#
cargo/core/compiler/build_runner/
compilation_files.rs

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    //! See [`CompilationFiles`].

    use std::collections::HashMap;
    use std::fmt;
    use std::hash::{Hash, Hasher};
    use std::path::{Path, PathBuf};
    use std::sync::Arc;

    use lazycell::LazyCell;
    use tracing::debug;

    use super::{BuildContext, BuildRunner, CompileKind, FileFlavor, Layout};
    use crate::core::compiler::{CompileMode, CompileTarget, CrateType, FileType, Unit};
    use crate::core::{Target, TargetKind, Workspace};
    use crate::util::{self, CargoResult, StableHasher};

    /// This is a generic version number that can be changed to make
    /// backwards-incompatible changes to any file structures in the output
    /// directory. For example, the fingerprint files or the build-script
    /// output files.
    ///
    /// Normally cargo updates ship with rustc updates which will
    /// cause a new hash due to the rustc version changing, but this allows
    /// cargo to be extra careful to deal with different versions of cargo that
    /// use the same rustc version.
    const METADATA_VERSION: u8 = 2;

    /// Uniquely identify a [`Unit`] under specific circumstances, see [`Metadata`] for more.
    #[derive(Copy, Clone, Hash, Eq, PartialEq, Ord, PartialOrd)]
    pub struct UnitHash(u64);

    impl fmt::Display for UnitHash {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(f, "{:016x}", self.0)
        }
    }

    impl fmt::Debug for UnitHash {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(f, "UnitHash({:016x})", self.0)
        }
    }

    /// [`Metadata`] tracks several [`UnitHash`]s, including
    /// [`Metadata::unit_id`], [`Metadata::c_metadata`], and [`Metadata::c_extra_filename`].
    ///
    /// We use a hash because it is an easy way to guarantee
    /// that all the inputs can be converted to a valid path.
    ///
    /// [`Metadata::unit_id`] is used to uniquely identify a unit in the build graph.
    /// This serves as a similar role as [`Metadata::c_extra_filename`] in that it uniquely identifies output
    /// on the filesystem except that its always present.
    ///
    /// [`Metadata::c_extra_filename`] is needed for cases like:
    /// - A project may depend on crate `A` and crate `B`, so the package name must be in the file name.
    /// - Similarly a project may depend on two versions of `A`, so the version must be in the file name.
    ///
    /// This also acts as the main layer of caching provided by Cargo
    /// so this must include all things that need to be distinguished in different parts of
    /// the same build. This is absolutely required or we override things before
    /// we get chance to use them.
    ///
    /// For example, we want to cache `cargo build` and `cargo doc` separately, so that running one
    /// does not invalidate the artifacts for the other. We do this by including [`CompileMode`] in the
    /// hash, thus the artifacts go in different folders and do not override each other.
    /// If we don't add something that we should have, for this reason, we get the
    /// correct output but rebuild more than is needed.
    ///
    /// Some things that need to be tracked to ensure the correct output should definitely *not*
    /// go in the `Metadata`. For example, the modification time of a file, should be tracked to make a
    /// rebuild when the file changes. However, it would be wasteful to include in the `Metadata`. The
    /// old artifacts are never going to be needed again. We can save space by just overwriting them.
    /// If we add something that we should not have, for this reason, we get the correct output but take
    /// more space than needed. This makes not including something in `Metadata`
    /// a form of cache invalidation.
    ///
    /// Note that the `Fingerprint` is in charge of tracking everything needed to determine if a
    /// rebuild is needed.
    ///
    /// [`Metadata::c_metadata`] is used for symbol mangling, because if you have two versions of
    /// the same crate linked together, their symbols need to be differentiated.
    ///
    /// You should avoid anything that would interfere with reproducible
    /// builds. For example, *any* absolute path should be avoided. This is one
    /// reason that `RUSTFLAGS` is not in [`Metadata::c_metadata`], because it often has
    /// absolute paths (like `--remap-path-prefix` which is fundamentally used for
    /// reproducible builds and has absolute paths in it). Also, in some cases the
    /// mangled symbols need to be stable between different builds with different
    /// settings. For example, profile-guided optimizations need to swap
    /// `RUSTFLAGS` between runs, but needs to keep the same symbol names.
    #[derive(Copy, Clone, Debug)]
    pub struct Metadata {
        unit_id: UnitHash,
        c_metadata: UnitHash,
        c_extra_filename: Option<UnitHash>,
    }

    impl Metadata {
        /// A hash to identify a given [`Unit`] in the build graph
        pub fn unit_id(&self) -> UnitHash {
            self.unit_id
        }

