
        get: fn(&Features) -> bool,
    }

    impl Features {
        /// Creates a new unstable features context.
        pub fn new(
            features: &[String],
            gctx: &GlobalContext,
            warnings: &mut Vec<String>,
            is_local: bool,
        ) -> CargoResult<Features> {
            let mut ret = Features::default();
            ret.nightly_features_allowed = gctx.nightly_features_allowed;
            ret.is_local = is_local;
            for feature in features {
                ret.add(feature, gctx, warnings)?;
                ret.activated.push(feature.to_string());
            }
            Ok(ret)
        }

        fn add(
            &mut self,
            feature_name: &str,
            gctx: &GlobalContext,
            warnings: &mut Vec<String>,
        ) -> CargoResult<()> {
            let nightly_features_allowed = self.nightly_features_allowed;
            let Some((slot, feature)) = self.status(feature_name) else {
                bail!("unknown cargo feature `{}`", feature_name)
            };

            if *slot {
                bail!(
                    "the cargo feature `{}` has already been activated",
                    feature_name
                );
            }

            let see_docs = || {
                format!(
                    "See {} for more information about using this feature.",
                    cargo_docs_link(feature.docs)
                )
            };

            match feature.stability {
                Status::Stable => {
                    let warning = format!(
                        "the cargo feature `{}` has been stabilized in the {} \
                             release and is no longer necessary to be listed in the \
                             manifest\n  {}",
                        feature_name,
                        feature.version,
                        see_docs()
                    );
                    warnings.push(warning);
                }
                Status::Unstable if !nightly_features_allowed => bail!(
                    "the cargo feature `{}` requires a nightly version of \
                     Cargo, but this is the `{}` channel\n\
                     {}\n{}",
                    feature_name,
                    channel(),
                    SEE_CHANNELS,
                    see_docs()
                ),
                Status::Unstable => {
                    if let Some(allow) = &gctx.cli_unstable().allow_features {
                        if !allow.contains(feature_name) {
                            bail!(
                                "the feature `{}` is not in the list of allowed features: [{}]",
                                feature_name,
                                itertools::join(allow, ", "),
                            );
                        }
                    }
                }
                Status::Removed => {
                    let mut msg = format!(
                        "the cargo feature `{}` has been removed in the {} release\n\n",
                        feature_name, feature.version
                    );
                    if self.is_local {
                        let _ = writeln!(
                            msg,
                            "Remove the feature from Cargo.toml to remove this error."
                        );
                    } else {
                        let _ = writeln!(
                            msg,
                            "This package cannot be used with this version of Cargo, \
                             as the unstable feature `{}` is no longer supported.",
                            feature_name
                        );
                    }
                    let _ = writeln!(msg, "{}", see_docs());
                    bail!(msg);
                }
            }

            *slot = true;

            Ok(())
        }

        /// Gets the current activated features.
        pub fn activated(&self) -> &[String] {
            &self.activated
        }

        /// Checks if the given feature is enabled.
        pub fn require(&self, feature: &Feature) -> CargoResult<()> {
            if feature.is_enabled(self) {
                return Ok(());
            }
            let feature_name = feature.name.replace("_", "-");
            let mut msg = format!(
                "feature `{}` is required\n\
                 \n\
                 The package requires the Cargo feature called `{}`, but \
                 that feature is not stabilized in this version of Cargo ({}).\n\
                ",
                feature_name,
                feature_name,
                crate::version(),
            );

            if self.nightly_features_allowed {
                if self.is_local {
                    let _ = writeln!(
                        msg,
                        "Consider adding `cargo-features = [\"{}\"]` \
                         to the top of Cargo.toml (above the [package] table) \
                         to tell Cargo you are opting in to use this unstable feature.",
                        feature_name
                    );
                } else {
                    let _ = writeln!(msg, "Consider trying a more recent nightly release.");
                }
            } else {
                let _ = writeln!(
                    msg,
                    "Consider trying a newer version of Cargo \
                     (this may require the nightly release)."
                );
            }
            let _ = writeln!(
                msg,
                "See https://doc.rust-lang.org/nightly/cargo/{} for more information \
                 about the status of this feature.",
                feature.docs
            );

            bail!("{}", msg);
        }

        /// Whether the given feature is allowed to use in this context.
        pub fn is_enabled(&self, feature: &Feature) -> bool {
            feature.is_enabled(self)
        }
    }

