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    //! Tracks changes to determine if something needs to be recompiled.
    //!
    //! This module implements change-tracking so that Cargo can know whether or
    //! not something needs to be recompiled. A Cargo [`Unit`] can be either "dirty"
    //! (needs to be recompiled) or "fresh" (it does not need to be recompiled).
    //!
    //! ## Mechanisms affecting freshness
    //!
    //! There are several mechanisms that influence a Unit's freshness:
    //!
    //! - The [`Fingerprint`] is a hash, saved to the filesystem in the
    //!   `.fingerprint` directory, that tracks information about the Unit. If the
    //!   fingerprint is missing (such as the first time the unit is being
    //!   compiled), then the unit is dirty. If any of the fingerprint fields
    //!   change (like the name of the source file), then the Unit is considered
    //!   dirty.
    //!
    //!   The `Fingerprint` also tracks the fingerprints of all its dependencies,
    //!   so a change in a dependency will propagate the "dirty" status up.
    //!
    //! - Filesystem mtime tracking is also used to check if a unit is dirty.
    //!   See the section below on "Mtime comparison" for more details. There
    //!   are essentially two parts to mtime tracking:
    //!
    //!   1. The mtime of a Unit's output files is compared to the mtime of all
    //!      its dependencies' output file mtimes (see
    //!      [`check_filesystem`]). If any output is missing, or is
    //!      older than a dependency's output, then the unit is dirty.
    //!   2. The mtime of a Unit's source files is compared to the mtime of its
    //!      dep-info file in the fingerprint directory (see [`find_stale_file`]).
    //!      The dep-info file is used as an anchor to know when the last build of
    //!      the unit was done. See the "dep-info files" section below for more
    //!      details. If any input files are missing, or are newer than the
    //!      dep-info, then the unit is dirty.
    //!
    //! Note: Fingerprinting is not a perfect solution. Filesystem mtime tracking
    //! is notoriously imprecise and problematic. Only a small part of the
    //! environment is captured. This is a balance of performance, simplicity, and
    //! completeness. Sandboxing, hashing file contents, tracking every file
    //! access, environment variable, and network operation would ensure more
    //! reliable and reproducible builds at the cost of being complex, slow, and
    //! platform-dependent.
    //!
    //! ## Fingerprints and Metadata
    //!
    //! The [`Metadata`] hash is a hash added to the output filenames to isolate
    //! each unit. See its documentationfor more details.
    //! NOTE: Not all output files are isolated via filename hashes (like dylibs).
    //! The fingerprint directory uses a hash, but sometimes units share the same
    //! fingerprint directory (when they don't have Metadata) so care should be
    //! taken to handle this!
    //!
    //! Fingerprints and Metadata are similar, and track some of the same things.
    //! The Metadata contains information that is required to keep Units separate.
    //! The Fingerprint includes additional information that should cause a
    //! recompile, but it is desired to reuse the same filenames. A comparison
    //! of what is tracked:
    //!
    //! Value                                      | Fingerprint | Metadata
    //! -------------------------------------------|-------------|----------
    //! rustc                                      | ✓           | ✓
    //! [`Profile`]                                | ✓           | ✓
    //! `cargo rustc` extra args                   | ✓           | ✓
    //! [`CompileMode`]                            | ✓           | ✓
    //! Target Name                                | ✓           | ✓
    //! TargetKind (bin/lib/etc.)                  | ✓           | ✓
    //! Enabled Features                           | ✓           | ✓
    //! Declared Features                          | ✓           |
    //! Immediate dependency’s hashes              | ✓[^1]       | ✓
    //! [`CompileKind`] (host/target)              | ✓           | ✓
    //! __CARGO_DEFAULT_LIB_METADATA[^4]           |             | ✓
    //! package_id                                 |             | ✓
    //! authors, description, homepage, repo       | ✓           |
    //! Target src path relative to ws             | ✓           |
    //! Target flags (test/bench/for_host/edition) | ✓           |
    //! -C incremental=… flag                      | ✓           |
    //! mtime of sources                           | ✓[^3]       |
    //! RUSTFLAGS/RUSTDOCFLAGS                     | ✓           |
    //! [`Lto`] flags                              | ✓           | ✓
    //! config settings[^5]                        | ✓           |
    //! is_std                                     |             | ✓
    //! `[lints]` table[^6]                        | ✓           |
