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    //! Constructs the dependency graph for compilation.
    //!
    //! Rust code is typically organized as a set of Cargo packages. The
    //! dependencies between the packages themselves are stored in the
    //! [`Resolve`] struct. However, we can't use that information as is for
    //! compilation! A package typically contains several targets, or crates,
    //! and these targets has inter-dependencies. For example, you need to
    //! compile the `lib` target before the `bin` one, and you need to compile
    //! `build.rs` before either of those.
    //!
    //! So, we need to lower the `Resolve`, which specifies dependencies between
    //! *packages*, to a graph of dependencies between their *targets*, and this
    //! is exactly what this module is doing! Well, almost exactly: another
    //! complication is that we might want to compile the same target several times
    //! (for example, with and without tests), so we actually build a dependency
    //! graph of [`Unit`]s, which capture these properties.

    use std::collections::{HashMap, HashSet};

    use tracing::trace;

    use crate::core::compiler::artifact::match_artifacts_kind_with_targets;
    use crate::core::compiler::unit_graph::{UnitDep, UnitGraph};
    use crate::core::compiler::{
        CompileKind, CompileMode, CrateType, RustcTargetData, Unit, UnitInterner,
    };
    use crate::core::dependency::{Artifact, ArtifactKind, ArtifactTarget, DepKind};
    use crate::core::profiles::{Profile, Profiles, UnitFor};
    use crate::core::resolver::features::{FeaturesFor, ResolvedFeatures};
    use crate::core::resolver::Resolve;
    use crate::core::{Dependency, Package, PackageId, PackageSet, Target, TargetKind, Workspace};
    use crate::ops::resolve_all_features;
    use crate::util::interning::InternedString;
    use crate::util::GlobalContext;
    use crate::CargoResult;

    const IS_NO_ARTIFACT_DEP: Option<&'static Artifact> = None;

    /// Collection of stuff used while creating the [`UnitGraph`].
    struct State<'a, 'gctx> {
        ws: &'a Workspace<'gctx>,
        gctx: &'gctx GlobalContext,
        /// Stores the result of building the [`UnitGraph`].
        unit_dependencies: UnitGraph,
        package_set: &'a PackageSet<'gctx>,
        usr_resolve: &'a Resolve,
        usr_features: &'a ResolvedFeatures,
        /// Like `usr_resolve` but for building standard library (`-Zbuild-std`).
        std_resolve: Option<&'a Resolve>,
        /// Like `usr_features` but for building standard library (`-Zbuild-std`).
        std_features: Option<&'a ResolvedFeatures>,
        /// `true` while generating the dependencies for the standard library.
        is_std: bool,
        /// The mode we are compiling in. Used for preventing from building lib thrice.
        global_mode: CompileMode,
        target_data: &'a RustcTargetData<'gctx>,
        profiles: &'a Profiles,
        interner: &'a UnitInterner,
        // Units for `-Zrustdoc-scrape-examples`.
        scrape_units: &'a [Unit],

        /// A set of edges in `unit_dependencies` where (a, b) means that the
        /// dependency from a to b was added purely because it was a dev-dependency.
        /// This is used during `connect_run_custom_build_deps`.
        dev_dependency_edges: HashSet<(Unit, Unit)>,
    }

    /// A boolean-like to indicate if a `Unit` is an artifact or not.
    #[derive(Copy, Clone, Hash, PartialEq, Eq, PartialOrd, Ord)]
    pub enum IsArtifact {
        Yes,
        No,
    }

    impl IsArtifact {
        pub fn is_true(&self) -> bool {
            matches!(self, IsArtifact::Yes)
        }
    }

    /// Then entry point for building a dependency graph of compilation units.
    ///
    /// You can find some information for arguments from doc of [`State`].
    #[tracing::instrument(skip_all)]
    pub fn build_unit_dependencies<'a, 'gctx>(
        ws: &'a Workspace<'gctx>,
        package_set: &'a PackageSet<'gctx>,
        resolve: &'a Resolve,
        features: &'a ResolvedFeatures,
        std_resolve: Option<&'a (Resolve, ResolvedFeatures)>,
        roots: &[Unit],
        scrape_units: &[Unit],
        std_roots: &HashMap<CompileKind, Vec<Unit>>,
        global_mode: CompileMode,
        target_data: &'a RustcTargetData<'gctx>,
        profiles: &'a Profiles,
        interner: &'a UnitInterner,
    ) -> CargoResult<UnitGraph> {
        if roots.is_empty() {
            // If -Zbuild-std, don't attach units if there is nothing to build.
            // Otherwise, other parts of the code may be confused by seeing units
            // in the dep graph without a root.
            return Ok(HashMap::new());
        }
        let (std_resolve, std_features) = match std_resolve {
            Some((r, f)) => (Some(r), Some(f)),
            None => (None, None),
        };
        let mut state = State {
            ws,
            gctx: ws.gctx(),
            unit_dependencies: HashMap::new(),
            package_set,
            usr_resolve: resolve,
            usr_features: features,
            std_resolve,
            std_features,
            is_std: false,
            global_mode,
            target_data,
            profiles,
            interner,
            scrape_units,
            dev_dependency_edges: HashSet::new(),
        };

        let std_unit_deps = calc_deps_of_std(&mut state, std_roots)?;

        deps_of_roots(roots, &mut state)?;
        super::links::validate_links(state.resolve(), &state.unit_dependencies)?;
        // Hopefully there aren't any links conflicts with the standard library?