        /// A hash to add to symbol naming through `-C metadata`
        pub fn c_metadata(&self) -> UnitHash {
            self.c_metadata
        }

        /// A hash to add to file names through `-C extra-filename`
        pub fn c_extra_filename(&self) -> Option<UnitHash> {
            self.c_extra_filename
        }
    }

    /// Collection of information about the files emitted by the compiler, and the
    /// output directory structure.
    pub struct CompilationFiles<'a, 'gctx> {
        /// The target directory layout for the host (and target if it is the same as host).
        pub(super) host: Layout,
        /// The target directory layout for the target (if different from then host).
        pub(super) target: HashMap<CompileTarget, Layout>,
        /// Additional directory to include a copy of the outputs.
        export_dir: Option<PathBuf>,
        /// The root targets requested by the user on the command line (does not
        /// include dependencies).
        roots: Vec<Unit>,
        ws: &'a Workspace<'gctx>,
        /// Metadata hash to use for each unit.
        metas: HashMap<Unit, Metadata>,
        /// For each Unit, a list all files produced.
        outputs: HashMap<Unit, LazyCell<Arc<Vec<OutputFile>>>>,
    }

    /// Info about a single file emitted by the compiler.
    #[derive(Debug)]
    pub struct OutputFile {
        /// Absolute path to the file that will be produced by the build process.
        pub path: PathBuf,
        /// If it should be linked into `target`, and what it should be called
        /// (e.g., without metadata).
        pub hardlink: Option<PathBuf>,
        /// If `--artifact-dir` is specified, the absolute path to the exported file.
        pub export_path: Option<PathBuf>,
        /// Type of the file (library / debug symbol / else).
        pub flavor: FileFlavor,
    }

    impl OutputFile {
        /// Gets the hard link if present; otherwise, returns the path.
        pub fn bin_dst(&self) -> &PathBuf {
            match self.hardlink {
                Some(ref link_dst) => link_dst,
                None => &self.path,
            }
        }
    }

    impl<'a, 'gctx: 'a> CompilationFiles<'a, 'gctx> {
        pub(super) fn new(
            build_runner: &BuildRunner<'a, 'gctx>,
            host: Layout,
            target: HashMap<CompileTarget, Layout>,
        ) -> CompilationFiles<'a, 'gctx> {
            let mut metas = HashMap::new();
            for unit in &build_runner.bcx.roots {
                metadata_of(unit, build_runner, &mut metas);
            }
            let outputs = metas
                .keys()
                .cloned()
                .map(|unit| (unit, LazyCell::new()))
                .collect();
            CompilationFiles {
                ws: build_runner.bcx.ws,
                host,
                target,
                export_dir: build_runner.bcx.build_config.export_dir.clone(),
                roots: build_runner.bcx.roots.clone(),
                metas,
                outputs,
            }
        }

        /// Returns the appropriate directory layout for either a plugin or not.
        pub fn layout(&self, kind: CompileKind) -> &Layout {
            match kind {
                CompileKind::Host => &self.host,
                CompileKind::Target(target) => &self.target[&target],
            }
        }

        /// Gets the metadata for the given unit.
        ///
        /// See [`Metadata`] and [`fingerprint`] module for more.
        ///
        /// [`fingerprint`]: super::super::fingerprint#fingerprints-and-metadata
        pub fn metadata(&self, unit: &Unit) -> Metadata {
            self.metas[unit]
        }

        /// Gets the short hash based only on the `PackageId`.
        /// Used for the metadata when `metadata` returns `None`.
        pub fn target_short_hash(&self, unit: &Unit) -> String {
            let hashable = unit.pkg.package_id().stable_hash(self.ws.root());
            util::short_hash(&(METADATA_VERSION, hashable))
        }