    /// Generates `-Z` flags as fields of [`CliUnstable`].
    ///
    /// See the [module-level documentation](self#-z-options) for details.
    macro_rules! unstable_cli_options {
        (
            $(
                $(#[$meta:meta])?
                $element: ident: $ty: ty$( = ($help:literal))?,
            )*
        ) => {
            /// A parsed representation of all unstable flags that Cargo accepts.
            ///
            /// Cargo, like `rustc`, accepts a suite of `-Z` flags which are intended for
            /// gating unstable functionality to Cargo. These flags are only available on
            /// the nightly channel of Cargo.
            #[derive(Default, Debug, Deserialize)]
            #[serde(default, rename_all = "kebab-case")]
            pub struct CliUnstable {
                $(
                    $(#[doc = $help])?
                    $(#[$meta])?
                    pub $element: $ty
                ),*
            }
            impl CliUnstable {
                /// Returns a list of `(<option-name>, <help-text>)`.
                pub fn help() -> Vec<(&'static str, Option<&'static str>)> {
                    let fields = vec![$((stringify!($element), None$(.or(Some($help)))?)),*];
                    fields
                }
            }

            #[cfg(test)]
            mod test {
                #[test]
                fn ensure_sorted() {
                    // This will be printed out if the fields are not sorted.
                    let location = std::panic::Location::caller();
                    println!(
                        "\nTo fix this test, sort the features inside the macro at {}:{}\n",
                        location.file(),
                        location.line()
                    );
                    let mut expected = vec![$(stringify!($element)),*];
                    expected[2..].sort();
                    let expected = format!("{:#?}", expected);
                    let actual = format!("{:#?}", vec![$(stringify!($element)),*]);
                    snapbox::assert_data_eq!(actual, expected);
                }
            }
        }
    }

    unstable_cli_options!(
        // Permanently unstable features:
        allow_features: Option<AllowFeatures> = ("Allow *only* the listed unstable features"),
        print_im_a_teapot: bool,

        // All other unstable features.
        // Please keep this list lexicographically ordered.
        advanced_env: bool,
        asymmetric_token: bool = ("Allows authenticating with asymmetric tokens"),
        avoid_dev_deps: bool = ("Avoid installing dev-dependencies if possible"),
        binary_dep_depinfo: bool = ("Track changes to dependency artifacts"),
        bindeps: bool = ("Allow Cargo packages to depend on bin, cdylib, and staticlib crates, and use the artifacts built by those crates"),
        build_std: Option<Vec<String>>  = ("Enable Cargo to compile the standard library itself as part of a crate graph compilation"),
        build_std_features: Option<Vec<String>>  = ("Configure features enabled for the standard library itself when building the standard library"),
        cargo_lints: bool = ("Enable the `[lints.cargo]` table"),
        checksum_freshness: bool = ("Use a checksum to determine if output is fresh rather than filesystem mtime"),
        codegen_backend: bool = ("Enable the `codegen-backend` option in profiles in .cargo/config.toml file"),
        config_include: bool = ("Enable the `include` key in config files"),
        direct_minimal_versions: bool = ("Resolve minimal dependency versions instead of maximum (direct dependencies only)"),
        doctest_xcompile: bool = ("Compile and run doctests for non-host target using runner config"),
        dual_proc_macros: bool = ("Build proc-macros for both the host and the target"),
        features: Option<Vec<String>>,
        gc: bool = ("Track cache usage and \"garbage collect\" unused files"),
        #[serde(deserialize_with = "deserialize_git_features")]
        git: Option<GitFeatures> = ("Enable support for shallow git fetch operations"),
        #[serde(deserialize_with = "deserialize_gitoxide_features")]
        gitoxide: Option<GitoxideFeatures> = ("Use gitoxide for the given git interactions, or all of them if no argument is given"),
        host_config: bool = ("Enable the `[host]` section in the .cargo/config.toml file"),
        minimal_versions: bool = ("Resolve minimal dependency versions instead of maximum"),
        msrv_policy: bool = ("Enable rust-version aware policy within cargo"),
        mtime_on_use: bool = ("Configure Cargo to update the mtime of used files"),
        next_lockfile_bump: bool,
        no_index_update: bool = ("Do not update the registry index even if the cache is outdated"),
        package_workspace: bool = ("Handle intra-workspace dependencies when packaging"),
        panic_abort_tests: bool = ("Enable support to run tests with -Cpanic=abort"),
        profile_rustflags: bool = ("Enable the `rustflags` option in profiles in .cargo/config.toml file"),
        public_dependency: bool = ("Respect a dependency's `public` field in Cargo.toml to control public/private dependencies"),
        publish_timeout: bool = ("Enable the `publish.timeout` key in .cargo/config.toml file"),
        root_dir: Option<PathBuf> = ("Set the root directory relative to which paths are printed (defaults to workspace root)"),
        rustdoc_map: bool = ("Allow passing external documentation mappings to rustdoc"),
        rustdoc_scrape_examples: bool = ("Allows Rustdoc to scrape code examples from reverse-dependencies"),
        script: bool = ("Enable support for single-file, `.rs` packages"),
        separate_nightlies: bool,
        skip_rustdoc_fingerprint: bool,
        target_applies_to_host: bool = ("Enable the `target-applies-to-host` key in the .cargo/config.toml file"),
        trim_paths: bool = ("Enable the `trim-paths` option in profiles"),
        unstable_options: bool = ("Allow the usage of unstable options"),
        warnings: bool = ("Allow use of the build.warnings config key"),
    );