    //! `[lints.rust.unexpected_cfgs.check-cfg]`   | ✓           |
    //!
    //! [^1]: Build script and bin dependencies are not included.
    //!
    //! [^3]: See below for details on mtime tracking.
    //!
    //! [^4]: `__CARGO_DEFAULT_LIB_METADATA` is set by rustbuild to embed the
    //!        release channel (bootstrap/stable/beta/nightly) in libstd.
    //!
    //! [^5]: Config settings that are not otherwise captured anywhere else.
    //!       Currently, this is only `doc.extern-map`.
    //!
    //! [^6]: Via [`Manifest::lint_rustflags`][crate::core::Manifest::lint_rustflags]
    //!
    //! When deciding what should go in the Metadata vs the Fingerprint, consider
    //! that some files (like dylibs) do not have a hash in their filename. Thus,
    //! if a value changes, only the fingerprint will detect the change (consider,
    //! for example, swapping between different features). Fields that are only in
    //! Metadata generally aren't relevant to the fingerprint because they
    //! fundamentally change the output (like target vs host changes the directory
    //! where it is emitted).
    //!
    //! ## Fingerprint files
    //!
    //! Fingerprint information is stored in the
    //! `target/{debug,release}/.fingerprint/` directory. Each Unit is stored in a
    //! separate directory. Each Unit directory contains:
    //!
    //! - A file with a 16 hex-digit hash. This is the Fingerprint hash, used for
    //!   quick loading and comparison.
    //! - A `.json` file that contains details about the Fingerprint. This is only
    //!   used to log details about *why* a fingerprint is considered dirty.
    //!   `CARGO_LOG=cargo::core::compiler::fingerprint=trace cargo build` can be
    //!   used to display this log information.
    //! - A "dep-info" file which is a translation of rustc's `*.d` dep-info files
    //!   to a Cargo-specific format that tweaks file names and is optimized for
    //!   reading quickly.
    //! - An `invoked.timestamp` file whose filesystem mtime is updated every time
    //!   the Unit is built. This is used for capturing the time when the build
    //!   starts, to detect if files are changed in the middle of the build. See
    //!   below for more details.
    //!
    //! Note that some units are a little different. A Unit for *running* a build
    //! script or for `rustdoc` does not have a dep-info file (it's not
    //! applicable). Build script `invoked.timestamp` files are in the build
    //! output directory.
    //!
    //! ## Fingerprint calculation
    //!
    //! After the list of Units has been calculated, the Units are added to the
    //! [`JobQueue`]. As each one is added, the fingerprint is calculated, and the
    //! dirty/fresh status is recorded. A closure is used to update the fingerprint
    //! on-disk when the Unit successfully finishes. The closure will recompute the
    //! Fingerprint based on the updated information. If the Unit fails to compile,
    //! the fingerprint is not updated.
    //!
    //! Fingerprints are cached in the [`BuildRunner`]. This makes computing
    //! Fingerprints faster, but also is necessary for properly updating
    //! dependency information. Since a Fingerprint includes the Fingerprints of
    //! all dependencies, when it is updated, by using `Arc` clones, it
    //! automatically picks up the updates to its dependencies.
    //!
    //! ### dep-info files
    //!
    //! Cargo has several kinds of "dep info" files:
    //!
    //! * dep-info files generated by `rustc`.
    //! * Fingerprint dep-info files translated from the first one.
    //! * dep-info for external build system integration.
    //! * Unstable `-Zbinary-dep-depinfo`.
    //!
    //! #### `rustc` dep-info files
    //!
    //! Cargo passes the `--emit=dep-info` flag to `rustc` so that `rustc` will
    //! generate a "dep info" file (with the `.d` extension). This is a
    //! Makefile-like syntax that includes all of the source files used to build
    //! the crate. This file is used by Cargo to know which files to check to see
    //! if the crate will need to be rebuilt. Example:
    //!
    //! ```makefile
    //! /path/to/target/debug/deps/cargo-b6219d178925203d: src/bin/main.rs src/bin/cargo/cli.rs # … etc.
    //! ```
    //!
    //! #### Fingerprint dep-info files
    //!
    //! After `rustc` exits successfully, Cargo will read the first kind of dep
    //! info file and translate it into a binary format that is stored in the
    //! fingerprint directory ([`translate_dep_info`]).
    //!
    //! These are used to quickly scan for any changed files. The mtime of the
    //! fingerprint dep-info file itself is used as the reference for comparing the
    //! source files to determine if any of the source files have been modified
    //! (see [below](#mtime-comparison) for more detail).
    //!
    //! Note that Cargo parses the special `# env-var:...` comments in dep-info
    //! files to learn about environment variables that the rustc compile depends on.
    //! Cargo then later uses this to trigger a recompile if a referenced env var