        /// Returns the directory where the artifacts for the given unit are
        /// initially created.
        pub fn out_dir(&self, unit: &Unit) -> PathBuf {
            // Docscrape units need to have doc/ set as the out_dir so sources for reverse-dependencies
            // will be put into doc/ and not into deps/ where the *.examples files are stored.
            if unit.mode.is_doc() || unit.mode.is_doc_scrape() {
                self.layout(unit.kind).doc().to_path_buf()
            } else if unit.mode.is_doc_test() {
                panic!("doc tests do not have an out dir");
            } else if unit.target.is_custom_build() {
                self.build_script_dir(unit)
            } else if unit.target.is_example() {
                self.layout(unit.kind).examples().to_path_buf()
            } else if unit.artifact.is_true() {
                self.artifact_dir(unit)
            } else {
                self.deps_dir(unit).to_path_buf()
            }
        }

        /// Additional export directory from `--artifact-dir`.
        pub fn export_dir(&self) -> Option<PathBuf> {
            self.export_dir.clone()
        }

        /// Directory name to use for a package in the form `NAME-HASH`.
        ///
        /// Note that some units may share the same directory, so care should be
        /// taken in those cases!
        fn pkg_dir(&self, unit: &Unit) -> String {
            let name = unit.pkg.package_id().name();
            let meta = self.metas[unit];
            if let Some(c_extra_filename) = meta.c_extra_filename() {
                format!("{}-{}", name, c_extra_filename)
            } else {
                format!("{}-{}", name, self.target_short_hash(unit))
            }
        }

        /// Returns the final artifact path for the host (`/…/target/debug`)
        pub fn host_dest(&self) -> &Path {
            self.host.dest()
        }

        /// Returns the root of the build output tree for the host (`/…/target`)
        pub fn host_root(&self) -> &Path {
            self.host.root()
        }

        /// Returns the host `deps` directory path.
        pub fn host_deps(&self) -> &Path {
            self.host.deps()
        }

        /// Returns the directories where Rust crate dependencies are found for the
        /// specified unit.
        pub fn deps_dir(&self, unit: &Unit) -> &Path {
            self.layout(unit.kind).deps()
        }

        /// Directory where the fingerprint for the given unit should go.
        pub fn fingerprint_dir(&self, unit: &Unit) -> PathBuf {
            let dir = self.pkg_dir(unit);
            self.layout(unit.kind).fingerprint().join(dir)
        }

        /// Returns the path for a file in the fingerprint directory.
        ///
        /// The "prefix" should be something to distinguish the file from other
        /// files in the fingerprint directory.
        pub fn fingerprint_file_path(&self, unit: &Unit, prefix: &str) -> PathBuf {
            // Different targets need to be distinguished in the
            let kind = unit.target.kind().description();
            let flavor = if unit.mode.is_any_test() {
                "test-"
            } else if unit.mode.is_doc() {
                "doc-"
            } else if unit.mode.is_run_custom_build() {
                "run-"
            } else {
                ""
            };
            let name = format!("{}{}{}-{}", prefix, flavor, kind, unit.target.name());
            self.fingerprint_dir(unit).join(name)
        }

        /// Path where compiler output is cached.
        pub fn message_cache_path(&self, unit: &Unit) -> PathBuf {
            self.fingerprint_file_path(unit, "output-")
        }

        /// Returns the directory where a compiled build script is stored.
        /// `/path/to/target/{debug,release}/build/PKG-HASH`
        pub fn build_script_dir(&self, unit: &Unit) -> PathBuf {
            assert!(unit.target.is_custom_build());
            assert!(!unit.mode.is_run_custom_build());
            assert!(self.metas.contains_key(unit));
            let dir = self.pkg_dir(unit);
            self.layout(CompileKind::Host).build().join(dir)
        }