    const STABILIZED_COMPILE_PROGRESS: &str = "The progress bar is now always \
        enabled when used on an interactive console.\n\
        See https://doc.rust-lang.org/cargo/reference/config.html#termprogresswhen \
        for information on controlling the progress bar.";

    const STABILIZED_OFFLINE: &str = "Offline mode is now available via the \
        --offline CLI option";

    const STABILIZED_CACHE_MESSAGES: &str = "Message caching is now always enabled.";

    const STABILIZED_INSTALL_UPGRADE: &str = "Packages are now always upgraded if \
        they appear out of date.\n\
        See https://doc.rust-lang.org/cargo/commands/cargo-install.html for more \
        information on how upgrading works.";

    const STABILIZED_CONFIG_PROFILE: &str = "See \
        https://doc.rust-lang.org/cargo/reference/config.html#profile for more \
        information about specifying profiles in config.";

    const STABILIZED_CRATE_VERSIONS: &str = "The crate version is now \
        automatically added to the documentation.";

    const STABILIZED_PACKAGE_FEATURES: &str = "Enhanced feature flag behavior is now \
        available in virtual workspaces, and `member/feature-name` syntax is also \
        always available. Other extensions require setting `resolver = \"2\"` in \
        Cargo.toml.\n\
        See https://doc.rust-lang.org/nightly/cargo/reference/features.html#resolver-version-2-command-line-flags \
        for more information.";

    const STABILIZED_FEATURES: &str = "The new feature resolver is now available \
        by specifying `resolver = \"2\"` in Cargo.toml.\n\
        See https://doc.rust-lang.org/nightly/cargo/reference/features.html#feature-resolver-version-2 \
        for more information.";

    const STABILIZED_EXTRA_LINK_ARG: &str = "Additional linker arguments are now \
        supported without passing this flag.";

    const STABILIZED_CONFIGURABLE_ENV: &str = "The [env] section is now always enabled.";

    const STABILIZED_PATCH_IN_CONFIG: &str = "The patch-in-config feature is now always enabled.";

    const STABILIZED_NAMED_PROFILES: &str = "The named-profiles feature is now always enabled.\n\
        See https://doc.rust-lang.org/nightly/cargo/reference/profiles.html#custom-profiles \
        for more information";

    const STABILIZED_DOCTEST_IN_WORKSPACE: &str =
        "The doctest-in-workspace feature is now always enabled.";

    const STABILIZED_FUTURE_INCOMPAT_REPORT: &str =
        "The future-incompat-report feature is now always enabled.";

    const STABILIZED_WEAK_DEP_FEATURES: &str = "Weak dependency features are now always available.";

    const STABILISED_NAMESPACED_FEATURES: &str = "Namespaced features are now always available.";

    const STABILIZED_TIMINGS: &str = "The -Ztimings option has been stabilized as --timings.";

    const STABILISED_MULTITARGET: &str = "Multiple `--target` options are now always available.";

    const STABILIZED_TERMINAL_WIDTH: &str =
        "The -Zterminal-width option is now always enabled for terminal output.";

    const STABILISED_SPARSE_REGISTRY: &str = "The sparse protocol is now the default for crates.io";

    const STABILIZED_CREDENTIAL_PROCESS: &str =
        "Authentication with a credential provider is always available.";

    const STABILIZED_REGISTRY_AUTH: &str =
        "Authenticated registries are available if a credential provider is configured.";

    const STABILIZED_LINTS: &str = "The `[lints]` table is now always available.";

    const STABILIZED_CHECK_CFG: &str =
        "Compile-time checking of conditional (a.k.a. `-Zcheck-cfg`) is now always enabled.";

