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    //! How to execute a build script and parse its output.
    //!
    //! ## Preparing a build script run
    //!
    //! A [build script] is an optional Rust script Cargo will run before building
    //! your package. As of this writing, two kinds of special [`Unit`]s will be
    //! constructed when there is a build script in a package.
    //!
    //! * Build script compilation --- This unit is generally the same as units
    //!   that would compile other Cargo targets. It will recursively creates units
    //!   of its dependencies. One biggest difference is that the [`Unit`] of
    //!   compiling a build script is flagged as [`TargetKind::CustomBuild`].
    //! * Build script execution --- During the construction of the [`UnitGraph`],
    //!   Cargo inserts a [`Unit`] with [`CompileMode::RunCustomBuild`]. This unit
    //!   depends on the unit of compiling the associated build script, to ensure
    //!   the executable is available before running. The [`Work`] of running the
    //!   build script is prepared in the function [`prepare`].
    //!
    //! ## Running a build script
    //!
    //! When running a build script, Cargo is aware of the progress and the result
    //! of a build script. Standard output is the chosen interprocess communication
    //! between Cargo and build script processes. A set of strings is defined for
    //! that purpose. These strings, a.k.a. instructions, are interpreted by
    //! [`BuildOutput::parse`] and stored in [`Context::build_script_outputs`].
    //! The entire execution work is constructed by [`build_work`].
    //!
    //! [build script]: https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html
    //! [`TargetKind::CustomBuild`]: crate::core::manifest::TargetKind::CustomBuild
    //! [`UnitGraph`]: super::unit_graph::UnitGraph
    //! [`CompileMode::RunCustomBuild`]: super::CompileMode
    //! [instructions]: https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script

    use super::{fingerprint, Context, Job, Unit, Work};
    use crate::core::compiler::artifact;
    use crate::core::compiler::context::Metadata;
    use crate::core::compiler::job_queue::JobState;
    use crate::core::{profiles::ProfileRoot, PackageId, Target};
    use crate::util::errors::CargoResult;
    use crate::util::machine_message::{self, Message};
    use crate::util::{internal, profile};
    use anyhow::{bail, Context as _};
    use cargo_platform::Cfg;
    use cargo_util::paths;
    use cargo_util_schemas::manifest::RustVersion;
    use std::collections::hash_map::{Entry, HashMap};
    use std::collections::{BTreeSet, HashSet};
    use std::path::{Path, PathBuf};
    use std::str::{self, FromStr};
    use std::sync::{Arc, Mutex};

    /// Deprecated: A build script instruction that tells Cargo to display a warning after the
    /// build script has finished running. Read [the doc] for more.
    ///
    /// [the doc]: https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#cargo-warning
    const OLD_CARGO_WARNING_SYNTAX: &str = "cargo:warning=";
    /// A build script instruction that tells Cargo to display a warning after the
    /// build script has finished running. Read [the doc] for more.
    ///
    /// [the doc]: https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#cargo-warning
    const NEW_CARGO_WARNING_SYNTAX: &str = "cargo::warning=";
    /// Contains the parsed output of a custom build script.
    #[derive(Clone, Debug, Hash, Default)]
    pub struct BuildOutput {
        /// Paths to pass to rustc with the `-L` flag.
        pub library_paths: Vec<PathBuf>,
        /// Names and link kinds of libraries, suitable for the `-l` flag.
        pub library_links: Vec<String>,
        /// Linker arguments suitable to be passed to `-C link-arg=<args>`
        pub linker_args: Vec<(LinkArgTarget, String)>,
        /// Various `--cfg` flags to pass to the compiler.
        pub cfgs: Vec<String>,
        /// Various `--check-cfg` flags to pass to the compiler.
        pub check_cfgs: Vec<String>,
        /// Additional environment variables to run the compiler with.
        pub env: Vec<(String, String)>,
        /// Metadata to pass to the immediate dependencies.
        pub metadata: Vec<(String, String)>,
        /// Paths to trigger a rerun of this build script.
        /// May be absolute or relative paths (relative to package root).
        pub rerun_if_changed: Vec<PathBuf>,
        /// Environment variables which, when changed, will cause a rebuild.
        pub rerun_if_env_changed: Vec<String>,
        /// Warnings generated by this build.
        ///
        /// These are only displayed if this is a "local" package, `-vv` is used,
        /// or there is a build error for any target in this package.
        pub warnings: Vec<String>,
    }

    /// Map of packages to build script output.
    ///
    /// This initially starts out as empty. Overridden build scripts get
    /// inserted during `build_map`. The rest of the entries are added
    /// immediately after each build script runs.
    ///
    /// The `Metadata` is the unique metadata hash for the RunCustomBuild Unit of
    /// the package. It needs a unique key, since the build script can be run
    /// multiple times with different profiles or features. We can't embed a
    /// `Unit` because this structure needs to be shareable between threads.
    #[derive(Default)]
    pub struct BuildScriptOutputs {
        outputs: HashMap<Metadata, BuildOutput>,
    }

    /// Linking information for a `Unit`.
    ///
    /// See [`build_map`] for more details.
    #[derive(Default)]
    pub struct BuildScripts {
        /// List of build script outputs this Unit needs to include for linking. Each
        /// element is an index into `BuildScriptOutputs`.
        ///
        /// Cargo will use this `to_link` vector to add `-L` flags to compiles as we
        /// propagate them upwards towards the final build. Note, however, that we
        /// need to preserve the ordering of `to_link` to be topologically sorted.
        /// This will ensure that build scripts which print their paths properly will
        /// correctly pick up the files they generated (if there are duplicates
        /// elsewhere).
        ///
        /// To preserve this ordering, the (id, metadata) is stored in two places, once
        /// in the `Vec` and once in `seen_to_link` for a fast lookup. We maintain
        /// this as we're building interactively below to ensure that the memory
        /// usage here doesn't blow up too much.
        ///
        /// For more information, see #2354.
        pub to_link: Vec<(PackageId, Metadata)>,
        /// This is only used while constructing `to_link` to avoid duplicates.
        seen_to_link: HashSet<(PackageId, Metadata)>,
        /// Host-only dependencies that have build scripts. Each element is an
        /// index into `BuildScriptOutputs`.
        ///
        /// This is the set of transitive dependencies that are host-only
        /// (proc-macro, plugin, build-dependency) that contain a build script.
        /// Any `BuildOutput::library_paths` path relative to `target` will be
        /// added to LD_LIBRARY_PATH so that the compiler can find any dynamic
        /// libraries a build script may have generated.
        pub plugins: BTreeSet<(PackageId, Metadata)>,
    }