        if let Some(std_unit_deps) = std_unit_deps {
            attach_std_deps(&mut state, std_roots, std_unit_deps);
        }

        connect_run_custom_build_deps(&mut state);

        // Dependencies are used in tons of places throughout the backend, many of
        // which affect the determinism of the build itself. As a result be sure
        // that dependency lists are always sorted to ensure we've always got a
        // deterministic output.
        for list in state.unit_dependencies.values_mut() {
            list.sort();
        }
        trace!("ALL UNIT DEPENDENCIES {:#?}", state.unit_dependencies);

        Ok(state.unit_dependencies)
    }

    /// Compute all the dependencies for the standard library.
    fn calc_deps_of_std(
        state: &mut State<'_, '_>,
        std_roots: &HashMap<CompileKind, Vec<Unit>>,
    ) -> CargoResult<Option<UnitGraph>> {
        if std_roots.is_empty() {
            return Ok(None);
        }
        // Compute dependencies for the standard library.
        state.is_std = true;
        for roots in std_roots.values() {
            deps_of_roots(roots, state)?;
        }
        state.is_std = false;
        Ok(Some(std::mem::take(&mut state.unit_dependencies)))
    }

    /// Add the standard library units to the `unit_dependencies`.
    fn attach_std_deps(
        state: &mut State<'_, '_>,
        std_roots: &HashMap<CompileKind, Vec<Unit>>,
        std_unit_deps: UnitGraph,
    ) {
        // Attach the standard library as a dependency of every target unit.
        let mut found = false;
        for (unit, deps) in state.unit_dependencies.iter_mut() {
            if !unit.kind.is_host() && !unit.mode.is_run_custom_build() {
                deps.extend(std_roots[&unit.kind].iter().map(|unit| UnitDep {
                    unit: unit.clone(),
                    unit_for: UnitFor::new_normal(unit.kind),
                    extern_crate_name: unit.pkg.name(),
                    dep_name: None,
                    // TODO: Does this `public` make sense?
                    public: true,
                    noprelude: true,
                }));
                found = true;
            }
        }
        // And also include the dependencies of the standard library itself. Don't
        // include these if no units actually needed the standard library.
        if found {
            for (unit, deps) in std_unit_deps.into_iter() {
                if let Some(other_unit) = state.unit_dependencies.insert(unit, deps) {
                    panic!("std unit collision with existing unit: {:?}", other_unit);
                }
            }
        }
    }

    /// Compute all the dependencies of the given root units.
    /// The result is stored in state.unit_dependencies.
    fn deps_of_roots(roots: &[Unit], state: &mut State<'_, '_>) -> CargoResult<()> {
        for unit in roots.iter() {
            // Dependencies of tests/benches should not have `panic` set.
            // We check the global test mode to see if we are running in `cargo
            // test` in which case we ensure all dependencies have `panic`
            // cleared, and avoid building the lib thrice (once with `panic`, once
            // without, once for `--test`). In particular, the lib included for
            // Doc tests and examples are `Build` mode here.
            let root_compile_kind = unit.kind;
            let unit_for = if unit.mode.is_any_test() || state.global_mode.is_rustc_test() {
                if unit.target.proc_macro() {
                    // Special-case for proc-macros, which are forced to for-host
                    // since they need to link with the proc_macro crate.
                    UnitFor::new_host_test(state.gctx, root_compile_kind)
                } else {
                    UnitFor::new_test(state.gctx, root_compile_kind)
                }
            } else if unit.target.is_custom_build() {
                // This normally doesn't happen, except `clean` aggressively
                // generates all units.
                UnitFor::new_host(false, root_compile_kind)
            } else if unit.target.proc_macro() {
                UnitFor::new_host(true, root_compile_kind)
            } else if unit.target.for_host() {
                // Plugin should never have panic set.
                UnitFor::new_compiler(root_compile_kind)
            } else {
                UnitFor::new_normal(root_compile_kind)
            };
            deps_of(unit, state, unit_for)?;
        }