    //! changes (even if the source didn't change).
    //!
    //! #### dep-info files for build system integration.
    //!
    //! There is also a third dep-info file. Cargo will extend the file created by
    //! rustc with some additional information and saves this into the output
    //! directory. This is intended for build system integration. See the
    //! [`output_depinfo`] function for more detail.
    //!
    //! #### -Zbinary-dep-depinfo
    //!
    //! `rustc` has an experimental flag `-Zbinary-dep-depinfo`. This causes
    //! `rustc` to include binary files (like rlibs) in the dep-info file. This is
    //! primarily to support rustc development, so that Cargo can check the
    //! implicit dependency to the standard library (which lives in the sysroot).
    //! We want Cargo to recompile whenever the standard library rlib/dylibs
    //! change, and this is a generic mechanism to make that work.
    //!
    //! ### Mtime comparison
    //!
    //! The use of modification timestamps is the most common way a unit will be
    //! determined to be dirty or fresh between builds. There are many subtle
    //! issues and edge cases with mtime comparisons. This gives a high-level
    //! overview, but you'll need to read the code for the gritty details. Mtime
    //! handling is different for different unit kinds. The different styles are
    //! driven by the [`Fingerprint::local`] field, which is set based on the unit
    //! kind.
    //!
    //! The status of whether or not the mtime is "stale" or "up-to-date" is
    //! stored in [`Fingerprint::fs_status`].
    //!
    //! All units will compare the mtime of its newest output file with the mtimes
    //! of the outputs of all its dependencies. If any output file is missing,
    //! then the unit is stale. If any dependency is newer, the unit is stale.
    //!
    //! #### Normal package mtime handling
    //!
    //! [`LocalFingerprint::CheckDepInfo`] is used for checking the mtime of
    //! packages. It compares the mtime of the input files (the source files) to
    //! the mtime of the dep-info file (which is written last after a build is
    //! finished). If the dep-info is missing, the unit is stale (it has never
    //! been built). The list of input files comes from the dep-info file. See the
    //! section above for details on dep-info files.
    //!
    //! Also note that although registry and git packages use [`CheckDepInfo`], none
    //! of their source files are included in the dep-info (see
    //! [`translate_dep_info`]), so for those kinds no mtime checking is done
    //! (unless `-Zbinary-dep-depinfo` is used). Repository and git packages are
    //! static, so there is no need to check anything.
    //!
    //! When a build is complete, the mtime of the dep-info file in the
    //! fingerprint directory is modified to rewind it to the time when the build
    //! started. This is done by creating an `invoked.timestamp` file when the
    //! build starts to capture the start time. The mtime is rewound to the start
    //! to handle the case where the user modifies a source file while a build is
    //! running. Cargo can't know whether or not the file was included in the
    //! build, so it takes a conservative approach of assuming the file was *not*
    //! included, and it should be rebuilt during the next build.
    //!
    //! #### Rustdoc mtime handling
    //!
    //! Rustdoc does not emit a dep-info file, so Cargo currently has a relatively
    //! simple system for detecting rebuilds. [`LocalFingerprint::Precalculated`] is
    //! used for rustdoc units. For registry packages, this is the package
    //! version. For git packages, it is the git hash. For path packages, it is
    //! the a string of the mtime of the newest file in the package.
    //!
    //! There are some known bugs with how this works, so it should be improved at
    //! some point.
    //!
    //! #### Build script mtime handling
    //!
    //! Build script mtime handling runs in different modes. There is the "old
    //! style" where the build script does not emit any `rerun-if` directives. In
    //! this mode, Cargo will use [`LocalFingerprint::Precalculated`]. See the
    //! "rustdoc" section above how it works.
    //!
    //! In the new-style, each `rerun-if` directive is translated to the
    //! corresponding [`LocalFingerprint`] variant. The [`RerunIfChanged`] variant
    //! compares the mtime of the given filenames against the mtime of the
    //! "output" file.
    //!
    //! Similar to normal units, the build script "output" file mtime is rewound
    //! to the time just before the build script is executed to handle mid-build
    //! modifications.
    //!
    //! ## Considerations for inclusion in a fingerprint
    //!
    //! Over time we've realized a few items which historically were included in
    //! fingerprint hashings should not actually be included. Examples are:
    //!
    //! * Modification time values. We strive to never include a modification time