    /// Dependency information as declared by a build script that might trigger
    /// a recompile of itself.
    #[derive(Debug)]
    pub struct BuildDeps {
        /// Absolute path to the file in the target directory that stores the
        /// output of the build script.
        pub build_script_output: PathBuf,
        /// Files that trigger a rebuild if they change.
        pub rerun_if_changed: Vec<PathBuf>,
        /// Environment variables that trigger a rebuild if they change.
        pub rerun_if_env_changed: Vec<String>,
    }

    /// Represents one of the instructions from `cargo::rustc-link-arg-*` build
    /// script instruction family.
    ///
    /// In other words, indicates targets that custom linker arguments applies to.
    ///
    /// See the [build script documentation][1] for more.
    ///
    /// [1]: https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#cargorustc-link-argflag
    #[derive(Clone, Hash, Debug, PartialEq, Eq)]
    pub enum LinkArgTarget {
        /// Represents `cargo::rustc-link-arg=FLAG`.
        All,
        /// Represents `cargo::rustc-cdylib-link-arg=FLAG`.
        Cdylib,
        /// Represents `cargo::rustc-link-arg-bins=FLAG`.
        Bin,
        /// Represents `cargo::rustc-link-arg-bin=BIN=FLAG`.
        SingleBin(String),
        /// Represents `cargo::rustc-link-arg-tests=FLAG`.
        Test,
        /// Represents `cargo::rustc-link-arg-benches=FLAG`.
        Bench,
        /// Represents `cargo::rustc-link-arg-examples=FLAG`.
        Example,
    }

    impl LinkArgTarget {
        /// Checks if this link type applies to a given [`Target`].
        pub fn applies_to(&self, target: &Target) -> bool {
            match self {
                LinkArgTarget::All => true,
                LinkArgTarget::Cdylib => target.is_cdylib(),
                LinkArgTarget::Bin => target.is_bin(),
                LinkArgTarget::SingleBin(name) => target.is_bin() && target.name() == name,
                LinkArgTarget::Test => target.is_test(),
                LinkArgTarget::Bench => target.is_bench(),
                LinkArgTarget::Example => target.is_exe_example(),
            }
        }
    }

    /// Prepares a `Work` that executes the target as a custom build script.
    pub fn prepare(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Job> {
        let _p = profile::start(format!(
            "build script prepare: {}/{}",
            unit.pkg,
            unit.target.name()
        ));

        let metadata = cx.get_run_build_script_metadata(unit);
        if cx
            .build_script_outputs
            .lock()
            .unwrap()
            .contains_key(metadata)
        {
            // The output is already set, thus the build script is overridden.
            fingerprint::prepare_target(cx, unit, false)
        } else {
            build_work(cx, unit)
        }
    }

    /// Emits the output of a build script as a [`machine_message::BuildScript`]
    /// JSON string to standard output.
    fn emit_build_output(
        state: &JobState<'_, '_>,
        output: &BuildOutput,
        out_dir: &Path,
        package_id: PackageId,
    ) -> CargoResult<()> {
        let library_paths = output
            .library_paths
            .iter()
            .map(|l| l.display().to_string())
            .collect::<Vec<_>>();

        let msg = machine_message::BuildScript {
            package_id: package_id.to_spec(),
            linked_libs: &output.library_links,
            linked_paths: &library_paths,
            cfgs: &output.cfgs,
            env: &output.env,
            out_dir,
        }
        .to_json_string();
        state.stdout(msg)?;
        Ok(())
    }

    /// Constructs the unit of work of running a build script.
    ///
    /// The construction includes:
    ///
    /// * Set environment variables for the build script run.
    /// * Create the output dir (`OUT_DIR`) for the build script output.
    /// * Determine if the build script needs a re-run.
    /// * Run the build script and store its output.
    fn build_work(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Job> {
        assert!(unit.mode.is_run_custom_build());
        let bcx = &cx.bcx;
        let dependencies = cx.unit_deps(unit);
        let build_script_unit = dependencies
            .iter()
            .find(|d| !d.unit.mode.is_run_custom_build() && d.unit.target.is_custom_build())
            .map(|d| &d.unit)
            .expect("running a script not depending on an actual script");
        let script_dir = cx.files().build_script_dir(build_script_unit);
        let script_out_dir = cx.files().build_script_out_dir(unit);
        let script_run_dir = cx.files().build_script_run_dir(unit);
        let build_plan = bcx.build_config.build_plan;
        let invocation_name = unit.buildkey();

        if let Some(deps) = unit.pkg.manifest().metabuild() {
            prepare_metabuild(cx, build_script_unit, deps)?;
        }

        // Building the command to execute
        let to_exec = script_dir.join(unit.target.name());

        // Start preparing the process to execute, starting out with some
        // environment variables. Note that the profile-related environment
        // variables are not set with this the build script's profile but rather the
        // package's library profile.
        // NOTE: if you add any profile flags, be sure to update
        // `Profiles::get_profile_run_custom_build` so that those flags get
        // carried over.
        let to_exec = to_exec.into_os_string();
        let mut cmd = cx.compilation.host_process(to_exec, &unit.pkg)?;
        let debug = unit.profile.debuginfo.is_turned_on();
        cmd.env("OUT_DIR", &script_out_dir)
            .env("CARGO_MANIFEST_DIR", unit.pkg.root())
            .env("NUM_JOBS", &bcx.jobs().to_string())
            .env("TARGET", bcx.target_data.short_name(&unit.kind))
            .env("DEBUG", debug.to_string())
            .env("OPT_LEVEL", &unit.profile.opt_level)
            .env(
                "PROFILE",
                match unit.profile.root {
                    ProfileRoot::Release => "release",
                    ProfileRoot::Debug => "debug",
                },
            )
            .env("HOST", &bcx.host_triple())
            .env("RUSTC", &bcx.rustc().path)
            .env("RUSTDOC", &*bcx.config.rustdoc()?)
            .inherit_jobserver(&cx.jobserver);

