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    //! Resolves conditional compilation for [`features` section] in the manifest.
    //!
    //! This is a [new feature resolver] that runs independently of the main
    //! dependency resolver. It has several options which can enable new feature
    //! resolution behavior.
    //!
    //! One of its key characteristics is that it can avoid unifying features for
    //! shared dependencies in some situations. See [`FeatureOpts`] for the
    //! different behaviors that can be enabled. If no extra options are enabled,
    //! then it should behave exactly the same as the dependency resolver's
    //! feature resolution.
    //!
    //! The preferred way to engage this new resolver is via [`resolve_ws_with_opts`].
    //!
    //! This does not *replace* feature resolution in the dependency resolver, but
    //! instead acts as a second pass which can *narrow* the features selected in
    //! the dependency resolver. The dependency resolver still needs to do its own
    //! feature resolution in order to avoid selecting optional dependencies that
    //! are never enabled. The dependency resolver could, in theory, just assume
    //! all optional dependencies on all packages are enabled (and remove all
    //! knowledge of features), but that could introduce new requirements that
    //! might change old behavior or cause conflicts. Maybe some day in the future
    //! we could experiment with that, but it seems unlikely to work or be all
    //! that helpful.
    //!
    //! ## Assumptions
    //!
    //! There are many assumptions made about the dependency resolver:
    //!
    //! * Assumes feature validation has already been done during the construction
    //!   of feature maps, so the feature resolver doesn't do that validation at all.
    //! * Assumes `dev-dependencies` within a dependency have been removed
    //!   in the given [`Resolve`].
    //!
    //! There are probably other assumptions that I am forgetting.
    //!
    //! [`features` section]: https://doc.rust-lang.org/nightly/cargo/reference/features.html
    //! [new feature resolver]: https://doc.rust-lang.org/nightly/cargo/reference/resolver.html#feature-resolver-version-2
    //! [`resolve_ws_with_opts`]: crate::ops::resolve_ws_with_opts

    use crate::core::compiler::{CompileKind, CompileTarget, RustcTargetData};
    use crate::core::dependency::{ArtifactTarget, DepKind, Dependency};
    use crate::core::resolver::types::FeaturesSet;
    use crate::core::resolver::{Resolve, ResolveBehavior};
    use crate::core::{FeatureValue, PackageId, PackageIdSpec, PackageSet, Workspace};
    use crate::util::interning::InternedString;
    use crate::util::CargoResult;
    use anyhow::{bail, Context};
    use itertools::Itertools;
    use std::collections::{BTreeMap, BTreeSet, HashMap, HashSet};
    use std::rc::Rc;

    /// The key used in various places to store features for a particular dependency.
    /// The actual discrimination happens with the [`FeaturesFor`] type.
    type PackageFeaturesKey = (PackageId, FeaturesFor);
    /// Map of activated features.
    type ActivateMap = HashMap<PackageFeaturesKey, BTreeSet<InternedString>>;

    /// Set of all activated features for all packages in the resolve graph.
    pub struct ResolvedFeatures {
        activated_features: ActivateMap,
        /// Optional dependencies that should be built.
        ///
        /// The value is the `name_in_toml` of the dependencies.
        activated_dependencies: ActivateMap,
        opts: FeatureOpts,
    }

    /// Options for how the feature resolver works.
    #[derive(Default)]
    pub struct FeatureOpts {
        /// Build deps and proc-macros will not share features with other dep kinds,
        /// and so won't artifact targets.
        /// In other terms, if true, features associated with certain kinds of dependencies
        /// will only be unified together.
        /// If false, there is only one namespace for features, unifying all features across
        /// all dependencies, no matter what kind.
        decouple_host_deps: bool,
        /// Dev dep features will not be activated unless needed.
        decouple_dev_deps: bool,
        /// Targets that are not in use will not activate features.
        ignore_inactive_targets: bool,
        /// If enabled, compare against old resolver (for testing).
        compare: bool,
    }

    /// Flag to indicate if Cargo is building *any* dev units (tests, examples, etc.).
    ///
    /// This disables decoupling of dev dependencies. It may be possible to relax
    /// this in the future, but it will require significant changes to how unit
    /// dependencies are computed, and can result in longer build times with
    /// `cargo test` because the lib may need to be built 3 times instead of
    /// twice.
    #[derive(Copy, Clone, PartialEq)]
    pub enum HasDevUnits {
        Yes,
        No,
    }