    //!   inside a `Fingerprint` to get hashed into an actual value. While
    //!   theoretically fine to do, in practice this causes issues with common
    //!   applications like Docker. Docker, after a layer is built, will zero out
    //!   the nanosecond part of all filesystem modification times. This means that
    //!   the actual modification time is different for all build artifacts, which
    //!   if we tracked the actual values of modification times would cause
    //!   unnecessary recompiles. To fix this we instead only track paths which are
    //!   relevant. These paths are checked dynamically to see if they're up to
    //!   date, and the modification time doesn't make its way into the fingerprint
    //!   hash.
    //!
    //! * Absolute path names. We strive to maintain a property where if you rename
    //!   a project directory Cargo will continue to preserve all build artifacts
    //!   and reuse the cache. This means that we can't ever hash an absolute path
    //!   name. Instead we always hash relative path names and the "root" is passed
    //!   in at runtime dynamically. Some of this is best effort, but the general
    //!   idea is that we assume all accesses within a crate stay within that
    //!   crate.
    //!
    //! These are pretty tricky to test for unfortunately, but we should have a good
    //! test suite nowadays and lord knows Cargo gets enough testing in the wild!
    //!
    //! ## Build scripts
    //!
    //! The *running* of a build script ([`CompileMode::RunCustomBuild`]) is treated
    //! significantly different than all other Unit kinds. It has its own function
    //! for calculating the Fingerprint ([`calculate_run_custom_build`]) and has some
    //! unique considerations. It does not track the same information as a normal
    //! Unit. The information tracked depends on the `rerun-if-changed` and
    //! `rerun-if-env-changed` statements produced by the build script. If the
    //! script does not emit either of these statements, the Fingerprint runs in
    //! "old style" mode where an mtime change of *any* file in the package will
    //! cause the build script to be re-run. Otherwise, the fingerprint *only*
    //! tracks the individual "rerun-if" items listed by the build script.
    //!
    //! The "rerun-if" statements from a *previous* build are stored in the build
    //! output directory in a file called `output`. Cargo parses this file when
    //! the Unit for that build script is prepared for the [`JobQueue`]. The
    //! Fingerprint code can then use that information to compute the Fingerprint
    //! and compare against the old fingerprint hash.
    //!
    //! Care must be taken with build script Fingerprints because the
    //! [`Fingerprint::local`] value may be changed after the build script runs
    //! (such as if the build script adds or removes "rerun-if" items).
    //!
    //! Another complication is if a build script is overridden. In that case, the
    //! fingerprint is the hash of the output of the override.
    //!
    //! ## Special considerations
    //!
    //! Registry dependencies do not track the mtime of files. This is because
    //! registry dependencies are not expected to change (if a new version is
    //! used, the Package ID will change, causing a rebuild). Cargo currently
    //! partially works with Docker caching. When a Docker image is built, it has
    //! normal mtime information. However, when a step is cached, the nanosecond
    //! portions of all files is zeroed out. Currently this works, but care must
    //! be taken for situations like these.
    //!
    //! HFS on macOS only supports 1 second timestamps. This causes a significant
    //! number of problems, particularly with Cargo's testsuite which does rapid
    //! builds in succession. Other filesystems have various degrees of
    //! resolution.
    //!
    //! Various weird filesystems (such as network filesystems) also can cause
    //! complications. Network filesystems may track the time on the server
    //! (except when the time is set manually such as with
    //! `filetime::set_file_times`). Not all filesystems support modifying the
    //! mtime.
    //!
    //! See the [`A-rebuild-detection`] label on the issue tracker for more.
    //!
    //! [`check_filesystem`]: Fingerprint::check_filesystem
    //! [`Metadata`]: crate::core::compiler::Metadata
    //! [`Profile`]: crate::core::profiles::Profile
    //! [`CompileMode`]: crate::core::compiler::CompileMode
    //! [`Lto`]: crate::core::compiler::Lto
    //! [`CompileKind`]: crate::core::compiler::CompileKind
    //! [`JobQueue`]: super::job_queue::JobQueue
    //! [`output_depinfo`]: super::output_depinfo()
    //! [`CheckDepInfo`]: LocalFingerprint::CheckDepInfo
    //! [`RerunIfChanged`]: LocalFingerprint::RerunIfChanged
    //! [`CompileMode::RunCustomBuild`]: crate::core::compiler::CompileMode::RunCustomBuild
    //! [`A-rebuild-detection`]: https://github.com/rust-lang/cargo/issues?q=is%3Aissue+is%3Aopen+label%3AA-rebuild-detection