    #[derive(Debug, Copy, Clone, Default, Deserialize, Ord, PartialOrd, Eq, PartialEq)]
    #[serde(default)]
    pub struct GitFeatures {
        /// When cloning the index, perform a shallow clone. Maintain shallowness upon subsequent fetches.
        pub shallow_index: bool,
        /// When cloning git dependencies, perform a shallow clone and maintain shallowness on subsequent fetches.
        pub shallow_deps: bool,
    }

    impl GitFeatures {
        pub fn all() -> Self {
            GitFeatures {
                shallow_index: true,
                shallow_deps: true,
            }
        }

        fn expecting() -> String {
            let fields = vec!["`shallow-index`", "`shallow-deps`"];
            format!(
                "unstable 'git' only takes {} as valid inputs",
                fields.join(" and ")
            )
        }
    }

    fn deserialize_git_features<'de, D>(deserializer: D) -> Result<Option<GitFeatures>, D::Error>
    where
        D: serde::de::Deserializer<'de>,
    {
        struct GitFeaturesVisitor;

        impl<'de> serde::de::Visitor<'de> for GitFeaturesVisitor {
            type Value = Option<GitFeatures>;

            fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
                formatter.write_str(&GitFeatures::expecting())
            }

            fn visit_bool<E>(self, v: bool) -> Result<Self::Value, E>
            where
                E: serde::de::Error,
            {
                if v {
                    Ok(Some(GitFeatures::all()))
                } else {
                    Ok(None)
                }
            }

            fn visit_str<E>(self, s: &str) -> Result<Self::Value, E>
            where
                E: serde::de::Error,
            {
                Ok(parse_git(s.split(",")).map_err(serde::de::Error::custom)?)
            }

            fn visit_some<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
            where
                D: serde::de::Deserializer<'de>,
            {
                let git = GitFeatures::deserialize(deserializer)?;
                Ok(Some(git))
            }

            fn visit_map<V>(self, map: V) -> Result<Self::Value, V::Error>
            where
                V: serde::de::MapAccess<'de>,
            {
                let mvd = serde::de::value::MapAccessDeserializer::new(map);
                Ok(Some(GitFeatures::deserialize(mvd)?))
            }
        }

        deserializer.deserialize_any(GitFeaturesVisitor)
    }

    fn parse_git(it: impl Iterator<Item = impl AsRef<str>>) -> CargoResult<Option<GitFeatures>> {
        let mut out = GitFeatures::default();
        let GitFeatures {
            shallow_index,
            shallow_deps,
        } = &mut out;

        for e in it {
            match e.as_ref() {
                "shallow-index" => *shallow_index = true,
                "shallow-deps" => *shallow_deps = true,
                _ => {
                    bail!(GitFeatures::expecting())
                }
            }
        }
        Ok(Some(out))
    }

    #[derive(Debug, Copy, Clone, Default, Deserialize, Ord, PartialOrd, Eq, PartialEq)]
    #[serde(default)]
    pub struct GitoxideFeatures {
        /// All fetches are done with `gitoxide`, which includes git dependencies as well as the crates index.
        pub fetch: bool,
        /// Checkout git dependencies using `gitoxide` (submodules are still handled by git2 ATM, and filters
        /// like linefeed conversions are unsupported).
        pub checkout: bool,
        /// A feature flag which doesn't have any meaning except for preventing
        /// `__CARGO_USE_GITOXIDE_INSTEAD_OF_GIT2=1` builds to enable all safe `gitoxide` features.
        /// That way, `gitoxide` isn't actually used even though it's enabled.
        pub internal_use_git2: bool,
    }

    impl GitoxideFeatures {
        pub fn all() -> Self {
            GitoxideFeatures {
                fetch: true,
                checkout: true,
                internal_use_git2: false,
            }
        }