    /// Flag to indicate that target-specific filtering should be disabled.
    #[derive(Copy, Clone, PartialEq)]
    pub enum ForceAllTargets {
        Yes,
        No,
    }

    /// Flag to indicate if features are requested for a certain type of dependency.
    ///
    /// This is primarily used for constructing a [`PackageFeaturesKey`] to decouple
    /// activated features of the same package with different types of dependency.
    #[derive(Default, Copy, Clone, Debug, PartialEq, Eq, Ord, PartialOrd, Hash)]
    pub enum FeaturesFor {
        /// Normal or dev dependency.
        #[default]
        NormalOrDev,
        /// Build dependency or proc-macro.
        HostDep,
        /// Any dependency with both artifact and target specified.
        ///
        /// That is, `dep = { …, artifact = <crate-type>, target = <triple> }`
        ArtifactDep(CompileTarget),
    }

    impl std::fmt::Display for FeaturesFor {
        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
            match self {
                FeaturesFor::HostDep => f.write_str("host"),
                FeaturesFor::ArtifactDep(target) => f.write_str(&target.rustc_target()),
                FeaturesFor::NormalOrDev => Ok(()),
            }
        }
    }

    impl FeaturesFor {
        pub fn from_for_host(for_host: bool) -> FeaturesFor {
            if for_host {
                FeaturesFor::HostDep
            } else {
                FeaturesFor::NormalOrDev
            }
        }

        pub fn from_for_host_or_artifact_target(
            for_host: bool,
            artifact_target: Option<CompileTarget>,
        ) -> FeaturesFor {
            match artifact_target {
                Some(target) => FeaturesFor::ArtifactDep(target),
                None => {
                    if for_host {
                        FeaturesFor::HostDep
                    } else {
                        FeaturesFor::NormalOrDev
                    }
                }
            }
        }

        fn apply_opts(self, opts: &FeatureOpts) -> Self {
            if opts.decouple_host_deps {
                self
            } else {
                FeaturesFor::default()
            }
        }
    }

    impl FeatureOpts {
        pub fn new(
            ws: &Workspace<'_>,
            has_dev_units: HasDevUnits,
            force_all_targets: ForceAllTargets,
        ) -> CargoResult<FeatureOpts> {
            let mut opts = FeatureOpts::default();
            let unstable_flags = ws.gctx().cli_unstable();
            let mut enable = |feat_opts: &Vec<String>| {
                for opt in feat_opts {
                    match opt.as_ref() {
                        "build_dep" | "host_dep" => opts.decouple_host_deps = true,
                        "dev_dep" => opts.decouple_dev_deps = true,
                        "itarget" => opts.ignore_inactive_targets = true,
                        "all" => {
                            opts.decouple_host_deps = true;
                            opts.decouple_dev_deps = true;
                            opts.ignore_inactive_targets = true;
                        }
                        "compare" => opts.compare = true,
                        "ws" => unimplemented!(),
                        s => bail!("-Zfeatures flag `{}` is not supported", s),
                    }
                }
                Ok(())
            };
            if let Some(feat_opts) = unstable_flags.features.as_ref() {
                enable(feat_opts)?;
            }
            match ws.resolve_behavior() {
                ResolveBehavior::V1 => {}
                ResolveBehavior::V2 | ResolveBehavior::V3 => {
                    enable(&vec!["all".to_string()]).unwrap();
                }
            }
            if let HasDevUnits::Yes = has_dev_units {
                // Dev deps cannot be decoupled when they are in use.
                opts.decouple_dev_deps = false;
            }
            if let ForceAllTargets::Yes = force_all_targets {
                opts.ignore_inactive_targets = false;
            }
            Ok(opts)
        }

        /// Creates a new FeatureOpts for the given behavior.
        pub fn new_behavior(behavior: ResolveBehavior, has_dev_units: HasDevUnits) -> FeatureOpts {
            match behavior {
                ResolveBehavior::V1 => FeatureOpts::default(),
                ResolveBehavior::V2 | ResolveBehavior::V3 => FeatureOpts {
                    decouple_host_deps: true,
                    decouple_dev_deps: has_dev_units == HasDevUnits::No,
                    ignore_inactive_targets: true,
                    compare: false,
                },
            }
        }
    }