        // Find all artifact dependencies and make their file and containing directory discoverable using environment variables.
        for (var, value) in artifact::get_env(cx, dependencies)? {
            cmd.env(&var, value);
        }

        if let Some(linker) = &cx.compilation.target_linker(unit.kind) {
            cmd.env("RUSTC_LINKER", linker);
        }

        if let Some(links) = unit.pkg.manifest().links() {
            cmd.env("CARGO_MANIFEST_LINKS", links);
        }

        if let Some(trim_paths) = unit.profile.trim_paths.as_ref() {
            cmd.env("CARGO_TRIM_PATHS", trim_paths.to_string());
        }

        // Be sure to pass along all enabled features for this package, this is the
        // last piece of statically known information that we have.
        for feat in &unit.features {
            cmd.env(&format!("CARGO_FEATURE_{}", super::envify(feat)), "1");
        }

        let mut cfg_map = HashMap::new();
        for cfg in bcx.target_data.cfg(unit.kind) {
            match *cfg {
                Cfg::Name(ref n) => {
                    cfg_map.insert(n.clone(), Vec::new());
                }
                Cfg::KeyPair(ref k, ref v) => {
                    let values = cfg_map.entry(k.clone()).or_default();
                    values.push(v.clone());
                }
            }
        }
        for (k, v) in cfg_map {
            if k == "debug_assertions" {
                // This cfg is always true and misleading, so avoid setting it.
                // That is because Cargo queries rustc without any profile settings.
                continue;
            }
            let k = format!("CARGO_CFG_{}", super::envify(&k));
            cmd.env(&k, v.join(","));
        }

        // Also inform the build script of the rustc compiler context.
        if let Some(wrapper) = bcx.rustc().wrapper.as_ref() {
            cmd.env("RUSTC_WRAPPER", wrapper);
        } else {
            cmd.env_remove("RUSTC_WRAPPER");
        }
        cmd.env_remove("RUSTC_WORKSPACE_WRAPPER");
        if cx.bcx.ws.is_member(&unit.pkg) {
            if let Some(wrapper) = bcx.rustc().workspace_wrapper.as_ref() {
                cmd.env("RUSTC_WORKSPACE_WRAPPER", wrapper);
            }
        }
        cmd.env(
            "CARGO_ENCODED_RUSTFLAGS",
            bcx.rustflags_args(unit).join("\x1f"),
        );
        cmd.env_remove("RUSTFLAGS");

        if cx.bcx.ws.config().extra_verbose() {
            cmd.display_env_vars();
        }

        // Gather the set of native dependencies that this package has along with
        // some other variables to close over.
        //
        // This information will be used at build-time later on to figure out which
        // sorts of variables need to be discovered at that time.
        let lib_deps = dependencies
            .iter()
            .filter_map(|dep| {
                if dep.unit.mode.is_run_custom_build() {
                    let dep_metadata = cx.get_run_build_script_metadata(&dep.unit);
                    Some((
                        dep.unit.pkg.manifest().links().unwrap().to_string(),
                        dep.unit.pkg.package_id(),
                        dep_metadata,
                    ))
                } else {
                    None
                }
            })
            .collect::<Vec<_>>();
        let library_name = unit.pkg.library().map(|t| t.crate_name());
        let pkg_descr = unit.pkg.to_string();
        let build_script_outputs = Arc::clone(&cx.build_script_outputs);
        let id = unit.pkg.package_id();
        let output_file = script_run_dir.join("output");
        let err_file = script_run_dir.join("stderr");
        let root_output_file = script_run_dir.join("root-output");
        let host_target_root = cx.files().host_dest().to_path_buf();
        let all = (
            id,
            library_name.clone(),
            pkg_descr.clone(),
            Arc::clone(&build_script_outputs),
            output_file.clone(),
            script_out_dir.clone(),
        );
        let build_scripts = cx.build_scripts.get(unit).cloned();
        let json_messages = bcx.build_config.emit_json();
        let extra_verbose = bcx.config.extra_verbose();
        let (prev_output, prev_script_out_dir) = prev_build_output(cx, unit);
        let metadata_hash = cx.get_run_build_script_metadata(unit);

        paths::create_dir_all(&script_dir)?;
        paths::create_dir_all(&script_out_dir)?;

        let nightly_features_allowed = cx.bcx.config.nightly_features_allowed;
        let extra_check_cfg = cx.bcx.config.cli_unstable().check_cfg;
        let targets: Vec<Target> = unit.pkg.targets().to_vec();
        let msrv = unit.pkg.rust_version().cloned();
        // Need a separate copy for the fresh closure.
        let targets_fresh = targets.clone();
        let msrv_fresh = msrv.clone();

        let env_profile_name = unit.profile.name.to_uppercase();
        let built_with_debuginfo = cx
            .bcx
            .unit_graph
            .get(unit)
            .and_then(|deps| deps.iter().find(|dep| dep.unit.target == unit.target))
            .map(|dep| dep.unit.profile.debuginfo.is_turned_on())
            .unwrap_or(false);

        // Prepare the unit of "dirty work" which will actually run the custom build
        // command.
        //
        // Note that this has to do some extra work just before running the command
        // to determine extra environment variables and such.
        let dirty = Work::new(move |state| {
            // Make sure that OUT_DIR exists.
            //
            // If we have an old build directory, then just move it into place,
            // otherwise create it!
            paths::create_dir_all(&script_out_dir)
                .with_context(|| "failed to create script output directory for build command")?;