    mod dirty_reason;

    use std::collections::hash_map::{Entry, HashMap};

    use std::env;
    use std::hash::{self, Hash, Hasher};
    use std::io;
    use std::path::{Path, PathBuf};
    use std::str;
    use std::sync::{Arc, Mutex};
    use std::time::SystemTime;

    use anyhow::{bail, format_err, Context as _};
    use cargo_util::{paths, ProcessBuilder};
    use filetime::FileTime;
    use serde::de;
    use serde::ser;
    use serde::{Deserialize, Serialize};
    use tracing::{debug, info};

    use crate::core::compiler::unit_graph::UnitDep;
    use crate::core::Package;
    use crate::util::errors::CargoResult;
    use crate::util::interning::InternedString;
    use crate::util::{self, try_canonicalize};
    use crate::util::{internal, path_args, StableHasher};
    use crate::{GlobalContext, CARGO_ENV};

    use super::custom_build::BuildDeps;
    use super::{BuildContext, BuildRunner, FileFlavor, Job, Unit, Work};

    pub use dirty_reason::DirtyReason;

    /// Determines if a [`Unit`] is up-to-date, and if not prepares necessary work to
    /// update the persisted fingerprint.
    ///
    /// This function will inspect `Unit`, calculate a fingerprint for it, and then
    /// return an appropriate [`Job`] to run. The returned `Job` will be a noop if
    /// `unit` is considered "fresh", or if it was previously built and cached.
    /// Otherwise the `Job` returned will write out the true fingerprint to the
    /// filesystem, to be executed after the unit's work has completed.
    ///
    /// The `force` flag is a way to force the `Job` to be "dirty", or always
    /// update the fingerprint. **Beware using this flag** because it does not
    /// transitively propagate throughout the dependency graph, it only forces this
    /// one unit which is very unlikely to be what you want unless you're
    /// exclusively talking about top-level units.
    #[tracing::instrument(
        skip(build_runner, unit),
        fields(package_id = %unit.pkg.package_id(), target = unit.target.name())
    )]
    pub fn prepare_target(
        build_runner: &mut BuildRunner<'_, '_>,
        unit: &Unit,
        force: bool,
    ) -> CargoResult<Job> {
        let bcx = build_runner.bcx;
        let loc = build_runner.files().fingerprint_file_path(unit, "");

        debug!("fingerprint at: {}", loc.display());