        /// Returns the directory for compiled artifacts files.
        /// `/path/to/target/{debug,release}/deps/artifact/KIND/PKG-HASH`
        fn artifact_dir(&self, unit: &Unit) -> PathBuf {
            assert!(self.metas.contains_key(unit));
            assert!(unit.artifact.is_true());
            let dir = self.pkg_dir(unit);
            let kind = match unit.target.kind() {
                TargetKind::Bin => "bin",
                TargetKind::Lib(lib_kinds) => match lib_kinds.as_slice() {
                    &[CrateType::Cdylib] => "cdylib",
                    &[CrateType::Staticlib] => "staticlib",
                    invalid => unreachable!(
                        "BUG: unexpected artifact library type(s): {:?} - these should have been split",
                        invalid
                    ),
                },
                invalid => unreachable!(
                    "BUG: {:?} are not supposed to be used as artifacts",
                    invalid
                ),
            };
            self.layout(unit.kind).artifact().join(dir).join(kind)
        }

        /// Returns the directory where information about running a build script
        /// is stored.
        /// `/path/to/target/{debug,release}/build/PKG-HASH`
        pub fn build_script_run_dir(&self, unit: &Unit) -> PathBuf {
            assert!(unit.target.is_custom_build());
            assert!(unit.mode.is_run_custom_build());
            let dir = self.pkg_dir(unit);
            self.layout(unit.kind).build().join(dir)
        }

        /// Returns the "`OUT_DIR`" directory for running a build script.
        /// `/path/to/target/{debug,release}/build/PKG-HASH/out`
        pub fn build_script_out_dir(&self, unit: &Unit) -> PathBuf {
            self.build_script_run_dir(unit).join("out")
        }

        /// Returns the path to the executable binary for the given bin target.
        ///
        /// This should only to be used when a `Unit` is not available.
        pub fn bin_link_for_target(
            &self,
            target: &Target,
            kind: CompileKind,
            bcx: &BuildContext<'_, '_>,
        ) -> CargoResult<PathBuf> {
            assert!(target.is_bin());
            let dest = self.layout(kind).dest();
            let info = bcx.target_data.info(kind);
            let (file_types, _) = info
                .rustc_outputs(
                    CompileMode::Build,
                    &TargetKind::Bin,
                    bcx.target_data.short_name(&kind),
                )
                .expect("target must support `bin`");

            let file_type = file_types
                .iter()
                .find(|file_type| file_type.flavor == FileFlavor::Normal)
                .expect("target must support `bin`");

            Ok(dest.join(file_type.uplift_filename(target)))
        }

        /// Returns the filenames that the given unit will generate.
        ///
        /// Note: It is not guaranteed that all of the files will be generated.
        pub(super) fn outputs(
            &self,
            unit: &Unit,
            bcx: &BuildContext<'a, 'gctx>,
        ) -> CargoResult<Arc<Vec<OutputFile>>> {
            self.outputs[unit]
                .try_borrow_with(|| self.calc_outputs(unit, bcx))
                .map(Arc::clone)
        }

        /// Returns the path where the output for the given unit and `FileType`
        /// should be uplifted to.
        ///
        /// Returns `None` if the unit shouldn't be uplifted (for example, a
        /// dependent rlib).
        fn uplift_to(&self, unit: &Unit, file_type: &FileType, from_path: &Path) -> Option<PathBuf> {
            // Tests, check, doc, etc. should not be uplifted.
            if unit.mode != CompileMode::Build || file_type.flavor == FileFlavor::Rmeta {
                return None;
            }

            // Artifact dependencies are never uplifted.
            if unit.artifact.is_true() {
                return None;
            }

            // - Binaries: The user always wants to see these, even if they are
            //   implicitly built (for example for integration tests).
            // - dylibs: This ensures that the dynamic linker pulls in all the
            //   latest copies (even if the dylib was built from a previous cargo
            //   build). There are complex reasons for this, see #8139, #6167, #6162.
            // - Things directly requested from the command-line (the "roots").
            //   This one is a little questionable for rlibs (see #6131), but is
            //   historically how Cargo has operated. This is primarily useful to
            //   give the user access to staticlibs and cdylibs.
            if !unit.target.is_bin()
                && !unit.target.is_custom_build()
                && file_type.crate_type != Some(CrateType::Dylib)
                && !self.roots.contains(unit)
            {
                return None;
            }

            let filename = file_type.uplift_filename(&unit.target);
            let uplift_path = if unit.target.is_example() {
                // Examples live in their own little world.
                self.layout(unit.kind).examples().join(filename)
            } else if unit.target.is_custom_build() {
                self.build_script_dir(unit).join(filename)
            } else {
                self.layout(unit.kind).dest().join(filename)
            };
            if from_path == uplift_path {
                // This can happen with things like examples that reside in the
                // same directory, do not have a metadata hash (like on Windows),
                // and do not have hyphens.
                return None;
            }
            Some(uplift_path)
        }