            // For all our native lib dependencies, pick up their metadata to pass
            // along to this custom build command. We're also careful to augment our
            // dynamic library search path in case the build script depended on any
            // native dynamic libraries.
            if !build_plan {
                let build_script_outputs = build_script_outputs.lock().unwrap();
                for (name, dep_id, dep_metadata) in lib_deps {
                    let script_output = build_script_outputs.get(dep_metadata).ok_or_else(|| {
                        internal(format!(
                            "failed to locate build state for env vars: {}/{}",
                            dep_id, dep_metadata
                        ))
                    })?;
                    let data = &script_output.metadata;
                    for (key, value) in data.iter() {
                        cmd.env(
                            &format!("DEP_{}_{}", super::envify(&name), super::envify(key)),
                            value,
                        );
                    }
                }
                if let Some(build_scripts) = build_scripts {
                    super::add_plugin_deps(
                        &mut cmd,
                        &build_script_outputs,
                        &build_scripts,
                        &host_target_root,
                    )?;
                }
            }

            if build_plan {
                state.build_plan(invocation_name, cmd.clone(), Arc::new(Vec::new()));
                return Ok(());
            }

            // And now finally, run the build command itself!
            state.running(&cmd);
            let timestamp = paths::set_invocation_time(&script_run_dir)?;
            let prefix = format!("[{} {}] ", id.name(), id.version());
            let mut warnings_in_case_of_panic = Vec::new();
            let output = cmd
                .exec_with_streaming(
                    &mut |stdout| {
                        if let Some(warning) = stdout
                            .strip_prefix(OLD_CARGO_WARNING_SYNTAX)
                            .or(stdout.strip_prefix(NEW_CARGO_WARNING_SYNTAX))
                        {
                            warnings_in_case_of_panic.push(warning.to_owned());
                        }
                        if extra_verbose {
                            state.stdout(format!("{}{}", prefix, stdout))?;
                        }
                        Ok(())
                    },
                    &mut |stderr| {
                        if extra_verbose {
                            state.stderr(format!("{}{}", prefix, stderr))?;
                        }
                        Ok(())
                    },
                    true,
                )
                .with_context(|| {
                    let mut build_error_context =
                        format!("failed to run custom build command for `{}`", pkg_descr);

                    // If we're opting into backtraces, mention that build dependencies' backtraces can
                    // be improved by requesting debuginfo to be built, if we're not building with
                    // debuginfo already.
                    //
                    // ALLOWED: Other tools like `rustc` might read it directly
                    // through `std::env`. We should make their behavior consistent.
                    #[allow(clippy::disallowed_methods)]
                    if let Ok(show_backtraces) = std::env::var("RUST_BACKTRACE") {
                        if !built_with_debuginfo && show_backtraces != "0" {
                            build_error_context.push_str(&format!(
                                "\n\
                                note: To improve backtraces for build dependencies, set the \
                                CARGO_PROFILE_{env_profile_name}_BUILD_OVERRIDE_DEBUG=true environment \
                                variable to enable debug information generation.",
                            ));
                        }
                    }

                    build_error_context
                });

            if let Err(error) = output {
                insert_warnings_in_build_outputs(
                    build_script_outputs,
                    id,
                    metadata_hash,
                    warnings_in_case_of_panic,
                );
                return Err(error);
            }

            let output = output.unwrap();

            // After the build command has finished running, we need to be sure to
            // remember all of its output so we can later discover precisely what it
            // was, even if we don't run the build command again (due to freshness).
            //
            // This is also the location where we provide feedback into the build
            // state informing what variables were discovered via our script as
            // well.
            paths::write(&output_file, &output.stdout)?;
            // This mtime shift allows Cargo to detect if a source file was
            // modified in the middle of the build.
            paths::set_file_time_no_err(output_file, timestamp);
            paths::write(&err_file, &output.stderr)?;
            paths::write(&root_output_file, paths::path2bytes(&script_out_dir)?)?;
            let parsed_output = BuildOutput::parse(
                &output.stdout,
                library_name,
                &pkg_descr,
                &script_out_dir,
                &script_out_dir,
                extra_check_cfg,
                nightly_features_allowed,
                &targets,
                &msrv,
            )?;

            if json_messages {
                emit_build_output(state, &parsed_output, script_out_dir.as_path(), id)?;
            }
            build_script_outputs
                .lock()
                .unwrap()
                .insert(id, metadata_hash, parsed_output);
            Ok(())
        });

        // Now that we've prepared our work-to-do, we need to prepare the fresh work
        // itself to run when we actually end up just discarding what we calculated
        // above.
        let fresh = Work::new(move |state| {
            let (id, library_name, pkg_descr, build_script_outputs, output_file, script_out_dir) = all;
            let output = match prev_output {
                Some(output) => output,
                None => BuildOutput::parse_file(
                    &output_file,
                    library_name,
                    &pkg_descr,
                    &prev_script_out_dir,
                    &script_out_dir,
                    extra_check_cfg,
                    nightly_features_allowed,
                    &targets_fresh,
                    &msrv_fresh,
                )?,
            };

            if json_messages {
                emit_build_output(state, &output, script_out_dir.as_path(), id)?;
            }

            build_script_outputs
                .lock()
                .unwrap()
                .insert(id, metadata_hash, output);
            Ok(())
        });

        let mut job = if cx.bcx.build_config.build_plan {
            Job::new_dirty(Work::noop(), None)
        } else {
            fingerprint::prepare_target(cx, unit, false)?
        };
        if job.freshness().is_dirty() {
            job.before(dirty);
        } else {
            job.before(fresh);
        }
        Ok(job)
    }

    /// When a build script run fails, store only warnings and nuke other outputs,
    /// as they are likely broken.
    fn insert_warnings_in_build_outputs(
        build_script_outputs: Arc<Mutex<BuildScriptOutputs>>,
        id: PackageId,
        metadata_hash: Metadata,
        warnings: Vec<String>,
    ) {
        let build_output_with_only_warnings = BuildOutput {
            warnings,
            ..BuildOutput::default()
        };
        build_script_outputs
            .lock()
            .unwrap()
            .insert(id, metadata_hash, build_output_with_only_warnings);
    }

    impl BuildOutput {
        /// Like [`BuildOutput::parse`] but from a file path.
        pub fn parse_file(
            path: &Path,
            library_name: Option<String>,
            pkg_descr: &str,
            script_out_dir_when_generated: &Path,
            script_out_dir: &Path,
            extra_check_cfg: bool,
            nightly_features_allowed: bool,
            targets: &[Target],
            msrv: &Option<RustVersion>,
        ) -> CargoResult<BuildOutput> {
            let contents = paths::read_bytes(path)?;
            BuildOutput::parse(
                &contents,
                library_name,
                pkg_descr,
                script_out_dir_when_generated,
                script_out_dir,
                extra_check_cfg,
                nightly_features_allowed,
                targets,
                msrv,
            )
        }