        // Figure out if this unit is up to date. After calculating the fingerprint
        // compare it to an old version, if any, and attempt to print diagnostic
        // information about failed comparisons to aid in debugging.
        let fingerprint = calculate(build_runner, unit)?;
        let mtime_on_use = build_runner.bcx.gctx.cli_unstable().mtime_on_use;
        let dirty_reason = compare_old_fingerprint(unit, &loc, &*fingerprint, mtime_on_use, force);

        let Some(dirty_reason) = dirty_reason else {
            return Ok(Job::new_fresh());
        };

        // We're going to rebuild, so ensure the source of the crate passes all
        // verification checks before we build it.
        //
        // The `Source::verify` method is intended to allow sources to execute
        // pre-build checks to ensure that the relevant source code is all
        // up-to-date and as expected. This is currently used primarily for
        // directory sources which will use this hook to perform an integrity check
        // on all files in the source to ensure they haven't changed. If they have
        // changed then an error is issued.
        let source_id = unit.pkg.package_id().source_id();
        let sources = bcx.packages.sources();
        let source = sources
            .get(source_id)
            .ok_or_else(|| internal("missing package source"))?;
        source.verify(unit.pkg.package_id())?;

        // Clear out the old fingerprint file if it exists. This protects when
        // compilation is interrupted leaving a corrupt file. For example, a
        // project with a lib.rs and integration test (two units):
        //
        // 1. Build the library and integration test.
        // 2. Make a change to lib.rs (NOT the integration test).
        // 3. Build the integration test, hit Ctrl-C while linking. With gcc, this
        //    will leave behind an incomplete executable (zero size, or partially
        //    written). NOTE: The library builds successfully, it is the linking
        //    of the integration test that we are interrupting.
        // 4. Build the integration test again.
        //
        // Without the following line, then step 3 will leave a valid fingerprint
        // on the disk. Then step 4 will think the integration test is "fresh"
        // because:
        //
        // - There is a valid fingerprint hash on disk (written in step 1).
        // - The mtime of the output file (the corrupt integration executable
        //   written in step 3) is newer than all of its dependencies.
        // - The mtime of the integration test fingerprint dep-info file (written
        //   in step 1) is newer than the integration test's source files, because
        //   we haven't modified any of its source files.
        //
        // But the executable is corrupt and needs to be rebuilt. Clearing the
        // fingerprint at step 3 ensures that Cargo never mistakes a partially
        // written output as up-to-date.
        if loc.exists() {
            // Truncate instead of delete so that compare_old_fingerprint will
            // still log the reason for the fingerprint failure instead of just
            // reporting "failed to read fingerprint" during the next build if
            // this build fails.
            paths::write(&loc, b"")?;
        }

        let write_fingerprint = if unit.mode.is_run_custom_build() {
            // For build scripts the `local` field of the fingerprint may change
            // while we're executing it. For example it could be in the legacy
            // "consider everything a dependency mode" and then we switch to "deps
            // are explicitly specified" mode.
            //
            // To handle this movement we need to regenerate the `local` field of a
            // build script's fingerprint after it's executed. We do this by
            // using the `build_script_local_fingerprints` function which returns a
            // thunk we can invoke on a foreign thread to calculate this.
            let build_script_outputs = Arc::clone(&build_runner.build_script_outputs);
            let metadata = build_runner.get_run_build_script_metadata(unit);
            let (gen_local, _overridden) = build_script_local_fingerprints(build_runner, unit);
            let output_path = build_runner.build_explicit_deps[unit]
                .build_script_output
                .clone();
            Work::new(move |_| {
                let outputs = build_script_outputs.lock().unwrap();
                let output = outputs
                    .get(metadata)
                    .expect("output must exist after running");
                let deps = BuildDeps::new(&output_path, Some(output));