    /// Features flags requested for a package.
    ///
    /// This should be cheap and fast to clone, it is used in the resolver for
    /// various caches.
    ///
    /// This is split into enum variants because the resolver needs to handle
    /// features coming from different places (command-line and dependency
    /// declarations), but those different places have different constraints on
    /// which syntax is allowed. This helps ensure that every place dealing with
    /// features is properly handling those syntax restrictions.
    #[derive(Debug, Clone, Eq, PartialEq, Hash)]
    pub enum RequestedFeatures {
        /// Features requested on the command-line with flags.
        CliFeatures(CliFeatures),
        /// Features specified in a dependency declaration.
        DepFeatures {
            /// The `features` dependency field.
            features: FeaturesSet,
            /// The `default-features` dependency field.
            uses_default_features: bool,
        },
    }

    /// Features specified on the command-line.
    #[derive(Debug, Clone, Eq, PartialEq, Hash)]
    pub struct CliFeatures {
        /// Features from the `--features` flag.
        pub features: Rc<BTreeSet<FeatureValue>>,
        /// The `--all-features` flag.
        pub all_features: bool,
        /// Inverse of `--no-default-features` flag.
        pub uses_default_features: bool,
    }

    impl CliFeatures {
        /// Creates a new CliFeatures from the given command-line flags.
        pub fn from_command_line(
            features: &[String],
            all_features: bool,
            uses_default_features: bool,
        ) -> CargoResult<CliFeatures> {
            let features = Rc::new(CliFeatures::split_features(features));
            // Some early validation to ensure correct syntax.
            for feature in features.iter() {
                match feature {
                    // Maybe call validate_feature_name here once it is an error?
                    FeatureValue::Feature(_) => {}
                    FeatureValue::Dep { .. } => {
                        bail!(
                            "feature `{}` is not allowed to use explicit `dep:` syntax",
                            feature
                        );
                    }
                    FeatureValue::DepFeature { dep_feature, .. } => {
                        if dep_feature.contains('/') {
                            bail!("multiple slashes in feature `{}` is not allowed", feature);
                        }
                    }
                }
            }
            Ok(CliFeatures {
                features,
                all_features,
                uses_default_features,
            })
        }

        /// Creates a new CliFeatures with the given `all_features` setting.
        pub fn new_all(all_features: bool) -> CliFeatures {
            CliFeatures {
                features: Rc::new(BTreeSet::new()),
                all_features,
                uses_default_features: true,
            }
        }

        fn split_features(features: &[String]) -> BTreeSet<FeatureValue> {
            features
                .iter()
                .flat_map(|s| s.split_whitespace())
                .flat_map(|s| s.split(','))
                .filter(|s| !s.is_empty())
                .map(InternedString::new)
                .map(FeatureValue::new)
                .collect()
        }
    }

    impl ResolvedFeatures {
        /// Returns the list of features that are enabled for the given package.
        pub fn activated_features(
            &self,
            pkg_id: PackageId,
            features_for: FeaturesFor,
        ) -> Vec<InternedString> {
            self.activated_features_int(pkg_id, features_for)
                .expect("activated_features for invalid package")
        }

        /// Returns if the given dependency should be included.
        ///
        /// This handles dependencies disabled via `cfg` expressions and optional
        /// dependencies which are not enabled.
        pub fn is_dep_activated(
            &self,
            pkg_id: PackageId,
            features_for: FeaturesFor,
            dep_name: InternedString,
        ) -> bool {
            let key = features_for.apply_opts(&self.opts);
            self.activated_dependencies
                .get(&(pkg_id, key))
                .map(|deps| deps.contains(&dep_name))
                .unwrap_or(false)
        }

        /// Variant of `activated_features` that returns `None` if this is
        /// not a valid pkg_id/is_build combination. Used in places which do
        /// not know which packages are activated (like `cargo clean`).
        pub fn activated_features_unverified(
            &self,
            pkg_id: PackageId,
            features_for: FeaturesFor,
        ) -> Option<Vec<InternedString>> {
            self.activated_features_int(pkg_id, features_for).ok()
        }

        fn activated_features_int(
            &self,
            pkg_id: PackageId,
            features_for: FeaturesFor,
        ) -> CargoResult<Vec<InternedString>> {
            let fk = features_for.apply_opts(&self.opts);
            if let Some(fs) = self.activated_features.get(&(pkg_id, fk)) {
                Ok(fs.iter().cloned().collect())
            } else {
                bail!("features did not find {:?} {:?}", pkg_id, fk)
            }
        }