        /// Calculates the filenames that the given unit will generate.
        /// Should use [`CompilationFiles::outputs`] instead
        /// as it caches the result of this function.
        fn calc_outputs(
            &self,
            unit: &Unit,
            bcx: &BuildContext<'a, 'gctx>,
        ) -> CargoResult<Arc<Vec<OutputFile>>> {
            let ret = match unit.mode {
                CompileMode::Doc { json, .. } => {
                    let path = if json {
                        self.out_dir(unit)
                            .join(format!("{}.json", unit.target.crate_name()))
                    } else {
                        self.out_dir(unit)
                            .join(unit.target.crate_name())
                            .join("index.html")
                    };

                    vec![OutputFile {
                        path,
                        hardlink: None,
                        export_path: None,
                        flavor: FileFlavor::Normal,
                    }]
                }
                CompileMode::RunCustomBuild => {
                    // At this time, this code path does not handle build script
                    // outputs.
                    vec![]
                }
                CompileMode::Doctest => {
                    // Doctests are built in a temporary directory and then
                    // deleted. There is the `--persist-doctests` unstable flag,
                    // but Cargo does not know about that.
                    vec![]
                }
                CompileMode::Docscrape => {
                    // The file name needs to be stable across Cargo sessions.
                    // This originally used unit.buildkey(), but that isn't stable,
                    // so we use metadata instead (prefixed with name for debugging).
                    let file_name = format!(
                        "{}-{}.examples",
                        unit.pkg.name(),
                        self.metadata(unit).unit_id()
                    );
                    let path = self.deps_dir(unit).join(file_name);
                    vec![OutputFile {
                        path,
                        hardlink: None,
                        export_path: None,
                        flavor: FileFlavor::Normal,
                    }]
                }
                CompileMode::Test
                | CompileMode::Build
                | CompileMode::Bench
                | CompileMode::Check { .. } => self.calc_outputs_rustc(unit, bcx)?,
            };
            debug!("Target filenames: {:?}", ret);

            Ok(Arc::new(ret))
        }

        /// Computes the actual, full pathnames for all the files generated by rustc.
        ///
        /// The `OutputFile` also contains the paths where those files should be
        /// "uplifted" to.
        fn calc_outputs_rustc(
            &self,
            unit: &Unit,
            bcx: &BuildContext<'a, 'gctx>,
        ) -> CargoResult<Vec<OutputFile>> {
            let out_dir = self.out_dir(unit);

            let info = bcx.target_data.info(unit.kind);
            let triple = bcx.target_data.short_name(&unit.kind);
            let (file_types, unsupported) =
                info.rustc_outputs(unit.mode, unit.target.kind(), triple)?;
            if file_types.is_empty() {
                if !unsupported.is_empty() {
                    let unsupported_strs: Vec<_> = unsupported.iter().map(|ct| ct.as_str()).collect();
                    anyhow::bail!(
                        "cannot produce {} for `{}` as the target `{}` \
                         does not support these crate types",
                        unsupported_strs.join(", "),
                        unit.pkg,
                        triple,
                    )
                }
                anyhow::bail!(
                    "cannot compile `{}` as the target `{}` does not \
                     support any of the output crate types",
                    unit.pkg,
                    triple,
                );
            }

            // Convert FileType to OutputFile.
            let mut outputs = Vec::new();
            for file_type in file_types {
                let meta = self.metas[unit];
                let meta_opt = meta.c_extra_filename().map(|h| h.to_string());
                let path = out_dir.join(file_type.output_filename(&unit.target, meta_opt.as_deref()));