                // FIXME: it's basically buggy that we pass `None` to `call_box`
                // here. See documentation on `build_script_local_fingerprints`
                // below for more information. Despite this just try to proceed and
                // hobble along if it happens to return `Some`.
                if let Some(new_local) = (gen_local)(&deps, None)? {
                    *fingerprint.local.lock().unwrap() = new_local;
                }

                write_fingerprint(&loc, &fingerprint)
            })
        } else {
            Work::new(move |_| write_fingerprint(&loc, &fingerprint))
        };

        Ok(Job::new_dirty(write_fingerprint, dirty_reason))
    }

    /// Dependency edge information for fingerprints. This is generated for each
    /// dependency and is stored in a [`Fingerprint`].
    #[derive(Clone)]
    struct DepFingerprint {
        /// The hash of the package id that this dependency points to
        pkg_id: u64,
        /// The crate name we're using for this dependency, which if we change we'll
        /// need to recompile!
        name: InternedString,
        /// Whether or not this dependency is flagged as a public dependency or not.
        public: bool,
        /// Whether or not this dependency is an rmeta dependency or a "full"
        /// dependency. In the case of an rmeta dependency our dependency edge only
        /// actually requires the rmeta from what we depend on, so when checking
        /// mtime information all files other than the rmeta can be ignored.
        only_requires_rmeta: bool,
        /// The dependency's fingerprint we recursively point to, containing all the
        /// other hash information we'd otherwise need.
        fingerprint: Arc<Fingerprint>,
    }

    /// A fingerprint can be considered to be a "short string" representing the
    /// state of a world for a package.
    ///
    /// If a fingerprint ever changes, then the package itself needs to be
    /// recompiled. Inputs to the fingerprint include source code modifications,
    /// compiler flags, compiler version, etc. This structure is not simply a
    /// `String` due to the fact that some fingerprints cannot be calculated lazily.
    ///
    /// Path sources, for example, use the mtime of the corresponding dep-info file
    /// as a fingerprint (all source files must be modified *before* this mtime).
    /// This dep-info file is not generated, however, until after the crate is
    /// compiled. As a result, this structure can be thought of as a fingerprint
    /// to-be. The actual value can be calculated via [`hash_u64()`], but the operation
    /// may fail as some files may not have been generated.
    ///
    /// Note that dependencies are taken into account for fingerprints because rustc
    /// requires that whenever an upstream crate is recompiled that all downstream
    /// dependents are also recompiled. This is typically tracked through
    /// [`DependencyQueue`], but it also needs to be retained here because Cargo can
    /// be interrupted while executing, losing the state of the [`DependencyQueue`]
    /// graph.
    ///
    /// [`hash_u64()`]: crate::core::compiler::fingerprint::Fingerprint::hash_u64
    /// [`DependencyQueue`]: crate::util::DependencyQueue
    #[derive(Serialize, Deserialize)]
    pub struct Fingerprint {
        /// Hash of the version of `rustc` used.
        rustc: u64,
        /// Sorted list of cfg features enabled.
        features: String,
        /// Sorted list of all the declared cfg features.
        declared_features: String,
        /// Hash of the `Target` struct, including the target name,
        /// package-relative source path, edition, etc.
        target: u64,
        /// Hash of the [`Profile`], [`CompileMode`], and any extra flags passed via
        /// `cargo rustc` or `cargo rustdoc`.
        ///
        /// [`Profile`]: crate::core::profiles::Profile
        /// [`CompileMode`]: crate::core::compiler::CompileMode
        profile: u64,
        /// Hash of the path to the base source file. This is relative to the
        /// workspace root for path members, or absolute for other sources.
        path: u64,
        /// Fingerprints of dependencies.
        deps: Vec<DepFingerprint>,
        /// Information about the inputs that affect this Unit (such as source
        /// file mtimes or build script environment variables).
        local: Mutex<Vec<LocalFingerprint>>,
        /// Cached hash of the [`Fingerprint`] struct. Used to improve performance
        /// for hashing.
        #[serde(skip)]
        memoized_hash: Mutex<Option<u64>>,
        /// RUSTFLAGS/RUSTDOCFLAGS environment variable value (or config value).
        rustflags: Vec<String>,
        /// Hash of some metadata from the manifest, such as "authors", or
        /// "description", which are exposed as environment variables during
        /// compilation.
        metadata: u64,
        /// Hash of various config settings that change how things are compiled.
        config: u64,
        /// The rustc target. This is only relevant for `.json` files, otherwise
        /// the metadata hash segregates the units.
        compile_kind: u64,
        /// Description of whether the filesystem status for this unit is up to date
        /// or should be considered stale.
        #[serde(skip)]
        fs_status: FsStatus,
        /// Files, relative to `target_root`, that are produced by the step that
        /// this `Fingerprint` represents. This is used to detect when the whole
        /// fingerprint is out of date if this is missing, or if previous
        /// fingerprints output files are regenerated and look newer than this one.
        #[serde(skip)]
        outputs: Vec<PathBuf>,
    }