        /// Compares the result against the original resolver behavior.
        ///
        /// Used by `cargo fix --edition` to display any differences.
        pub fn compare_legacy(&self, legacy: &ResolvedFeatures) -> DiffMap {
            self.activated_features
                .iter()
                .filter_map(|((pkg_id, for_host), new_features)| {
                    let old_features = legacy
                        .activated_features
                        .get(&(*pkg_id, *for_host))
                        // The new features may have for_host entries where the old one does not.
                        .or_else(|| {
                            legacy
                                .activated_features
                                .get(&(*pkg_id, FeaturesFor::default()))
                        })
                        .map(|feats| feats.iter().cloned().collect())
                        .unwrap_or_else(|| BTreeSet::new());
                    // The new resolver should never add features.
                    assert_eq!(new_features.difference(&old_features).next(), None);
                    let removed_features: BTreeSet<_> =
                        old_features.difference(new_features).cloned().collect();
                    if removed_features.is_empty() {
                        None
                    } else {
                        Some(((*pkg_id, *for_host), removed_features))
                    }
                })
                .collect()
        }
    }

    /// Map of differences.
    ///
    /// Key is `(pkg_id, for_host)`. Value is a set of features or dependencies removed.
    pub type DiffMap = BTreeMap<PackageFeaturesKey, BTreeSet<InternedString>>;

    /// The new feature resolver that [`resolve`]s your project.
    ///
    /// For more information, please see the [module-level documentation].
    ///
    /// [`resolve`]: Self::resolve
    /// [module-level documentation]: crate::core::resolver::features
    pub struct FeatureResolver<'a, 'gctx> {
        ws: &'a Workspace<'gctx>,
        target_data: &'a mut RustcTargetData<'gctx>,
        /// The platforms to build for, requested by the user.
        requested_targets: &'a [CompileKind],
        resolve: &'a Resolve,
        package_set: &'a PackageSet<'gctx>,
        /// Options that change how the feature resolver operates.
        opts: FeatureOpts,
        /// Map of features activated for each package.
        activated_features: ActivateMap,
        /// Map of optional dependencies activated for each package.
        activated_dependencies: ActivateMap,
        /// Keeps track of which packages have had its dependencies processed.
        /// Used to avoid cycles, and to speed up processing.
        processed_deps: HashSet<PackageFeaturesKey>,
        /// If this is `true`, then a non-default `feature_key` needs to be tracked while
        /// traversing the graph.
        ///
        /// This is only here to avoid calling `is_proc_macro` when all feature
        /// options are disabled (because `is_proc_macro` can trigger downloads).
        /// This has to be separate from `FeatureOpts.decouple_host_deps` because
        /// `for_host` tracking is also needed for `itarget` to work properly.
        track_for_host: bool,
        /// `dep_name?/feat_name` features that will be activated if `dep_name` is
        /// ever activated.
        ///
        /// The key is the `(package, for_host, dep_name)` of the package whose
        /// dependency will trigger the addition of new features. The value is the
        /// set of features to activate.
        deferred_weak_dependencies:
            HashMap<(PackageId, FeaturesFor, InternedString), HashSet<InternedString>>,
    }

    impl<'a, 'gctx> FeatureResolver<'a, 'gctx> {
        /// Runs the resolution algorithm and returns a new [`ResolvedFeatures`]
        /// with the result.
        #[tracing::instrument(skip_all)]
        pub fn resolve(
            ws: &Workspace<'gctx>,
            target_data: &'a mut RustcTargetData<'gctx>,
            resolve: &Resolve,
            package_set: &'a PackageSet<'gctx>,
            cli_features: &CliFeatures,
            specs: &[PackageIdSpec],
            requested_targets: &[CompileKind],
            opts: FeatureOpts,
        ) -> CargoResult<ResolvedFeatures> {
            let track_for_host = opts.decouple_host_deps || opts.ignore_inactive_targets;
            let mut r = FeatureResolver {
                ws,
                target_data,
                requested_targets,
                resolve,
                package_set,
                opts,
                activated_features: HashMap::new(),
                activated_dependencies: HashMap::new(),
                processed_deps: HashSet::new(),
                track_for_host,
                deferred_weak_dependencies: HashMap::new(),
            };
            r.do_resolve(specs, cli_features)?;
            tracing::debug!("features={:#?}", r.activated_features);
            if r.opts.compare {
                r.compare();
            }
            Ok(ResolvedFeatures {
                activated_features: r.activated_features,
                activated_dependencies: r.activated_dependencies,
                opts: r.opts,
            })
        }