        /// Features we deem safe for everyday use - typically true when all tests pass with them
        /// AND they are backwards compatible.
        fn safe() -> Self {
            GitoxideFeatures {
                fetch: true,
                checkout: true,
                internal_use_git2: false,
            }
        }

        fn expecting() -> String {
            let fields = vec!["`fetch`", "`checkout`", "`internal-use-git2`"];
            format!(
                "unstable 'gitoxide' only takes {} as valid inputs, for shallow fetches see `-Zgit=shallow-index,shallow-deps`",
                fields.join(" and ")
            )
        }
    }

    fn deserialize_gitoxide_features<'de, D>(
        deserializer: D,
    ) -> Result<Option<GitoxideFeatures>, D::Error>
    where
        D: serde::de::Deserializer<'de>,
    {
        struct GitoxideFeaturesVisitor;

        impl<'de> serde::de::Visitor<'de> for GitoxideFeaturesVisitor {
            type Value = Option<GitoxideFeatures>;

            fn expecting(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
                formatter.write_str(&GitoxideFeatures::expecting())
            }

            fn visit_str<E>(self, s: &str) -> Result<Self::Value, E>
            where
                E: serde::de::Error,
            {
                Ok(parse_gitoxide(s.split(",")).map_err(serde::de::Error::custom)?)
            }

            fn visit_bool<E>(self, v: bool) -> Result<Self::Value, E>
            where
                E: serde::de::Error,
            {
                if v {
                    Ok(Some(GitoxideFeatures::all()))
                } else {
                    Ok(None)
                }
            }

            fn visit_some<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
            where
                D: serde::de::Deserializer<'de>,
            {
                let gitoxide = GitoxideFeatures::deserialize(deserializer)?;
                Ok(Some(gitoxide))
            }

            fn visit_map<V>(self, map: V) -> Result<Self::Value, V::Error>
            where
                V: serde::de::MapAccess<'de>,
            {
                let mvd = serde::de::value::MapAccessDeserializer::new(map);
                Ok(Some(GitoxideFeatures::deserialize(mvd)?))
            }
        }

        deserializer.deserialize_any(GitoxideFeaturesVisitor)
    }

    fn parse_gitoxide(
        it: impl Iterator<Item = impl AsRef<str>>,
    ) -> CargoResult<Option<GitoxideFeatures>> {
        let mut out = GitoxideFeatures::default();
        let GitoxideFeatures {
            fetch,
            checkout,
            internal_use_git2,
        } = &mut out;

        for e in it {
            match e.as_ref() {
                "fetch" => *fetch = true,
                "checkout" => *checkout = true,
                "internal-use-git2" => *internal_use_git2 = true,
                _ => {
                    bail!(GitoxideFeatures::expecting())
                }
            }
        }
        Ok(Some(out))
    }

    impl CliUnstable {
        /// Parses `-Z` flags from the command line, and returns messages that warn
        /// if any flag has alreardy been stabilized.
        pub fn parse(
            &mut self,
            flags: &[String],
            nightly_features_allowed: bool,
        ) -> CargoResult<Vec<String>> {
            if !flags.is_empty() && !nightly_features_allowed {
                bail!(
                    "the `-Z` flag is only accepted on the nightly channel of Cargo, \
                     but this is the `{}` channel\n\
                     {}",
                    channel(),
                    SEE_CHANNELS
                );
            }
            let mut warnings = Vec::new();
            // We read flags twice, first to get allowed-features (if specified),
            // and then to read the remaining unstable flags.
            for flag in flags {
                if flag.starts_with("allow-features=") {
                    self.add(flag, &mut warnings)?;
                }
            }
            for flag in flags {
                self.add(flag, &mut warnings)?;
            }

            if self.gitoxide.is_none() && cargo_use_gitoxide_instead_of_git2() {
                self.gitoxide = GitoxideFeatures::safe().into();
            }
            Ok(warnings)
        }

        fn add(&mut self, flag: &str, warnings: &mut Vec<String>) -> CargoResult<()> {
            let mut parts = flag.splitn(2, '=');
            let k = parts.next().unwrap();
            let v = parts.next();

            fn parse_bool(key: &str, value: Option<&str>) -> CargoResult<bool> {
                match value {
                    None | Some("yes") => Ok(true),
                    Some("no") => Ok(false),
                    Some(s) => bail!("flag -Z{} expected `no` or `yes`, found: `{}`", key, s),
                }
            }