        /// Parses the output instructions of a build script.
        ///
        /// * `pkg_descr` --- for error messages
        /// * `library_name` --- for determining if `RUSTC_BOOTSTRAP` should be allowed
        /// * `extra_check_cfg` --- for unstable feature [`-Zcheck-cfg`]
        ///
        /// [`-Zcheck-cfg`]: https://doc.rust-lang.org/cargo/reference/unstable.html#check-cfg
        pub fn parse(
            input: &[u8],
            // Takes String instead of InternedString so passing `unit.pkg.name()` will give a compile error.
            library_name: Option<String>,
            pkg_descr: &str,
            script_out_dir_when_generated: &Path,
            script_out_dir: &Path,
            extra_check_cfg: bool,
            nightly_features_allowed: bool,
            targets: &[Target],
            msrv: &Option<RustVersion>,
        ) -> CargoResult<BuildOutput> {
            let mut library_paths = Vec::new();
            let mut library_links = Vec::new();
            let mut linker_args = Vec::new();
            let mut cfgs = Vec::new();
            let mut check_cfgs = Vec::new();
            let mut env = Vec::new();
            let mut metadata = Vec::new();
            let mut rerun_if_changed = Vec::new();
            let mut rerun_if_env_changed = Vec::new();
            let mut warnings = Vec::new();
            let whence = format!("build script of `{}`", pkg_descr);
            // Old syntax:
            //    cargo:rustc-flags=VALUE
            //    cargo:KEY=VALUE (for other unreserved keys)
            // New syntax:
            //    cargo::rustc-flags=VALUE
            //    cargo::metadata=KEY=VALUE (for other unreserved keys)
            // Due to backwards compatibility, no new keys can be added to this old format.
            const RESERVED_PREFIXES: &[&str] = &[
                "rustc-flags=",
                "rustc-link-lib=",
                "rustc-link-search=",
                "rustc-link-arg-cdylib=",
                "rustc-cdylib-link-arg=",
                "rustc-link-arg-bins=",
                "rustc-link-arg-bin=",
                "rustc-link-arg-tests=",
                "rustc-link-arg-benches=",
                "rustc-link-arg-examples=",
                "rustc-link-arg=",
                "rustc-cfg=",
                "rustc-check-cfg=",
                "rustc-env=",
                "warning=",
                "rerun-if-changed=",
                "rerun-if-env-changed=",
            ];
            const DOCS_LINK_SUGGESTION: &str = "See https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script \
                    for more information about build script outputs.";

            fn check_minimum_supported_rust_version_for_new_syntax(
                pkg_descr: &str,
                msrv: &Option<RustVersion>,
            ) -> CargoResult<()> {
                let new_syntax_added_in = &RustVersion::from_str("1.77.0")?;

                if let Some(msrv) = msrv {
                    if msrv < new_syntax_added_in {
                        bail!(
                            "the `cargo::` syntax for build script output instructions was added in \
                            Rust 1.77.0, but the minimum supported Rust version of `{pkg_descr}` is {msrv}.\n\
                            {DOCS_LINK_SUGGESTION}"
                        );
                    }
                }

                Ok(())
            }

            fn parse_directive<'a>(
                whence: &str,
                line: &str,
                data: &'a str,
                old_syntax: bool,
            ) -> CargoResult<(&'a str, &'a str)> {
                let mut iter = data.splitn(2, "=");
                let key = iter.next();
                let value = iter.next();
                match (key, value) {
                    (Some(a), Some(b)) => Ok((a, b.trim_end())),
                    _ => bail!(
                        "invalid output in {whence}: `{line}`\n\
                        Expected a line with `{syntax}KEY=VALUE` with an `=` character, \
                        but none was found.\n\
                        {DOCS_LINK_SUGGESTION}",
                        syntax = if old_syntax { "cargo:" } else { "cargo::" },
                    ),
                }
            }

            fn parse_metadata<'a>(
                whence: &str,
                line: &str,
                data: &'a str,
                old_syntax: bool,
            ) -> CargoResult<(&'a str, &'a str)> {
                let mut iter = data.splitn(2, "=");
                let key = iter.next();
                let value = iter.next();
                match (key, value) {
                    (Some(a), Some(b)) => Ok((a, b.trim_end())),
                    _ => bail!(
                        "invalid output in {whence}: `{line}`\n\
                        Expected a line with `{syntax}KEY=VALUE` with an `=` character, \
                        but none was found.\n\
                        {DOCS_LINK_SUGGESTION}",
                        syntax = if old_syntax {
                            "cargo:"
                        } else {
                            "cargo::metadata="
                        },
                    ),
                }
            }

            for line in input.split(|b| *b == b'\n') {
                let line = match str::from_utf8(line) {
                    Ok(line) => line.trim(),
                    Err(..) => continue,
                };
                let mut old_syntax = false;
                let (key, value) = if let Some(data) = line.strip_prefix("cargo::") {
                    check_minimum_supported_rust_version_for_new_syntax(pkg_descr, msrv)?;
                    // For instance, `cargo::rustc-flags=foo` or `cargo::metadata=foo=bar`.
                    parse_directive(whence.as_str(), line, data, old_syntax)?
                } else if let Some(data) = line.strip_prefix("cargo:") {
                    old_syntax = true;
                    // For instance, `cargo:rustc-flags=foo`.
                    if RESERVED_PREFIXES
                        .iter()
                        .any(|prefix| data.starts_with(prefix))
                    {
                        parse_directive(whence.as_str(), line, data, old_syntax)?
                    } else {
                        // For instance, `cargo:foo=bar`.
                        ("metadata", data)
                    }
                } else {
                    // Skip this line since it doesn't start with "cargo:" or "cargo::".
                    continue;
                };
                // This will rewrite paths if the target directory has been moved.
                let value = value.replace(
                    script_out_dir_when_generated.to_str().unwrap(),
                    script_out_dir.to_str().unwrap(),
                );