        /// Performs the process of resolving all features for the resolve graph.
        fn do_resolve(
            &mut self,
            specs: &[PackageIdSpec],
            cli_features: &CliFeatures,
        ) -> CargoResult<()> {
            let member_features = self.ws.members_with_features(specs, cli_features)?;
            for (member, cli_features) in &member_features {
                let fvs = self.fvs_from_requested(member.package_id(), cli_features);
                let fk = if self.track_for_host && self.is_proc_macro(member.package_id()) {
                    // Also activate for normal dependencies. This is needed if the
                    // proc-macro includes other targets (like binaries or tests),
                    // or running in `cargo test`. Note that in a workspace, if
                    // the proc-macro is selected on the command like (like with
                    // `--workspace`), this forces feature unification with normal
                    // dependencies. This is part of the bigger problem where
                    // features depend on which packages are built.
                    self.activate_pkg(member.package_id(), FeaturesFor::default(), &fvs)?;
                    FeaturesFor::HostDep
                } else {
                    FeaturesFor::default()
                };
                self.activate_pkg(member.package_id(), fk, &fvs)?;
            }
            Ok(())
        }

        /// Activates [`FeatureValue`]s on the given package.
        ///
        /// This is the main entrance into the recursion of feature activation
        /// for a package.
        fn activate_pkg(
            &mut self,
            pkg_id: PackageId,
            fk: FeaturesFor,
            fvs: &[FeatureValue],
        ) -> CargoResult<()> {
            tracing::trace!("activate_pkg {} {}", pkg_id.name(), fk);
            // Add an empty entry to ensure everything is covered. This is intended for
            // finding bugs where the resolver missed something it should have visited.
            // Remove this in the future if `activated_features` uses an empty default.
            self.activated_features
                .entry((pkg_id, fk.apply_opts(&self.opts)))
                .or_insert_with(BTreeSet::new);
            for fv in fvs {
                self.activate_fv(pkg_id, fk, fv)?;
            }
            if !self.processed_deps.insert((pkg_id, fk)) {
                // Already processed dependencies. There's no need to process them
                // again. This is primarily to avoid cycles, but also helps speed
                // things up.
                //
                // This is safe because if another package comes along and adds a
                // feature on this package, it will immediately add it (in
                // `activate_fv`), and recurse as necessary right then and there.
                // For example, consider we've already processed our dependencies,
                // and another package comes along and enables one of our optional
                // dependencies, it will do so immediately in the
                // `FeatureValue::DepFeature` branch, and then immediately
                // recurse into that optional dependency. This also holds true for
                // features that enable other features.
                return Ok(());
            }
            for (dep_pkg_id, deps) in self.deps(pkg_id, fk)? {
                for (dep, dep_fk) in deps {
                    if dep.is_optional() {
                        // Optional dependencies are enabled in `activate_fv` when
                        // a feature enables it.
                        continue;
                    }
                    // Recurse into the dependency.
                    let fvs = self.fvs_from_dependency(dep_pkg_id, dep);
                    self.activate_pkg(dep_pkg_id, dep_fk, &fvs)?;
                }
            }
            Ok(())
        }

        /// Activate a single FeatureValue for a package.
        fn activate_fv(
            &mut self,
            pkg_id: PackageId,
            fk: FeaturesFor,
            fv: &FeatureValue,
        ) -> CargoResult<()> {
            tracing::trace!("activate_fv {} {} {}", pkg_id.name(), fk, fv);
            match fv {
                FeatureValue::Feature(f) => {
                    self.activate_rec(pkg_id, fk, *f)?;
                }
                FeatureValue::Dep { dep_name } => {
                    self.activate_dependency(pkg_id, fk, *dep_name)?;
                }
                FeatureValue::DepFeature {
                    dep_name,
                    dep_feature,
                    weak,
                } => {
                    self.activate_dep_feature(pkg_id, fk, *dep_name, *dep_feature, *weak)?;
                }
            }
            Ok(())
        }