        Ok(())
    }

    /// Compute the dependencies of a single unit, recursively computing all
    /// transitive dependencies.
    ///
    /// The result is stored in `state.unit_dependencies`.
    fn deps_of(unit: &Unit, state: &mut State<'_, '_>, unit_for: UnitFor) -> CargoResult<()> {
        // Currently the `unit_dependencies` map does not include `unit_for`. This should
        // be safe for now. `TestDependency` only exists to clear the `panic`
        // flag, and you'll never ask for a `unit` with `panic` set as a
        // `TestDependency`. `CustomBuild` should also be fine since if the
        // requested unit's settings are the same as `Any`, `CustomBuild` can't
        // affect anything else in the hierarchy.
        if !state.unit_dependencies.contains_key(unit) {
            let unit_deps = compute_deps(unit, state, unit_for)?;
            state
                .unit_dependencies
                .insert(unit.clone(), unit_deps.clone());
            for unit_dep in unit_deps {
                deps_of(&unit_dep.unit, state, unit_dep.unit_for)?;
            }
        }
        Ok(())
    }

    /// Returns the direct unit dependencies for the given `Unit`.
    fn compute_deps(
        unit: &Unit,
        state: &mut State<'_, '_>,
        unit_for: UnitFor,
    ) -> CargoResult<Vec<UnitDep>> {
        if unit.mode.is_run_custom_build() {
            return compute_deps_custom_build(unit, unit_for, state);
        } else if unit.mode.is_doc() {
            // Note: this does not include doc test.
            return compute_deps_doc(unit, state, unit_for);
        }

        let mut ret = Vec::new();
        let mut dev_deps = Vec::new();
        for (dep_pkg_id, deps) in state.deps(unit, unit_for) {
            let Some(dep_lib) = calc_artifact_deps(unit, unit_for, dep_pkg_id, &deps, state, &mut ret)?
            else {
                continue;
            };
            let dep_pkg = state.get(dep_pkg_id);
            let mode = check_or_build_mode(unit.mode, dep_lib);
            let dep_unit_for = unit_for.with_dependency(unit, dep_lib, unit_for.root_compile_kind());

            let start = ret.len();
            if state.gctx.cli_unstable().dual_proc_macros
                && dep_lib.proc_macro()
                && !unit.kind.is_host()
            {
                let unit_dep = new_unit_dep(
                    state,
                    unit,
                    dep_pkg,
                    dep_lib,
                    dep_unit_for,
                    unit.kind,
                    mode,
                    IS_NO_ARTIFACT_DEP,
                )?;
                ret.push(unit_dep);
                let unit_dep = new_unit_dep(
                    state,
                    unit,
                    dep_pkg,
                    dep_lib,
                    dep_unit_for,
                    CompileKind::Host,
                    mode,
                    IS_NO_ARTIFACT_DEP,
                )?;
                ret.push(unit_dep);
            } else {
                let unit_dep = new_unit_dep(
                    state,
                    unit,
                    dep_pkg,
                    dep_lib,
                    dep_unit_for,
                    unit.kind.for_target(dep_lib),
                    mode,
                    IS_NO_ARTIFACT_DEP,
                )?;
                ret.push(unit_dep);
            }

            // If the unit added was a dev-dependency unit, then record that in the
            // dev-dependencies array. We'll add this to
            // `state.dev_dependency_edges` at the end and process it later in
            // `connect_run_custom_build_deps`.
            if deps.iter().all(|d| !d.is_transitive()) {
                for dep in ret[start..].iter() {
                    dev_deps.push((unit.clone(), dep.unit.clone()));
                }
            }
        }
        state.dev_dependency_edges.extend(dev_deps);

        // If this target is a build script, then what we've collected so far is
        // all we need. If this isn't a build script, then it depends on the
        // build script if there is one.
        if unit.target.is_custom_build() {
            return Ok(ret);
        }
        ret.extend(dep_build_script(unit, unit_for, state)?);

        // If this target is a binary, test, example, etc, then it depends on
        // the library of the same package. The call to `resolve.deps` above
        // didn't include `pkg` in the return values, so we need to special case
        // it here and see if we need to push `(pkg, pkg_lib_target)`.
        if unit.target.is_lib() && unit.mode != CompileMode::Doctest {
            return Ok(ret);
        }
        ret.extend(maybe_lib(unit, state, unit_for)?);

        // If any integration tests/benches are being run, make sure that
        // binaries are built as well.
        if !unit.mode.is_check()
            && unit.mode.is_any_test()
            && (unit.target.is_test() || unit.target.is_bench())
        {
            let id = unit.pkg.package_id();
            ret.extend(
                unit.pkg
                    .targets()
                    .iter()
                    .filter(|t| {
                        // Skip binaries with required features that have not been selected.
                        match t.required_features() {
                            Some(rf) if t.is_bin() => {
                                let features = resolve_all_features(
                                    state.resolve(),
                                    state.features(),
                                    state.package_set,
                                    id,
                                );
                                rf.iter().all(|f| features.contains(f))
                            }
                            None if t.is_bin() => true,
                            _ => false,
                        }
                    })
                    .map(|t| {
                        new_unit_dep(
                            state,
                            unit,
                            &unit.pkg,
                            t,
                            UnitFor::new_normal(unit_for.root_compile_kind()),
                            unit.kind.for_target(t),
                            CompileMode::Build,
                            IS_NO_ARTIFACT_DEP,
                        )
                    })
                    .collect::<CargoResult<Vec<UnitDep>>>()?,
            );
        }