                let syntax_prefix = if old_syntax { "cargo:" } else { "cargo::" };
                macro_rules! check_and_add_target {
                    ($target_kind: expr, $is_target_kind: expr, $link_type: expr) => {
                        if !targets.iter().any(|target| $is_target_kind(target)) {
                            bail!(
                                "invalid instruction `{}{}` from {}\n\
                                    The package {} does not have a {} target.",
                                syntax_prefix,
                                key,
                                whence,
                                pkg_descr,
                                $target_kind
                            );
                        }
                        linker_args.push(($link_type, value));
                    };
                }

                // Keep in sync with TargetConfig::parse_links_overrides.
                match key {
                    "rustc-flags" => {
                        let (paths, links) = BuildOutput::parse_rustc_flags(&value, &whence)?;
                        library_links.extend(links.into_iter());
                        library_paths.extend(paths.into_iter());
                    }
                    "rustc-link-lib" => library_links.push(value.to_string()),
                    "rustc-link-search" => library_paths.push(PathBuf::from(value)),
                    "rustc-link-arg-cdylib" | "rustc-cdylib-link-arg" => {
                        if !targets.iter().any(|target| target.is_cdylib()) {
                            warnings.push(format!(
                                "{}{} was specified in the build script of {}, \
                                 but that package does not contain a cdylib target\n\
                                 \n\
                                 Allowing this was an unintended change in the 1.50 \
                                 release, and may become an error in the future. \
                                 For more information, see \
                                 <https://github.com/rust-lang/cargo/issues/9562>.",
                                syntax_prefix, key, pkg_descr
                            ));
                        }
                        linker_args.push((LinkArgTarget::Cdylib, value))
                    }
                    "rustc-link-arg-bins" => {
                        check_and_add_target!("bin", Target::is_bin, LinkArgTarget::Bin);
                    }
                    "rustc-link-arg-bin" => {
                        let (bin_name, arg) = value.split_once('=').ok_or_else(|| {
                            anyhow::format_err!(
                                "invalid instruction `{}{}={}` from {}\n\
                                    The instruction should have the form {}{}=BIN=ARG",
                                syntax_prefix,
                                key,
                                value,
                                whence,
                                syntax_prefix,
                                key
                            )
                        })?;
                        if !targets
                            .iter()
                            .any(|target| target.is_bin() && target.name() == bin_name)
                        {
                            bail!(
                                "invalid instruction `{}{}` from {}\n\
                                    The package {} does not have a bin target with the name `{}`.",
                                syntax_prefix,
                                key,
                                whence,
                                pkg_descr,
                                bin_name
                            );
                        }
                        linker_args.push((
                            LinkArgTarget::SingleBin(bin_name.to_owned()),
                            arg.to_string(),
                        ));
                    }
                    "rustc-link-arg-tests" => {
                        check_and_add_target!("test", Target::is_test, LinkArgTarget::Test);
                    }
                    "rustc-link-arg-benches" => {
                        check_and_add_target!("benchmark", Target::is_bench, LinkArgTarget::Bench);
                    }
                    "rustc-link-arg-examples" => {
                        check_and_add_target!("example", Target::is_example, LinkArgTarget::Example);
                    }
                    "rustc-link-arg" => {
                        linker_args.push((LinkArgTarget::All, value));
                    }
                    "rustc-cfg" => cfgs.push(value.to_string()),
                    "rustc-check-cfg" => {
                        if extra_check_cfg {
                            check_cfgs.push(value.to_string());
                        } else {
                            // silently ignoring the instruction to try to
                            // minimise MSRV annoyance when stabilizing -Zcheck-cfg
                        }
                    }
                    "rustc-env" => {
                        let (key, val) = BuildOutput::parse_rustc_env(&value, &whence)?;
                        // Build scripts aren't allowed to set RUSTC_BOOTSTRAP.
                        // See https://github.com/rust-lang/cargo/issues/7088.
                        if key == "RUSTC_BOOTSTRAP" {
                            // If RUSTC_BOOTSTRAP is already set, the user of Cargo knows about
                            // bootstrap and still wants to override the channel. Give them a way to do
                            // so, but still emit a warning that the current crate shouldn't be trying
                            // to set RUSTC_BOOTSTRAP.
                            // If this is a nightly build, setting RUSTC_BOOTSTRAP wouldn't affect the
                            // behavior, so still only give a warning.
                            // NOTE: cargo only allows nightly features on RUSTC_BOOTSTRAP=1, but we
                            // want setting any value of RUSTC_BOOTSTRAP to downgrade this to a warning
                            // (so that `RUSTC_BOOTSTRAP=library_name` will work)
                            let rustc_bootstrap_allows = |name: Option<&str>| {
                                let name = match name {
                                    // as of 2021, no binaries on crates.io use RUSTC_BOOTSTRAP, so
                                    // fine-grained opt-outs aren't needed. end-users can always use
                                    // RUSTC_BOOTSTRAP=1 from the top-level if it's really a problem.
                                    None => return false,
                                    Some(n) => n,
                                };
                                // 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)]
                                std::env::var("RUSTC_BOOTSTRAP")
                                    .map_or(false, |var| var.split(',').any(|s| s == name))
                            };
                            if nightly_features_allowed
                                || rustc_bootstrap_allows(library_name.as_deref())
                            {
                                warnings.push(format!("Cannot set `RUSTC_BOOTSTRAP={}` from {}.\n\
                                    note: Crates cannot set `RUSTC_BOOTSTRAP` themselves, as doing so would subvert the stability guarantees of Rust for your project.",
                                    val, whence
                                ));
                            } else {
                                // Setting RUSTC_BOOTSTRAP would change the behavior of the crate.
                                // Abort with an error.
                                bail!("Cannot set `RUSTC_BOOTSTRAP={}` from {}.\n\
                                    note: Crates cannot set `RUSTC_BOOTSTRAP` themselves, as doing so would subvert the stability guarantees of Rust for your project.\n\
                                    help: If you're sure you want to do this in your project, set the environment variable `RUSTC_BOOTSTRAP={}` before running cargo instead.",
                                    val,
                                    whence,
                                    library_name.as_deref().unwrap_or("1"),
                                );
                            }
                        } else {
                            env.push((key, val));
                        }
                    }
                    "warning" => warnings.push(value.to_string()),
                    "rerun-if-changed" => rerun_if_changed.push(PathBuf::from(value)),
                    "rerun-if-env-changed" => rerun_if_env_changed.push(value.to_string()),
                    "metadata" => {
                        let (key, value) = parse_metadata(whence.as_str(), line, &value, old_syntax)?;
                        metadata.push((key.to_owned(), value.to_owned()));
                    }
                    _ => bail!(
                        "invalid output in {whence}: `{line}`\n\
                        Unknown key: `{key}`.\n\
                        {DOCS_LINK_SUGGESTION}",
                    ),
                }
            }