        /// Activate the given feature for the given package, and then recursively
        /// activate any other features that feature enables.
        fn activate_rec(
            &mut self,
            pkg_id: PackageId,
            fk: FeaturesFor,
            feature_to_enable: InternedString,
        ) -> CargoResult<()> {
            tracing::trace!(
                "activate_rec {} {} feat={}",
                pkg_id.name(),
                fk,
                feature_to_enable
            );
            let enabled = self
                .activated_features
                .entry((pkg_id, fk.apply_opts(&self.opts)))
                .or_insert_with(BTreeSet::new);
            if !enabled.insert(feature_to_enable) {
                // Already enabled.
                return Ok(());
            }
            let summary = self.resolve.summary(pkg_id);
            let feature_map = summary.features();
            let Some(fvs) = feature_map.get(&feature_to_enable) else {
                // TODO: this should only happen for optional dependencies.
                // Other cases should be validated by Summary's `build_feature_map`.
                // Figure out some way to validate this assumption.
                tracing::debug!(
                    "pkg {:?} does not define feature {}",
                    pkg_id,
                    feature_to_enable
                );
                return Ok(());
            };
            for fv in fvs {
                self.activate_fv(pkg_id, fk, fv)?;
            }
            Ok(())
        }

        /// Activate a dependency (`dep:dep_name` syntax).
        fn activate_dependency(
            &mut self,
            pkg_id: PackageId,
            fk: FeaturesFor,
            dep_name: InternedString,
        ) -> CargoResult<()> {
            // Mark this dependency as activated.
            let save_decoupled = fk.apply_opts(&self.opts);
            self.activated_dependencies
                .entry((pkg_id, save_decoupled))
                .or_default()
                .insert(dep_name);
            // Check for any deferred features.
            let to_enable = self
                .deferred_weak_dependencies
                .remove(&(pkg_id, fk, dep_name));
            // Activate the optional dep.
            for (dep_pkg_id, deps) in self.deps(pkg_id, fk)? {
                for (dep, dep_fk) in deps {
                    if dep.name_in_toml() != dep_name {
                        continue;
                    }
                    if let Some(to_enable) = &to_enable {
                        for dep_feature in to_enable {
                            tracing::trace!(
                                "activate deferred {} {} -> {}/{}",
                                pkg_id.name(),
                                fk,
                                dep_name,
                                dep_feature
                            );
                            let fv = FeatureValue::new(*dep_feature);
                            self.activate_fv(dep_pkg_id, dep_fk, &fv)?;
                        }
                    }
                    let fvs = self.fvs_from_dependency(dep_pkg_id, dep);
                    self.activate_pkg(dep_pkg_id, dep_fk, &fvs)?;
                }
            }
            Ok(())
        }

        /// Activate a feature within a dependency (`dep_name/feat_name` syntax).
        fn activate_dep_feature(
            &mut self,
            pkg_id: PackageId,
            fk: FeaturesFor,
            dep_name: InternedString,
            dep_feature: InternedString,
            weak: bool,
        ) -> CargoResult<()> {
            for (dep_pkg_id, deps) in self.deps(pkg_id, fk)? {
                for (dep, dep_fk) in deps {
                    if dep.name_in_toml() != dep_name {
                        continue;
                    }
                    if dep.is_optional() {
                        let save_for_host = fk.apply_opts(&self.opts);
                        if weak
                            && !self
                                .activated_dependencies
                                .get(&(pkg_id, save_for_host))
                                .map(|deps| deps.contains(&dep_name))
                                .unwrap_or(false)
                        {
                            // This is weak, but not yet activated. Defer in case
                            // something comes along later and enables it.
                            tracing::trace!(
                                "deferring feature {} {} -> {}/{}",
                                pkg_id.name(),
                                fk,
                                dep_name,
                                dep_feature
                            );
                            self.deferred_weak_dependencies
                                .entry((pkg_id, fk, dep_name))
                                .or_default()
                                .insert(dep_feature);
                            continue;
                        }