        Ok(ret)
    }

    /// Find artifacts for all `deps` of `unit` and add units that build these artifacts
    /// to `ret`.
    fn calc_artifact_deps<'a>(
        unit: &Unit,
        unit_for: UnitFor,
        dep_id: PackageId,
        deps: &[&Dependency],
        state: &State<'a, '_>,
        ret: &mut Vec<UnitDep>,
    ) -> CargoResult<Option<&'a Target>> {
        let mut has_artifact_lib = false;
        let mut maybe_non_artifact_lib = false;
        let artifact_pkg = state.get(dep_id);
        for dep in deps {
            let Some(artifact) = dep.artifact() else {
                maybe_non_artifact_lib = true;
                continue;
            };
            has_artifact_lib |= artifact.is_lib();
            // Custom build scripts (build/compile) never get artifact dependencies,
            // but the run-build-script step does (where it is handled).
            if !unit.target.is_custom_build() {
                debug_assert!(
                    !unit.mode.is_run_custom_build(),
                    "BUG: This should be handled in a separate branch"
                );
                ret.extend(artifact_targets_to_unit_deps(
                    unit,
                    unit_for.with_artifact_features(artifact),
                    state,
                    artifact
                        .target()
                        .and_then(|t| match t {
                            ArtifactTarget::BuildDependencyAssumeTarget => None,
                            ArtifactTarget::Force(kind) => Some(CompileKind::Target(kind)),
                        })
                        .unwrap_or(unit.kind),
                    artifact_pkg,
                    dep,
                )?);
            }
        }
        if has_artifact_lib || maybe_non_artifact_lib {
            Ok(artifact_pkg.targets().iter().find(|t| t.is_lib()))
        } else {
            Ok(None)
        }
    }

    /// Returns the dependencies needed to run a build script.
    ///
    /// The `unit` provided must represent an execution of a build script, and
    /// the returned set of units must all be run before `unit` is run.
    fn compute_deps_custom_build(
        unit: &Unit,
        unit_for: UnitFor,
        state: &State<'_, '_>,
    ) -> CargoResult<Vec<UnitDep>> {
        if let Some(links) = unit.pkg.manifest().links() {
            if state
                .target_data
                .script_override(links, unit.kind)
                .is_some()
            {
                // Overridden build scripts don't have any dependencies.
                return Ok(Vec::new());
            }
        }
        // All dependencies of this unit should use profiles for custom builds.
        // If this is a build script of a proc macro, make sure it uses host
        // features.
        let script_unit_for = unit_for.for_custom_build();
        // When not overridden, then the dependencies to run a build script are:
        //
        // 1. Compiling the build script itself.
        // 2. For each immediate dependency of our package which has a `links`
        //    key, the execution of that build script.
        //
        // We don't have a great way of handling (2) here right now so this is
        // deferred until after the graph of all unit dependencies has been
        // constructed.
        let compile_script_unit = new_unit_dep(
            state,
            unit,
            &unit.pkg,
            &unit.target,
            script_unit_for,
            // Build scripts always compiled for the host.
            CompileKind::Host,
            CompileMode::Build,
            IS_NO_ARTIFACT_DEP,
        )?;

        let mut result = vec![compile_script_unit];

        // Include any artifact dependencies.
        //
        // This is essentially the same as `calc_artifact_deps`, but there are some
        // subtle differences that require this to be implemented differently.
        //
        // Produce units that build all required artifact kinds (like binaries,
        // static libraries, etc) with the correct compile target.
        //
        // Computing the compile target for artifact units is more involved as it has to handle
        // various target configurations specific to artifacts, like `target = "target"` and
        // `target = "<triple>"`, which makes knowing the root units compile target
        // `root_unit_compile_target` necessary.
        let root_unit_compile_target = unit_for.root_compile_kind();
        let unit_for = UnitFor::new_host(/*host_features*/ true, root_unit_compile_target);
        for (dep_pkg_id, deps) in state.deps(unit, script_unit_for) {
            for dep in deps {
                if dep.kind() != DepKind::Build || dep.artifact().is_none() {
                    continue;
                }
                let artifact_pkg = state.get(dep_pkg_id);
                let artifact = dep.artifact().expect("artifact dep");
                let resolved_artifact_compile_kind = artifact
                    .target()
                    .map(|target| target.to_resolved_compile_kind(root_unit_compile_target));

                result.extend(artifact_targets_to_unit_deps(
                    unit,
                    unit_for.with_artifact_features_from_resolved_compile_kind(
                        resolved_artifact_compile_kind,
                    ),
                    state,
                    resolved_artifact_compile_kind.unwrap_or(CompileKind::Host),
                    artifact_pkg,
                    dep,
                )?);
            }
        }