            Ok(BuildOutput {
                library_paths,
                library_links,
                linker_args,
                cfgs,
                check_cfgs,
                env,
                metadata,
                rerun_if_changed,
                rerun_if_env_changed,
                warnings,
            })
        }

        /// Parses [`cargo::rustc-flags`] instruction.
        ///
        /// [`cargo::rustc-flags`]: https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#cargorustc-flagsflags
        pub fn parse_rustc_flags(
            value: &str,
            whence: &str,
        ) -> CargoResult<(Vec<PathBuf>, Vec<String>)> {
            let value = value.trim();
            let mut flags_iter = value
                .split(|c: char| c.is_whitespace())
                .filter(|w| w.chars().any(|c| !c.is_whitespace()));
            let (mut library_paths, mut library_links) = (Vec::new(), Vec::new());

            while let Some(flag) = flags_iter.next() {
                if flag.starts_with("-l") || flag.starts_with("-L") {
                    // Check if this flag has no space before the value as is
                    // common with tools like pkg-config
                    // e.g. -L/some/dir/local/lib or -licui18n
                    let (flag, mut value) = flag.split_at(2);
                    if value.is_empty() {
                        value = match flags_iter.next() {
                            Some(v) => v,
                            None => bail! {
                                "Flag in rustc-flags has no value in {}: {}",
                                whence,
                                value
                            },
                        }
                    }

                    match flag {
                        "-l" => library_links.push(value.to_string()),
                        "-L" => library_paths.push(PathBuf::from(value)),

                        // This was already checked above
                        _ => unreachable!(),
                    };
                } else {
                    bail!(
                        "Only `-l` and `-L` flags are allowed in {}: `{}`",
                        whence,
                        value
                    )
                }
            }
            Ok((library_paths, library_links))
        }

        /// Parses [`cargo::rustc-env`] instruction.
        ///
        /// [`cargo::rustc-env`]: https://doc.rust-lang.org/nightly/cargo/reference/build-scripts.html#rustc-env
        pub fn parse_rustc_env(value: &str, whence: &str) -> CargoResult<(String, String)> {
            match value.split_once('=') {
                Some((n, v)) => Ok((n.to_owned(), v.to_owned())),
                _ => bail!("Variable rustc-env has no value in {whence}: {value}"),
            }
        }
    }

    /// Prepares the Rust script for the unstable feature [metabuild].
    ///
    /// [metabuild]: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#metabuild
    fn prepare_metabuild(cx: &Context<'_, '_>, unit: &Unit, deps: &[String]) -> CargoResult<()> {
        let mut output = Vec::new();
        let available_deps = cx.unit_deps(unit);
        // Filter out optional dependencies, and look up the actual lib name.
        let meta_deps: Vec<_> = deps
            .iter()
            .filter_map(|name| {
                available_deps
                    .iter()
                    .find(|d| d.unit.pkg.name().as_str() == name.as_str())
                    .map(|d| d.unit.target.crate_name())
            })
            .collect();
        output.push("fn main() {\n".to_string());
        for dep in &meta_deps {
            output.push(format!("    {}::metabuild();\n", dep));
        }
        output.push("}\n".to_string());
        let output = output.join("");
        let path = unit.pkg.manifest().metabuild_path(cx.bcx.ws.target_dir());
        paths::create_dir_all(path.parent().unwrap())?;
        paths::write_if_changed(path, &output)?;
        Ok(())
    }

    impl BuildDeps {
        /// Creates a build script dependency information from a previous
        /// build script output path and the content.
        pub fn new(output_file: &Path, output: Option<&BuildOutput>) -> BuildDeps {
            BuildDeps {
                build_script_output: output_file.to_path_buf(),
                rerun_if_changed: output
                    .map(|p| &p.rerun_if_changed)
                    .cloned()
                    .unwrap_or_default(),
                rerun_if_env_changed: output
                    .map(|p| &p.rerun_if_env_changed)
                    .cloned()
                    .unwrap_or_default(),
            }
        }
    }

    /// Computes several maps in [`Context`].
    ///
    /// - [`build_scripts`]: A map that tracks which build scripts each package
    ///   depends on.
    /// - [`build_explicit_deps`]: Dependency statements emitted by build scripts
    ///   from a previous run.
    /// - [`build_script_outputs`]: Pre-populates this with any overridden build
    ///   scripts.
    ///
    /// The important one here is [`build_scripts`], which for each `(package,
    /// metadata)` stores a [`BuildScripts`] object which contains a list of
    /// dependencies with build scripts that the unit should consider when linking.
    /// For example this lists all dependencies' `-L` flags which need to be
    /// propagated transitively.
    ///
    /// The given set of units to this function is the initial set of
    /// targets/profiles which are being built.
    ///
    /// [`build_scripts`]: Context::build_scripts
    /// [`build_explicit_deps`]: Context::build_explicit_deps
    /// [`build_script_outputs`]: Context::build_script_outputs
    pub fn build_map(cx: &mut Context<'_, '_>) -> CargoResult<()> {
        let mut ret = HashMap::new();
        for unit in &cx.bcx.roots {
            build(&mut ret, cx, unit)?;
        }
        cx.build_scripts
            .extend(ret.into_iter().map(|(k, v)| (k, Arc::new(v))));
        return Ok(());