                        // Activate the dependency on self.
                        let fv = FeatureValue::Dep { dep_name };
                        self.activate_fv(pkg_id, fk, &fv)?;
                        if !weak {
                            // The old behavior before weak dependencies were
                            // added is to also enables a feature of the same
                            // name.
                            //
                            // Don't enable if the implicit optional dependency
                            // feature wasn't created due to `dep:` hiding.
                            // See rust-lang/cargo#10788 and rust-lang/cargo#12130
                            let summary = self.resolve.summary(pkg_id);
                            let feature_map = summary.features();
                            if feature_map.contains_key(&dep_name) {
                                self.activate_rec(pkg_id, fk, dep_name)?;
                            }
                        }
                    }
                    // Activate the feature on the dependency.
                    let fv = FeatureValue::new(dep_feature);
                    self.activate_fv(dep_pkg_id, dep_fk, &fv)?;
                }
            }
            Ok(())
        }

        /// Returns Vec of FeatureValues from a Dependency definition.
        fn fvs_from_dependency(&self, dep_id: PackageId, dep: &Dependency) -> Vec<FeatureValue> {
            let summary = self.resolve.summary(dep_id);
            let feature_map = summary.features();
            let mut result: Vec<FeatureValue> = dep
                .features()
                .iter()
                .map(|f| FeatureValue::new(*f))
                .collect();
            let default = InternedString::new("default");
            if dep.uses_default_features() && feature_map.contains_key(&default) {
                result.push(FeatureValue::Feature(default));
            }
            result
        }

        /// Returns Vec of FeatureValues from a set of command-line features.
        fn fvs_from_requested(
            &self,
            pkg_id: PackageId,
            cli_features: &CliFeatures,
        ) -> Vec<FeatureValue> {
            let summary = self.resolve.summary(pkg_id);
            let feature_map = summary.features();

            let mut result: Vec<FeatureValue> = cli_features.features.iter().cloned().collect();
            let default = InternedString::new("default");
            if cli_features.uses_default_features && feature_map.contains_key(&default) {
                result.push(FeatureValue::Feature(default));
            }

            if cli_features.all_features {
                result.extend(feature_map.keys().map(|k| FeatureValue::Feature(*k)))
            }

            result
        }

        /// Returns the dependencies for a package, filtering out inactive targets.
        fn deps(
            &mut self,
            pkg_id: PackageId,
            fk: FeaturesFor,
        ) -> CargoResult<Vec<(PackageId, Vec<(&'a Dependency, FeaturesFor)>)>> {
            // Helper for determining if a platform is activated.
            fn platform_activated(
                dep: &Dependency,
                fk: FeaturesFor,
                target_data: &RustcTargetData<'_>,
                requested_targets: &[CompileKind],
            ) -> bool {
                // We always count platforms as activated if the target stems from an artifact
                // dependency's target specification. This triggers in conjunction with
                // `[target.'cfg(…)'.dependencies]` manifest sections.
                match (dep.is_build(), fk) {
                    (true, _) | (_, FeaturesFor::HostDep) => {
                        // We always care about build-dependencies, and they are always
                        // Host. If we are computing dependencies "for a build script",
                        // even normal dependencies are host-only.
                        target_data.dep_platform_activated(dep, CompileKind::Host)
                    }
                    (_, FeaturesFor::NormalOrDev) => requested_targets
                        .iter()
                        .any(|kind| target_data.dep_platform_activated(dep, *kind)),
                    (_, FeaturesFor::ArtifactDep(target)) => {
                        target_data.dep_platform_activated(dep, CompileKind::Target(target))
                    }
                }
            }

            self.resolve
                .deps(pkg_id)
                .map(|(dep_id, deps)| {
                    let deps = deps
                        .iter()
                        .filter(|dep| {
                            if dep.platform().is_some()
                                && self.opts.ignore_inactive_targets
                                && !platform_activated(
                                    dep,
                                    fk,
                                    self.target_data,
                                    self.requested_targets,
                                )
                            {
                                return false;
                            }
                            if self.opts.decouple_dev_deps && dep.kind() == DepKind::Development {
                                return false;
                            }
                            true
                        })
                        .collect_vec() // collect because the next closure mutably borrows `self.target_data`
                        .into_iter()
                        .map(|dep| {
                            // Each `dep`endency can be built for multiple targets. For one, it
                            // may be a library target which is built as initially configured
                            // by `fk`. If it appears as build dependency, it must be built
                            // for the host.
                            //
                            // It may also be an artifact dependency,
                            // which could be built either
                            //
                            //  - for a specified (aka 'forced') target, specified by
                            //    `dep = { …, target = <triple>` }`
                            //  - as an artifact for use in build dependencies that should
                            //    build for whichever `--target`s are specified
                            //  - like a library would be built
                            //
                            // Generally, the logic for choosing a target for dependencies is
                            // unaltered and used to determine how to build non-artifacts,
                            // artifacts without target specification and no library,
                            // or an artifacts library.
                            //
                            // All this may result in a dependency being built multiple times
                            // for various targets which are either specified in the manifest
                            // or on the cargo command-line.
                            let lib_fk = if fk == FeaturesFor::default() {
                                (self.track_for_host && (dep.is_build() || self.is_proc_macro(dep_id)))
                                    .then(|| FeaturesFor::HostDep)
                                    .unwrap_or_default()
                            } else {
                                fk
                            };