        Ok(result)
    }

    /// Given a `parent` unit containing a dependency `dep` whose package is `artifact_pkg`,
    /// find all targets in `artifact_pkg` which refer to the `dep`s artifact declaration
    /// and turn them into units.
    /// Due to the nature of artifact dependencies, a single dependency in a manifest can
    /// cause one or more targets to be build, for instance with
    /// `artifact = ["bin:a", "bin:b", "staticlib"]`, which is very different from normal
    /// dependencies which cause only a single unit to be created.
    ///
    /// `compile_kind` is the computed kind for the future artifact unit
    /// dependency, only the caller can pick the correct one.
    fn artifact_targets_to_unit_deps(
        parent: &Unit,
        parent_unit_for: UnitFor,
        state: &State<'_, '_>,
        compile_kind: CompileKind,
        artifact_pkg: &Package,
        dep: &Dependency,
    ) -> CargoResult<Vec<UnitDep>> {
        let ret =
            match_artifacts_kind_with_targets(dep, artifact_pkg.targets(), parent.pkg.name().as_str())?
                .into_iter()
                .flat_map(|(artifact_kind, target)| {
                    // We split target libraries into individual units, even though rustc is able
                    // to produce multiple kinds in a single invocation for the sole reason that
                    // each artifact kind has its own output directory, something we can't easily
                    // teach rustc for now.
                    match target.kind() {
                        TargetKind::Lib(kinds) => Box::new(
                            kinds
                                .iter()
                                .filter(move |tk| match (tk, artifact_kind) {
                                    (CrateType::Cdylib, ArtifactKind::Cdylib) => true,
                                    (CrateType::Staticlib, ArtifactKind::Staticlib) => true,
                                    _ => false,
                                })
                                .map(|target_kind| {
                                    new_unit_dep(
                                        state,
                                        parent,
                                        artifact_pkg,
                                        target
                                            .clone()
                                            .set_kind(TargetKind::Lib(vec![target_kind.clone()])),
                                        parent_unit_for,
                                        compile_kind,
                                        CompileMode::Build,
                                        dep.artifact(),
                                    )
                                }),
                        ) as Box<dyn Iterator<Item = _>>,
                        _ => Box::new(std::iter::once(new_unit_dep(
                            state,
                            parent,
                            artifact_pkg,
                            target,
                            parent_unit_for,
                            compile_kind,
                            CompileMode::Build,
                            dep.artifact(),
                        ))),
                    }
                })
                .collect::<Result<Vec<_>, _>>()?;
        Ok(ret)
    }

    /// Returns the dependencies necessary to document a package.
    fn compute_deps_doc(
        unit: &Unit,
        state: &mut State<'_, '_>,
        unit_for: UnitFor,
    ) -> CargoResult<Vec<UnitDep>> {
        // To document a library, we depend on dependencies actually being
        // built. If we're documenting *all* libraries, then we also depend on
        // the documentation of the library being built.
        let mut ret = Vec::new();
        for (id, deps) in state.deps(unit, unit_for) {
            let Some(dep_lib) = calc_artifact_deps(unit, unit_for, id, &deps, state, &mut ret)? else {
                continue;
            };
            let dep_pkg = state.get(id);
            // Rustdoc only needs rmeta files for regular dependencies.
            // However, for plugins/proc macros, deps should be built like normal.
            let mode = check_or_build_mode(unit.mode, dep_lib);
            let dep_unit_for = unit_for.with_dependency(unit, dep_lib, unit_for.root_compile_kind());
            let lib_unit_dep = new_unit_dep(
                state,
                unit,
                dep_pkg,
                dep_lib,
                dep_unit_for,
                unit.kind.for_target(dep_lib),
                mode,
                IS_NO_ARTIFACT_DEP,
            )?;
            ret.push(lib_unit_dep);
            if dep_lib.documented() {
                if let CompileMode::Doc { deps: true, .. } = unit.mode {
                    // Document this lib as well.
                    let doc_unit_dep = new_unit_dep(
                        state,
                        unit,
                        dep_pkg,
                        dep_lib,
                        dep_unit_for,
                        unit.kind.for_target(dep_lib),
                        unit.mode,
                        IS_NO_ARTIFACT_DEP,
                    )?;
                    ret.push(doc_unit_dep);
                }
            }
        }

        // Be sure to build/run the build script for documented libraries.
        ret.extend(dep_build_script(unit, unit_for, state)?);