        // Recursive function to build up the map we're constructing. This function
        // memoizes all of its return values as it goes along.
        fn build<'a>(
            out: &'a mut HashMap<Unit, BuildScripts>,
            cx: &mut Context<'_, '_>,
            unit: &Unit,
        ) -> CargoResult<&'a BuildScripts> {
            // Do a quick pre-flight check to see if we've already calculated the
            // set of dependencies.
            if out.contains_key(unit) {
                return Ok(&out[unit]);
            }

            // If there is a build script override, pre-fill the build output.
            if unit.mode.is_run_custom_build() {
                if let Some(links) = unit.pkg.manifest().links() {
                    if let Some(output) = cx.bcx.target_data.script_override(links, unit.kind) {
                        let metadata = cx.get_run_build_script_metadata(unit);
                        cx.build_script_outputs.lock().unwrap().insert(
                            unit.pkg.package_id(),
                            metadata,
                            output.clone(),
                        );
                    }
                }
            }

            let mut ret = BuildScripts::default();

            // If a package has a build script, add itself as something to inspect for linking.
            if !unit.target.is_custom_build() && unit.pkg.has_custom_build() {
                let script_meta = cx
                    .find_build_script_metadata(unit)
                    .expect("has_custom_build should have RunCustomBuild");
                add_to_link(&mut ret, unit.pkg.package_id(), script_meta);
            }

            if unit.mode.is_run_custom_build() {
                parse_previous_explicit_deps(cx, unit);
            }

            // We want to invoke the compiler deterministically to be cache-friendly
            // to rustc invocation caching schemes, so be sure to generate the same
            // set of build script dependency orderings via sorting the targets that
            // come out of the `Context`.
            let mut dependencies: Vec<Unit> =
                cx.unit_deps(unit).iter().map(|d| d.unit.clone()).collect();
            dependencies.sort_by_key(|u| u.pkg.package_id());

            for dep_unit in dependencies.iter() {
                let dep_scripts = build(out, cx, dep_unit)?;

                if dep_unit.target.for_host() {
                    ret.plugins.extend(dep_scripts.to_link.iter().cloned());
                } else if dep_unit.target.is_linkable() {
                    for &(pkg, metadata) in dep_scripts.to_link.iter() {
                        add_to_link(&mut ret, pkg, metadata);
                    }
                }
            }

            match out.entry(unit.clone()) {
                Entry::Vacant(entry) => Ok(entry.insert(ret)),
                Entry::Occupied(_) => panic!("cyclic dependencies in `build_map`"),
            }
        }

        // When adding an entry to 'to_link' we only actually push it on if the
        // script hasn't seen it yet (e.g., we don't push on duplicates).
        fn add_to_link(scripts: &mut BuildScripts, pkg: PackageId, metadata: Metadata) {
            if scripts.seen_to_link.insert((pkg, metadata)) {
                scripts.to_link.push((pkg, metadata));
            }
        }

        /// Load any dependency declarations from a previous build script run.
        fn parse_previous_explicit_deps(cx: &mut Context<'_, '_>, unit: &Unit) {
            let script_run_dir = cx.files().build_script_run_dir(unit);
            let output_file = script_run_dir.join("output");
            let (prev_output, _) = prev_build_output(cx, unit);
            let deps = BuildDeps::new(&output_file, prev_output.as_ref());
            cx.build_explicit_deps.insert(unit.clone(), deps);
        }
    }

    /// Returns the previous parsed `BuildOutput`, if any, from a previous
    /// execution.
    ///
    /// Also returns the directory containing the output, typically used later in
    /// processing.
    fn prev_build_output(cx: &mut Context<'_, '_>, unit: &Unit) -> (Option<BuildOutput>, PathBuf) {
        let script_out_dir = cx.files().build_script_out_dir(unit);
        let script_run_dir = cx.files().build_script_run_dir(unit);
        let root_output_file = script_run_dir.join("root-output");
        let output_file = script_run_dir.join("output");

        let prev_script_out_dir = paths::read_bytes(&root_output_file)
            .and_then(|bytes| paths::bytes2path(&bytes))
            .unwrap_or_else(|_| script_out_dir.clone());

        (
            BuildOutput::parse_file(
                &output_file,
                unit.pkg.library().map(|t| t.crate_name()),
                &unit.pkg.to_string(),
                &prev_script_out_dir,
                &script_out_dir,
                cx.bcx.config.cli_unstable().check_cfg,
                cx.bcx.config.nightly_features_allowed,
                unit.pkg.targets(),
                &unit.pkg.rust_version().cloned(),
            )
            .ok(),
            prev_script_out_dir,
        )
    }

    impl BuildScriptOutputs {
        /// Inserts a new entry into the map.
        fn insert(&mut self, pkg_id: PackageId, metadata: Metadata, parsed_output: BuildOutput) {
            match self.outputs.entry(metadata) {
                Entry::Vacant(entry) => {
                    entry.insert(parsed_output);
                }
                Entry::Occupied(entry) => panic!(
                    "build script output collision for {}/{}\n\
                    old={:?}\nnew={:?}",
                    pkg_id,
                    metadata,
                    entry.get(),
                    parsed_output
                ),
            }
        }

        /// Returns `true` if the given key already exists.
        fn contains_key(&self, metadata: Metadata) -> bool {
            self.outputs.contains_key(&metadata)
        }

        /// Gets the build output for the given key.
        pub fn get(&self, meta: Metadata) -> Option<&BuildOutput> {
            self.outputs.get(&meta)
        }

        /// Returns an iterator over all entries.
        pub fn iter(&self) -> impl Iterator<Item = (&Metadata, &BuildOutput)> {
            self.outputs.iter()
        }
    }