                            // `artifact_target_keys` are produced to fulfil the needs of artifacts that have a target specification.
                            let artifact_target_keys = dep
                                .artifact()
                                .map(|artifact| {
                                    let host_triple = self.target_data.rustc.host;
                                    // not all targets may be queried before resolution since artifact dependencies
                                    // and per-pkg-targets are not immediately known.
                                    let mut activate_target = |target| {
                                        let name = dep.name_in_toml();
                                        self.target_data
                                            .merge_compile_kind(CompileKind::Target(target))
                                            .with_context(|| format!("failed to determine target information for target `{target}`.\n  \
                                            Artifact dependency `{name}` in package `{pkg_id}` requires building for `{target}`", target = target.rustc_target()))
                                    };
                                    CargoResult::Ok((
                                        artifact.is_lib(),
                                        artifact
                                            .target()
                                            .map(|target| {
                                                CargoResult::Ok(match target {
                                                    ArtifactTarget::Force(target) => {
                                                        activate_target(target)?;
                                                        vec![FeaturesFor::ArtifactDep(target)]
                                                    }
                                                    // FIXME: this needs to interact with the `default-target` and `forced-target` values
                                                    // of the dependency
                                                    ArtifactTarget::BuildDependencyAssumeTarget => self
                                                        .requested_targets
                                                        .iter()
                                                        .map(|kind| match kind {
                                                            CompileKind::Host => {
                                                                CompileTarget::new(&host_triple)
                                                                    .unwrap()
                                                            }
                                                            CompileKind::Target(target) => *target,
                                                        })
                                                        .map(|target| {
                                                            activate_target(target)?;
                                                            Ok(FeaturesFor::ArtifactDep(target))
                                                        })
                                                        .collect::<CargoResult<_>>()?,
                                                })
                                            })
                                            .transpose()?,
                                    ))
                                })
                                .transpose()?;

                            let dep_fks = match artifact_target_keys {
                                // The artifact is also a library and does specify custom
                                // targets.
                                // The library's feature key needs to be used alongside
                                // the keys artifact targets.
                                Some((is_lib, Some(mut dep_fks))) if is_lib => {
                                    dep_fks.push(lib_fk);
                                    dep_fks
                                }
                                // The artifact is not a library, but does specify
                                // custom targets.
                                // Use only these targets feature keys.
                                Some((_, Some(dep_fks))) => dep_fks,
                                // There is no artifact in the current dependency
                                // or there is no target specified on the artifact.
                                // Use the standard feature key without any alteration.
                                Some((_, None)) | None => vec![lib_fk],
                            };
                            Ok(dep_fks.into_iter().map(move |dep_fk| (dep, dep_fk)))
                        })
                        .flatten_ok()
                        .collect::<CargoResult<Vec<_>>>()?;
                    Ok((dep_id, deps))
                })
                .filter(|res| res.as_ref().map_or(true, |(_id, deps)| !deps.is_empty()))
                .collect()
        }

        /// Compare the activated features to the resolver. Used for testing.
        fn compare(&self) {
            let mut found = false;
            for ((pkg_id, dep_kind), features) in &self.activated_features {
                let r_features = self.resolve.features(*pkg_id);
                if !r_features.iter().eq(features.iter()) {
                    crate::drop_eprintln!(
                        self.ws.gctx(),
                        "{}/{:?} features mismatch\nresolve: {:?}\nnew: {:?}\n",
                        pkg_id,
                        dep_kind,
                        r_features,
                        features
                    );
                    found = true;
                }
            }
            if found {
                panic!("feature mismatch");
            }
        }

        fn is_proc_macro(&self, package_id: PackageId) -> bool {
            self.package_set
                .get_one(package_id)
                .expect("packages downloaded")
                .proc_macro()
        }
    }