            /// Parse a comma-separated list
            fn parse_list(value: Option<&str>) -> Vec<String> {
                match value {
                    None => Vec::new(),
                    Some("") => Vec::new(),
                    Some(v) => v.split(',').map(|s| s.to_string()).collect(),
                }
            }

            // Asserts that there is no argument to the flag.
            fn parse_empty(key: &str, value: Option<&str>) -> CargoResult<bool> {
                if let Some(v) = value {
                    bail!("flag -Z{} does not take a value, found: `{}`", key, v);
                }
                Ok(true)
            }

            let mut stabilized_warn = |key: &str, version: &str, message: &str| {
                warnings.push(format!(
                    "flag `-Z {}` has been stabilized in the {} release, \
                     and is no longer necessary\n{}",
                    key,
                    version,
                    indented_lines(message)
                ));
            };

            // Use this if the behavior now requires another mechanism to enable.
            let stabilized_err = |key: &str, version: &str, message: &str| {
                Err(anyhow::format_err!(
                    "flag `-Z {}` has been stabilized in the {} release\n{}",
                    key,
                    version,
                    indented_lines(message)
                ))
            };

            if let Some(allowed) = &self.allow_features {
                if k != "allow-features" && !allowed.contains(k) {
                    bail!(
                        "the feature `{}` is not in the list of allowed features: [{}]",
                        k,
                        itertools::join(allowed, ", ")
                    );
                }
            }

            match k {
                // Permanently unstable features
                // Sorted alphabetically:
                "allow-features" => self.allow_features = Some(parse_list(v).into_iter().collect()),
                "print-im-a-teapot" => self.print_im_a_teapot = parse_bool(k, v)?,

                // Stabilized features
                // Sorted by version, then alphabetically:
                "compile-progress" => stabilized_warn(k, "1.30", STABILIZED_COMPILE_PROGRESS),
                "offline" => stabilized_err(k, "1.36", STABILIZED_OFFLINE)?,
                "cache-messages" => stabilized_warn(k, "1.40", STABILIZED_CACHE_MESSAGES),
                "install-upgrade" => stabilized_warn(k, "1.41", STABILIZED_INSTALL_UPGRADE),
                "config-profile" => stabilized_warn(k, "1.43", STABILIZED_CONFIG_PROFILE),
                "crate-versions" => stabilized_warn(k, "1.47", STABILIZED_CRATE_VERSIONS),
                "features" => {
                    // `-Z features` has been stabilized since 1.51,
                    // but `-Z features=compare` is still allowed for convenience
                    // to validate that the feature resolver resolves features
                    // in the same way as the dependency resolver,
                    // until we feel confident to remove entirely.
                    //
                    // See rust-lang/cargo#11168
                    let feats = parse_list(v);
                    let stab_is_not_empty = feats.iter().any(|feat| {
                        matches!(
                            feat.as_str(),
                            "build_dep" | "host_dep" | "dev_dep" | "itarget" | "all"
                        )
                    });
                    if stab_is_not_empty || feats.is_empty() {
                        // Make this stabilized_err once -Zfeature support is removed.
                        stabilized_warn(k, "1.51", STABILIZED_FEATURES);
                    }
                    self.features = Some(feats);
                }
                "package-features" => stabilized_warn(k, "1.51", STABILIZED_PACKAGE_FEATURES),
                "configurable-env" => stabilized_warn(k, "1.56", STABILIZED_CONFIGURABLE_ENV),
                "extra-link-arg" => stabilized_warn(k, "1.56", STABILIZED_EXTRA_LINK_ARG),
                "patch-in-config" => stabilized_warn(k, "1.56", STABILIZED_PATCH_IN_CONFIG),
                "named-profiles" => stabilized_warn(k, "1.57", STABILIZED_NAMED_PROFILES),
                "future-incompat-report" => {
                    stabilized_warn(k, "1.59.0", STABILIZED_FUTURE_INCOMPAT_REPORT)
                }
                "namespaced-features" => stabilized_warn(k, "1.60", STABILISED_NAMESPACED_FEATURES),
                "timings" => stabilized_warn(k, "1.60", STABILIZED_TIMINGS),
                "weak-dep-features" => stabilized_warn(k, "1.60", STABILIZED_WEAK_DEP_FEATURES),
                "multitarget" => stabilized_warn(k, "1.64", STABILISED_MULTITARGET),
                "sparse-registry" => stabilized_warn(k, "1.68", STABILISED_SPARSE_REGISTRY),
                "terminal-width" => stabilized_warn(k, "1.68", STABILIZED_TERMINAL_WIDTH),
                "doctest-in-workspace" => stabilized_warn(k, "1.72", STABILIZED_DOCTEST_IN_WORKSPACE),
                "credential-process" => stabilized_warn(k, "1.74", STABILIZED_CREDENTIAL_PROCESS),
                "lints" => stabilized_warn(k, "1.74", STABILIZED_LINTS),
                "registry-auth" => stabilized_warn(k, "1.74", STABILIZED_REGISTRY_AUTH),
                "check-cfg" => stabilized_warn(k, "1.80", STABILIZED_CHECK_CFG),