                // If, the `different_binary_name` feature is enabled, the name of the hardlink will
                // be the name of the binary provided by the user in `Cargo.toml`.
                let hardlink = self.uplift_to(unit, &file_type, &path);
                let export_path = if unit.target.is_custom_build() {
                    None
                } else {
                    self.export_dir.as_ref().and_then(|export_dir| {
                        hardlink
                            .as_ref()
                            .map(|hardlink| export_dir.join(hardlink.file_name().unwrap()))
                    })
                };
                outputs.push(OutputFile {
                    path,
                    hardlink,
                    export_path,
                    flavor: file_type.flavor,
                });
            }
            Ok(outputs)
        }
    }

    /// Gets the metadata hash for the given [`Unit`].
    ///
    /// When a metadata hash doesn't exist for the given unit,
    /// this calls itself recursively to compute metadata hashes of all its dependencies.
    /// See [`compute_metadata`] for how a single metadata hash is computed.
    fn metadata_of<'a>(
        unit: &Unit,
        build_runner: &BuildRunner<'_, '_>,
        metas: &'a mut HashMap<Unit, Metadata>,
    ) -> &'a Metadata {
        if !metas.contains_key(unit) {
            let meta = compute_metadata(unit, build_runner, metas);
            metas.insert(unit.clone(), meta);
            for dep in build_runner.unit_deps(unit) {
                metadata_of(&dep.unit, build_runner, metas);
            }
        }
        &metas[unit]
    }

    /// Computes the metadata hash for the given [`Unit`].
    fn compute_metadata(
        unit: &Unit,
        build_runner: &BuildRunner<'_, '_>,
        metas: &mut HashMap<Unit, Metadata>,
    ) -> Metadata {
        let bcx = &build_runner.bcx;
        let deps_metadata = build_runner
            .unit_deps(unit)
            .iter()
            .map(|dep| *metadata_of(&dep.unit, build_runner, metas))
            .collect::<Vec<_>>();
        let use_extra_filename = use_extra_filename(bcx, unit);

        let mut shared_hasher = StableHasher::new();

        METADATA_VERSION.hash(&mut shared_hasher);

        let ws_root = if unit.is_std {
            // SourceId for stdlib crates is an absolute path inside the sysroot.
            // Pass the sysroot as workspace root so that we hash a relative path.
            // This avoids the metadata hash changing depending on where the user installed rustc.
            &bcx.target_data.get_info(unit.kind).unwrap().sysroot
        } else {
            bcx.ws.root()
        };

        // Unique metadata per (name, source, version) triple. This'll allow us
        // to pull crates from anywhere without worrying about conflicts.
        unit.pkg
            .package_id()
            .stable_hash(ws_root)
            .hash(&mut shared_hasher);

        // Also mix in enabled features to our metadata. This'll ensure that
        // when changing feature sets each lib is separately cached.
        unit.features.hash(&mut shared_hasher);

        // Throw in the profile we're compiling with. This helps caching
        // `panic=abort` and `panic=unwind` artifacts, additionally with various
        // settings like debuginfo and whatnot.
        unit.profile.hash(&mut shared_hasher);
        unit.mode.hash(&mut shared_hasher);
        build_runner.lto[unit].hash(&mut shared_hasher);

        // Artifacts compiled for the host should have a different
        // metadata piece than those compiled for the target, so make sure
        // we throw in the unit's `kind` as well.  Use `fingerprint_hash`
        // so that the StableHash doesn't change based on the pathnames
        // of the custom target JSON spec files.
        unit.kind.fingerprint_hash().hash(&mut shared_hasher);

        // Finally throw in the target name/kind. This ensures that concurrent
        // compiles of targets in the same crate don't collide.
        unit.target.name().hash(&mut shared_hasher);
        unit.target.kind().hash(&mut shared_hasher);

        hash_rustc_version(bcx, &mut shared_hasher, unit);

        if build_runner.bcx.ws.is_member(&unit.pkg) {
            // This is primarily here for clippy. This ensures that the clippy
            // artifacts are separate from the `check` ones.
            if let Some(path) = &build_runner.bcx.rustc().workspace_wrapper {
                path.hash(&mut shared_hasher);
            }
        }

        // Seed the contents of `__CARGO_DEFAULT_LIB_METADATA` to the hasher if present.
        // This should be the release channel, to get a different hash for each channel.
        if let Ok(ref channel) = build_runner
            .bcx
            .gctx
            .get_env("__CARGO_DEFAULT_LIB_METADATA")
        {
            channel.hash(&mut shared_hasher);
        }