        // If we document a binary/example, we need the library available.
        if unit.target.is_bin() || unit.target.is_example() {
            // build the lib
            ret.extend(maybe_lib(unit, state, unit_for)?);
            // and also the lib docs for intra-doc links
            if let Some(lib) = unit
                .pkg
                .targets()
                .iter()
                .find(|t| t.is_linkable() && t.documented())
            {
                let dep_unit_for = unit_for.with_dependency(unit, lib, unit_for.root_compile_kind());
                let lib_doc_unit = new_unit_dep(
                    state,
                    unit,
                    &unit.pkg,
                    lib,
                    dep_unit_for,
                    unit.kind.for_target(lib),
                    unit.mode,
                    IS_NO_ARTIFACT_DEP,
                )?;
                ret.push(lib_doc_unit);
            }
        }

        // Add all units being scraped for examples as a dependency of top-level Doc units.
        if state.ws.unit_needs_doc_scrape(unit) {
            for scrape_unit in state.scrape_units.iter() {
                let scrape_unit_for = UnitFor::new_normal(scrape_unit.kind);
                deps_of(scrape_unit, state, scrape_unit_for)?;
                ret.push(new_unit_dep(
                    state,
                    scrape_unit,
                    &scrape_unit.pkg,
                    &scrape_unit.target,
                    scrape_unit_for,
                    scrape_unit.kind,
                    scrape_unit.mode,
                    IS_NO_ARTIFACT_DEP,
                )?);
            }
        }

        Ok(ret)
    }

    fn maybe_lib(
        unit: &Unit,
        state: &mut State<'_, '_>,
        unit_for: UnitFor,
    ) -> CargoResult<Option<UnitDep>> {
        unit.pkg
            .targets()
            .iter()
            .find(|t| t.is_linkable())
            .map(|t| {
                let mode = check_or_build_mode(unit.mode, t);
                let dep_unit_for = unit_for.with_dependency(unit, t, unit_for.root_compile_kind());
                new_unit_dep(
                    state,
                    unit,
                    &unit.pkg,
                    t,
                    dep_unit_for,
                    unit.kind.for_target(t),
                    mode,
                    IS_NO_ARTIFACT_DEP,
                )
            })
            .transpose()
    }

    /// If a build script is scheduled to be run for the package specified by
    /// `unit`, this function will return the unit to run that build script.
    ///
    /// Overriding a build script simply means that the running of the build
    /// script itself doesn't have any dependencies, so even in that case a unit
    /// of work is still returned. `None` is only returned if the package has no
    /// build script.
    fn dep_build_script(
        unit: &Unit,
        unit_for: UnitFor,
        state: &State<'_, '_>,
    ) -> CargoResult<Option<UnitDep>> {
        unit.pkg
            .targets()
            .iter()
            .find(|t| t.is_custom_build())
            .map(|t| {
                // The profile stored in the Unit is the profile for the thing
                // the custom build script is running for.
                let profile = state.profiles.get_profile_run_custom_build(&unit.profile);
                // UnitFor::for_custom_build is used because we want the `host` flag set
                // for all of our build dependencies (so they all get
                // build-override profiles), including compiling the build.rs
                // script itself.
                //
                // If `is_for_host_features` here is `false`, that means we are a
                // build.rs script for a normal dependency and we want to set the
                // CARGO_FEATURE_* environment variables to the features as a
                // normal dep.
                //
                // If `is_for_host_features` here is `true`, that means that this
                // package is being used as a build dependency or proc-macro, and
                // so we only want to set CARGO_FEATURE_* variables for the host
                // side of the graph.
                //
                // Keep in mind that the RunCustomBuild unit and the Compile
                // build.rs unit use the same features. This is because some
                // people use `cfg!` and `#[cfg]` expressions to check for enabled
                // features instead of just checking `CARGO_FEATURE_*` at runtime.
                // In the case with the new feature resolver (decoupled host
                // deps), and a shared dependency has different features enabled
                // for normal vs. build, then the build.rs script will get
                // compiled twice. I believe it is not feasible to only build it
                // once because it would break a large number of scripts (they
                // would think they have the wrong set of features enabled).
                let script_unit_for = unit_for.for_custom_build();
                new_unit_dep_with_profile(
                    state,
                    unit,
                    &unit.pkg,
                    t,
                    script_unit_for,
                    unit.kind,
                    CompileMode::RunCustomBuild,
                    profile,
                    IS_NO_ARTIFACT_DEP,
                )
            })
            .transpose()
    }

    /// Choose the correct mode for dependencies.
    fn check_or_build_mode(mode: CompileMode, target: &Target) -> CompileMode {
        match mode {
            CompileMode::Check { .. } | CompileMode::Doc { .. } | CompileMode::Docscrape => {
                if target.for_host() {
                    // Plugin and proc macro targets should be compiled like
                    // normal.
                    CompileMode::Build
                } else {
                    // Regular dependencies should not be checked with --test.
                    // Regular dependencies of doc targets should emit rmeta only.
                    CompileMode::Check { test: false }
                }
            }
            _ => CompileMode::Build,
        }
    }