                // Unstable features
                // Sorted alphabetically:
                "advanced-env" => self.advanced_env = parse_empty(k, v)?,
                "asymmetric-token" => self.asymmetric_token = parse_empty(k, v)?,
                "avoid-dev-deps" => self.avoid_dev_deps = parse_empty(k, v)?,
                "binary-dep-depinfo" => self.binary_dep_depinfo = parse_empty(k, v)?,
                "bindeps" => self.bindeps = parse_empty(k, v)?,
                "build-std" => self.build_std = Some(parse_list(v)),
                "build-std-features" => self.build_std_features = Some(parse_list(v)),
                "cargo-lints" => self.cargo_lints = parse_empty(k, v)?,
                "codegen-backend" => self.codegen_backend = parse_empty(k, v)?,
                "config-include" => self.config_include = parse_empty(k, v)?,
                "direct-minimal-versions" => self.direct_minimal_versions = parse_empty(k, v)?,
                "doctest-xcompile" => self.doctest_xcompile = parse_empty(k, v)?,
                "dual-proc-macros" => self.dual_proc_macros = parse_empty(k, v)?,
                "gc" => self.gc = parse_empty(k, v)?,
                "git" => {
                    self.git = v.map_or_else(
                        || Ok(Some(GitFeatures::all())),
                        |v| parse_git(v.split(',')),
                    )?
                }
                "gitoxide" => {
                    self.gitoxide = v.map_or_else(
                        || Ok(Some(GitoxideFeatures::all())),
                        |v| parse_gitoxide(v.split(',')),
                    )?
                }
                "host-config" => self.host_config = parse_empty(k, v)?,
                "next-lockfile-bump" => self.next_lockfile_bump = parse_empty(k, v)?,
                "minimal-versions" => self.minimal_versions = parse_empty(k, v)?,
                "msrv-policy" => self.msrv_policy = parse_empty(k, v)?,
                // can also be set in .cargo/config or with and ENV
                "mtime-on-use" => self.mtime_on_use = parse_empty(k, v)?,
                "no-index-update" => self.no_index_update = parse_empty(k, v)?,
                "package-workspace" => self.package_workspace= parse_empty(k, v)?,
                "panic-abort-tests" => self.panic_abort_tests = parse_empty(k, v)?,
                "public-dependency" => self.public_dependency = parse_empty(k, v)?,
                "profile-rustflags" => self.profile_rustflags = parse_empty(k, v)?,
                "trim-paths" => self.trim_paths = parse_empty(k, v)?,
                "publish-timeout" => self.publish_timeout = parse_empty(k, v)?,
                "root-dir" => self.root_dir = v.map(|v| v.into()),
                "rustdoc-map" => self.rustdoc_map = parse_empty(k, v)?,
                "rustdoc-scrape-examples" => self.rustdoc_scrape_examples = parse_empty(k, v)?,
                "separate-nightlies" => self.separate_nightlies = parse_empty(k, v)?,
                "checksum-freshness" => self.checksum_freshness = parse_empty(k, v)?,
                "skip-rustdoc-fingerprint" => self.skip_rustdoc_fingerprint = parse_empty(k, v)?,
                "script" => self.script = parse_empty(k, v)?,
                "target-applies-to-host" => self.target_applies_to_host = parse_empty(k, v)?,
                "unstable-options" => self.unstable_options = parse_empty(k, v)?,
                "warnings" => self.warnings = parse_empty(k, v)?,
                _ => bail!("\
                unknown `-Z` flag specified: {k}\n\n\
                For available unstable features, see https://doc.rust-lang.org/nightly/cargo/reference/unstable.html\n\
                If you intended to use an unstable rustc feature, try setting `RUSTFLAGS=\"-Z{k}\"`"),
            }