        // std units need to be kept separate from user dependencies. std crates
        // are differentiated in the Unit with `is_std` (for things like
        // `-Zforce-unstable-if-unmarked`), so they are always built separately.
        // This isn't strictly necessary for build dependencies which probably
        // don't need unstable support. A future experiment might be to set
        // `is_std` to false for build dependencies so that they can be shared
        // with user dependencies.
        unit.is_std.hash(&mut shared_hasher);

        // While we don't hash RUSTFLAGS because it may contain absolute paths that
        // hurts reproducibility, we track whether a unit's RUSTFLAGS is from host
        // config, so that we can generate a different metadata hash for runtime
        // and compile-time units.
        //
        // HACK: This is a temporary hack for fixing rust-lang/cargo#14253
        // Need to find a long-term solution to replace this fragile workaround.
        // See https://github.com/rust-lang/cargo/pull/14432#discussion_r1725065350
        if unit.kind.is_host() && !bcx.gctx.target_applies_to_host().unwrap_or_default() {
            let host_info = bcx.target_data.info(CompileKind::Host);
            let target_configs_are_different = unit.rustflags != host_info.rustflags
                || unit.rustdocflags != host_info.rustdocflags
                || bcx
                    .target_data
                    .target_config(CompileKind::Host)
                    .links_overrides
                    != unit.links_overrides;
            target_configs_are_different.hash(&mut shared_hasher);
        }

        let mut c_metadata_hasher = shared_hasher.clone();
        // Mix in the target-metadata of all the dependencies of this target.
        let mut dep_c_metadata_hashes = deps_metadata
            .iter()
            .map(|m| m.c_metadata)
            .collect::<Vec<_>>();
        dep_c_metadata_hashes.sort();
        dep_c_metadata_hashes.hash(&mut c_metadata_hasher);

        let mut c_extra_filename_hasher = shared_hasher.clone();
        // Mix in the target-metadata of all the dependencies of this target.
        let mut dep_c_extra_filename_hashes = deps_metadata
            .iter()
            .map(|m| m.c_extra_filename)
            .collect::<Vec<_>>();
        dep_c_extra_filename_hashes.sort();
        dep_c_extra_filename_hashes.hash(&mut c_extra_filename_hasher);
        // Avoid trashing the caches on RUSTFLAGS changing via `c_extra_filename`
        //
        // Limited to `c_extra_filename` to help with reproducible build / PGO issues.
        let default = Vec::new();
        let extra_args = build_runner.bcx.extra_args_for(unit).unwrap_or(&default);
        if !has_remap_path_prefix(&extra_args) {
            extra_args.hash(&mut c_extra_filename_hasher);
        }
        if unit.mode.is_doc() || unit.mode.is_doc_scrape() {
            if !has_remap_path_prefix(&unit.rustdocflags) {
                unit.rustdocflags.hash(&mut c_extra_filename_hasher);
            }
        } else {
            if !has_remap_path_prefix(&unit.rustflags) {
                unit.rustflags.hash(&mut c_extra_filename_hasher);
            }
        }

        let c_metadata = UnitHash(Hasher::finish(&c_metadata_hasher));
        let c_extra_filename = UnitHash(Hasher::finish(&c_extra_filename_hasher));
        let unit_id = c_extra_filename;

        let c_extra_filename = use_extra_filename.then_some(c_extra_filename);

        Metadata {
            unit_id,
            c_metadata,
            c_extra_filename,
        }
    }

    /// HACK: Detect the *potential* presence of `--remap-path-prefix`
    ///
    /// As CLI parsing is contextual and dependent on the CLI definition to understand the context, we
    /// can't say for sure whether `--remap-path-prefix` is present, so we guess if anything looks like
    /// it.
    /// If we could, we'd strip it out for hashing.
    /// Instead, we use this to avoid hashing rustflags if it might be present to avoid the risk of taking
    /// a flag that is trying to make things reproducible and making things less reproducible by the
    /// `-Cextra-filename` showing up in the rlib, even with `split-debuginfo`.
    fn has_remap_path_prefix(args: &[String]) -> bool {
        args.iter()
            .any(|s| s.starts_with("--remap-path-prefix=") || s == "--remap-path-prefix")
    }