    /// Create a new Unit for a dependency from `parent` to `pkg` and `target`.
    fn new_unit_dep(
        state: &State<'_, '_>,
        parent: &Unit,
        pkg: &Package,
        target: &Target,
        unit_for: UnitFor,
        kind: CompileKind,
        mode: CompileMode,
        artifact: Option<&Artifact>,
    ) -> CargoResult<UnitDep> {
        let is_local = pkg.package_id().source_id().is_path() && !state.is_std;
        let profile = state.profiles.get_profile(
            pkg.package_id(),
            state.ws.is_member(pkg),
            is_local,
            unit_for,
            kind,
        );
        new_unit_dep_with_profile(
            state, parent, pkg, target, unit_for, kind, mode, profile, artifact,
        )
    }

    fn new_unit_dep_with_profile(
        state: &State<'_, '_>,
        parent: &Unit,
        pkg: &Package,
        target: &Target,
        unit_for: UnitFor,
        kind: CompileKind,
        mode: CompileMode,
        profile: Profile,
        artifact: Option<&Artifact>,
    ) -> CargoResult<UnitDep> {
        let (extern_crate_name, dep_name) = state.resolve().extern_crate_name_and_dep_name(
            parent.pkg.package_id(),
            pkg.package_id(),
            target,
        )?;
        let public = state
            .resolve()
            .is_public_dep(parent.pkg.package_id(), pkg.package_id());
        let features_for = unit_for.map_to_features_for(artifact);
        let artifact_target = match features_for {
            FeaturesFor::ArtifactDep(target) => Some(target),
            _ => None,
        };
        let features = state.activated_features(pkg.package_id(), features_for);
        let unit = state.interner.intern(
            pkg,
            target,
            profile,
            kind,
            mode,
            features,
            state.is_std,
            /*dep_hash*/ 0,
            artifact.map_or(IsArtifact::No, |_| IsArtifact::Yes),
            artifact_target,
        );
        Ok(UnitDep {
            unit,
            unit_for,
            extern_crate_name,
            dep_name,
            public,
            noprelude: false,
        })
    }

    /// Fill in missing dependencies for units of the `RunCustomBuild`
    ///
    /// As mentioned above in `compute_deps_custom_build` each build script
    /// execution has two dependencies. The first is compiling the build script
    /// itself (already added) and the second is that all crates the package of the
    /// build script depends on with `links` keys, their build script execution. (a
    /// bit confusing eh?)
    ///
    /// Here we take the entire `deps` map and add more dependencies from execution
    /// of one build script to execution of another build script.
    fn connect_run_custom_build_deps(state: &mut State<'_, '_>) {
        let mut new_deps = Vec::new();

        {
            let state = &*state;
            // First up build a reverse dependency map. This is a mapping of all
            // `RunCustomBuild` known steps to the unit which depends on them. For
            // example a library might depend on a build script, so this map will
            // have the build script as the key and the library would be in the
            // value's set.
            let mut reverse_deps_map = HashMap::new();
            for (unit, deps) in state.unit_dependencies.iter() {
                for dep in deps {
                    if dep.unit.mode == CompileMode::RunCustomBuild {
                        reverse_deps_map
                            .entry(dep.unit.clone())
                            .or_insert_with(HashSet::new)
                            .insert(unit);
                    }
                }
            }

            // Next, we take a look at all build scripts executions listed in the
            // dependency map. Our job here is to take everything that depends on
            // this build script (from our reverse map above) and look at the other
            // package dependencies of these parents.
            //
            // If we depend on a linkable target and the build script mentions
            // `links`, then we depend on that package's build script! Here we use
            // `dep_build_script` to manufacture an appropriate build script unit to
            // depend on.
            for unit in state
                .unit_dependencies
                .keys()
                .filter(|k| k.mode == CompileMode::RunCustomBuild)
            {
                // This list of dependencies all depend on `unit`, an execution of
                // the build script.
                let Some(reverse_deps) = reverse_deps_map.get(unit) else {
                    continue;
                };