            Ok(())
        }

        /// Generates an error if `-Z unstable-options` was not used for a new,
        /// unstable command-line flag.
        pub fn fail_if_stable_opt(&self, flag: &str, issue: u32) -> CargoResult<()> {
            self.fail_if_stable_opt_custom_z(flag, issue, "unstable-options", self.unstable_options)
        }

        pub fn fail_if_stable_opt_custom_z(
            &self,
            flag: &str,
            issue: u32,
            z_name: &str,
            enabled: bool,
        ) -> CargoResult<()> {
            if !enabled {
                let see = format!(
                    "See https://github.com/rust-lang/cargo/issues/{issue} for more \
                     information about the `{flag}` flag."
                );
                // NOTE: a `config` isn't available here, check the channel directly
                let channel = channel();
                if channel == "nightly" || channel == "dev" {
                    bail!(
                        "the `{flag}` flag is unstable, pass `-Z {z_name}` to enable it\n\
                         {see}"
                    );
                } else {
                    bail!(
                        "the `{flag}` flag is unstable, and only available on the nightly channel \
                         of Cargo, but this is the `{channel}` channel\n\
                         {SEE_CHANNELS}\n\
                         {see}"
                    );
                }
            }
            Ok(())
        }

        /// Generates an error if `-Z unstable-options` was not used for a new,
        /// unstable subcommand.
        pub fn fail_if_stable_command(
            &self,
            gctx: &GlobalContext,
            command: &str,
            issue: u32,
            z_name: &str,
            enabled: bool,
        ) -> CargoResult<()> {
            if enabled {
                return Ok(());
            }
            let see = format!(
                "See https://github.com/rust-lang/cargo/issues/{} for more \
                information about the `cargo {}` command.",
                issue, command
            );
            if gctx.nightly_features_allowed {
                bail!(
                    "the `cargo {command}` command is unstable, pass `-Z {z_name}` \
                     to enable it\n\
                     {see}",
                );
            } else {
                bail!(
                    "the `cargo {}` command is unstable, and only available on the \
                     nightly channel of Cargo, but this is the `{}` channel\n\
                     {}\n\
                     {}",
                    command,
                    channel(),
                    SEE_CHANNELS,
                    see
                );
            }
        }
    }

    /// Returns the current release channel ("stable", "beta", "nightly", "dev").
    pub fn channel() -> String {
        // ALLOWED: For testing cargo itself only.
        #[allow(clippy::disallowed_methods)]
        if let Ok(override_channel) = env::var("__CARGO_TEST_CHANNEL_OVERRIDE_DO_NOT_USE_THIS") {
            return override_channel;
        }
        // ALLOWED: the process of rustc bootstrapping reads this through
        // `std::env`. We should make the behavior consistent. Also, we
        // don't advertise this for bypassing nightly.
        #[allow(clippy::disallowed_methods)]
        if let Ok(staging) = env::var("RUSTC_BOOTSTRAP") {
            if staging == "1" {
                return "dev".to_string();
            }
        }
        // Debian: always return dev channel
        String::from("dev")
    }

    /// Only for testing and developing. See ["Running with gitoxide as default git backend in tests"][1].
    ///
    /// [1]: https://doc.crates.io/contrib/tests/running.html#running-with-gitoxide-as-default-git-backend-in-tests
    // ALLOWED: For testing cargo itself only.
    #[allow(clippy::disallowed_methods)]
    fn cargo_use_gitoxide_instead_of_git2() -> bool {
        std::env::var_os("__CARGO_USE_GITOXIDE_INSTEAD_OF_GIT2").map_or(false, |value| value == "1")
    }

    /// Generate a link to Cargo documentation for the current release channel
    /// `path` is the URL component after `https://doc.rust-lang.org/{channel}/cargo/`
    pub fn cargo_docs_link(path: &str) -> String {
        let url_channel = match channel().as_str() {
            "dev" | "nightly" => "nightly/",
            "beta" => "beta/",
            _ => "",
        };
        format!("https://doc.rust-lang.org/{url_channel}cargo/{path}")
    }