    /// Hash the version of rustc being used during the build process.
    fn hash_rustc_version(bcx: &BuildContext<'_, '_>, hasher: &mut StableHasher, unit: &Unit) {
        let vers = &bcx.rustc().version;
        if vers.pre.is_empty() || bcx.gctx.cli_unstable().separate_nightlies {
            // For stable, keep the artifacts separate. This helps if someone is
            // testing multiple versions, to avoid recompiles. Note though that for
            // cross-compiled builds the `host:` line of `verbose_version` is
            // omitted since rustc should produce the same output for each target
            // regardless of the host.
            for line in bcx.rustc().verbose_version.lines() {
                if unit.kind.is_host() || !line.starts_with("host: ") {
                    line.hash(hasher);
                }
            }
            return;
        }
        // On "nightly"/"beta"/"dev"/etc, keep each "channel" separate. Don't hash
        // the date/git information, so that whenever someone updates "nightly",
        // they won't have a bunch of stale artifacts in the target directory.
        //
        // This assumes that the first segment is the important bit ("nightly",
        // "beta", "dev", etc.). Skip other parts like the `.3` in `-beta.3`.
        vers.pre.split('.').next().hash(hasher);
        // Keep "host" since some people switch hosts to implicitly change
        // targets, (like gnu vs musl or gnu vs msvc). In the future, we may want
        // to consider hashing `unit.kind.short_name()` instead.
        if unit.kind.is_host() {
            bcx.rustc().host.hash(hasher);
        }
        // None of the other lines are important. Currently they are:
        // binary: rustc  <-- or "rustdoc"
        // commit-hash: 38114ff16e7856f98b2b4be7ab4cd29b38bed59a
        // commit-date: 2020-03-21
        // host: x86_64-apple-darwin
        // release: 1.44.0-nightly
        // LLVM version: 9.0
        //
        // The backend version ("LLVM version") might become more relevant in
        // the future when cranelift sees more use, and people want to switch
        // between different backends without recompiling.
    }

    /// Returns whether or not this unit should use a hash in the filename to make it unique.
    fn use_extra_filename(bcx: &BuildContext<'_, '_>, unit: &Unit) -> bool {
        if unit.mode.is_doc_test() || unit.mode.is_doc() {
            // Doc tests do not have metadata.
            return false;
        }
        if unit.mode.is_any_test() || unit.mode.is_check() {
            // These always use metadata.
            return true;
        }
        // No metadata in these cases:
        //
        // - dylibs:
        //   - if any dylib names are encoded in executables, so they can't be renamed.
        //   - TODO: Maybe use `-install-name` on macOS or `-soname` on other UNIX systems
        //     to specify the dylib name to be used by the linker instead of the filename.
        // - Windows MSVC executables: The path to the PDB is embedded in the
        //   executable, and we don't want the PDB path to include the hash in it.
        // - wasm32-unknown-emscripten executables: When using emscripten, the path to the
        //   .wasm file is embedded in the .js file, so we don't want the hash in there.
        //
        // This is only done for local packages, as we don't expect to export
        // dependencies.
        //
        // The __CARGO_DEFAULT_LIB_METADATA env var is used to override this to
        // force metadata in the hash. This is only used for building libstd. For
        // example, if libstd is placed in a common location, we don't want a file
        // named /usr/lib/libstd.so which could conflict with other rustc
        // installs. In addition it prevents accidentally loading a libstd of a
        // different compiler at runtime.
        // See https://github.com/rust-lang/cargo/issues/3005
        let short_name = bcx.target_data.short_name(&unit.kind);
        if (unit.target.is_dylib()
            || unit.target.is_cdylib()
            || (unit.target.is_executable() && short_name == "wasm32-unknown-emscripten")
            || (unit.target.is_executable() && short_name.contains("msvc")))
            && unit.pkg.package_id().source_id().is_path()
            && bcx.gctx.get_env("__CARGO_DEFAULT_LIB_METADATA").is_err()
        {
            return false;
        }
        true
    }