                let to_add = reverse_deps
                    .iter()
                    // Get all sibling dependencies of `unit`
                    .flat_map(|reverse_dep| {
                        state.unit_dependencies[reverse_dep]
                            .iter()
                            .map(move |a| (reverse_dep, a))
                    })
                    // Only deps with `links`.
                    .filter(|(_parent, other)| {
                        other.unit.pkg != unit.pkg
                            && other.unit.target.is_linkable()
                            && other.unit.pkg.manifest().links().is_some()
                    })
                    // Avoid cycles when using the doc --scrape-examples feature:
                    // Say a workspace has crates A and B where A has a build-dependency on B.
                    // The Doc units for A and B will have a dependency on the Docscrape for both A and B.
                    // So this would add a dependency from B-build to A-build, causing a cycle:
                    //   B (build) -> A (build) -> B(build)
                    // See the test scrape_examples_avoid_build_script_cycle for a concrete example.
                    // To avoid this cycle, we filter out the B -> A (docscrape) dependency.
                    .filter(|(_parent, other)| !other.unit.mode.is_doc_scrape())
                    // Skip dependencies induced via dev-dependencies since
                    // connections between `links` and build scripts only happens
                    // via normal dependencies. Otherwise since dev-dependencies can
                    // be cyclic we could have cyclic build-script executions.
                    .filter_map(move |(parent, other)| {
                        if state
                            .dev_dependency_edges
                            .contains(&((*parent).clone(), other.unit.clone()))
                        {
                            None
                        } else {
                            Some(other)
                        }
                    })
                    // Get the RunCustomBuild for other lib.
                    .filter_map(|other| {
                        state.unit_dependencies[&other.unit]
                            .iter()
                            .find(|other_dep| other_dep.unit.mode == CompileMode::RunCustomBuild)
                            .cloned()
                    })
                    .collect::<HashSet<_>>();

                if !to_add.is_empty() {
                    // (RunCustomBuild, set(other RunCustomBuild))
                    new_deps.push((unit.clone(), to_add));
                }
            }
        }

        // And finally, add in all the missing dependencies!
        for (unit, new_deps) in new_deps {
            state
                .unit_dependencies
                .get_mut(&unit)
                .unwrap()
                .extend(new_deps);
        }
    }

    impl<'a, 'gctx> State<'a, 'gctx> {
        /// Gets `std_resolve` during building std, otherwise `usr_resolve`.
        fn resolve(&self) -> &'a Resolve {
            if self.is_std {
                self.std_resolve.unwrap()
            } else {
                self.usr_resolve
            }
        }

        /// Gets `std_features` during building std, otherwise `usr_features`.
        fn features(&self) -> &'a ResolvedFeatures {
            if self.is_std {
                self.std_features.unwrap()
            } else {
                self.usr_features
            }
        }

        fn activated_features(
            &self,
            pkg_id: PackageId,
            features_for: FeaturesFor,
        ) -> Vec<InternedString> {
            let features = self.features();
            features.activated_features(pkg_id, features_for)
        }

        fn is_dep_activated(
            &self,
            pkg_id: PackageId,
            features_for: FeaturesFor,
            dep_name: InternedString,
        ) -> bool {
            self.features()
                .is_dep_activated(pkg_id, features_for, dep_name)
        }

        fn get(&self, id: PackageId) -> &'a Package {
            self.package_set
                .get_one(id)
                .unwrap_or_else(|_| panic!("expected {} to be downloaded", id))
        }

        /// Returns a filtered set of dependencies for the given unit.
        fn deps(&self, unit: &Unit, unit_for: UnitFor) -> Vec<(PackageId, Vec<&Dependency>)> {
            let pkg_id = unit.pkg.package_id();
            let kind = unit.kind;
            self.resolve()
                .deps(pkg_id)
                .filter_map(|(id, deps)| {
                    assert!(!deps.is_empty());
                    let deps: Vec<_> = deps
                        .iter()
                        .filter(|dep| {
                            // If this target is a build command, then we only want build
                            // dependencies, otherwise we want everything *other than* build
                            // dependencies.
                            if unit.target.is_custom_build() != dep.is_build() {
                                return false;
                            }

                            // If this dependency is **not** a transitive dependency, then it
                            // only applies to test/example targets.
                            if !dep.is_transitive()
                                && !unit.target.is_test()
                                && !unit.target.is_example()
                                && !unit.mode.is_any_test()
                            {
                                return false;
                            }

                            // If this dependency is only available for certain platforms,
                            // make sure we're only enabling it for that platform.
                            if !self.target_data.dep_platform_activated(dep, kind) {
                                return false;
                            }

                            // If this is an optional dependency, and the new feature resolver
                            // did not enable it, don't include it.
                            if dep.is_optional() {
                                // This `unit_for` is from parent dep and *SHOULD* contains its own
                                // artifact dep information inside `artifact_target_for_features`.
                                // So, no need to map any artifact info from an incorrect `dep.artifact()`.
                                let features_for = unit_for.map_to_features_for(IS_NO_ARTIFACT_DEP);
                                if !self.is_dep_activated(pkg_id, features_for, dep.name_in_toml()) {
                                    return false;
                                }
                            }

                            // If we've gotten past all that, then this dependency is
                            // actually used!
                            true
                        })
                        .collect();
                    if deps.is_empty() {
                        None
                    } else {
                        Some((id, deps))
                    }
                })
                .collect()
        }
    }