    /// Indication of the status on the filesystem for a particular unit.
    #[derive(Clone, Default, Debug)]
    pub enum FsStatus {
        /// This unit is to be considered stale, even if hash information all
        /// matches.
        #[default]
        Stale,

        /// File system inputs have changed (or are missing), or there were
        /// changes to the environment variables that affect this unit. See
        /// the variants of [`StaleItem`] for more information.
        StaleItem(StaleItem),

        /// A dependency was stale.
        StaleDependency {
            name: InternedString,
            dep_mtime: FileTime,
            max_mtime: FileTime,
        },

        /// A dependency was stale.
        StaleDepFingerprint { name: InternedString },

        /// This unit is up-to-date. All outputs and their corresponding mtime are
        /// listed in the payload here for other dependencies to compare against.
        UpToDate { mtimes: HashMap<PathBuf, FileTime> },
    }

    impl FsStatus {
        fn up_to_date(&self) -> bool {
            match self {
                FsStatus::UpToDate { .. } => true,
                FsStatus::Stale
                | FsStatus::StaleItem(_)
                | FsStatus::StaleDependency { .. }
                | FsStatus::StaleDepFingerprint { .. } => false,
            }
        }
    }

    impl Serialize for DepFingerprint {
        fn serialize<S>(&self, ser: S) -> Result<S::Ok, S::Error>
        where
            S: ser::Serializer,
        {
            (
                &self.pkg_id,
                &self.name,
                &self.public,
                &self.fingerprint.hash_u64(),
            )
                .serialize(ser)
        }
    }

    impl<'de> Deserialize<'de> for DepFingerprint {
        fn deserialize<D>(d: D) -> Result<DepFingerprint, D::Error>
        where
            D: de::Deserializer<'de>,
        {
            let (pkg_id, name, public, hash) = <(u64, String, bool, u64)>::deserialize(d)?;
            Ok(DepFingerprint {
                pkg_id,
                name: InternedString::new(&name),
                public,
                fingerprint: Arc::new(Fingerprint {
                    memoized_hash: Mutex::new(Some(hash)),
                    ..Fingerprint::new()
                }),
                // This field is never read since it's only used in
                // `check_filesystem` which isn't used by fingerprints loaded from
                // disk.
                only_requires_rmeta: false,
            })
        }
    }

    /// A `LocalFingerprint` represents something that we use to detect direct
    /// changes to a `Fingerprint`.
    ///
    /// This is where we track file information, env vars, etc. This
    /// `LocalFingerprint` struct is hashed and if the hash changes will force a
    /// recompile of any fingerprint it's included into. Note that the "local"
    /// terminology comes from the fact that it only has to do with one crate, and
    /// `Fingerprint` tracks the transitive propagation of fingerprint changes.
    ///
    /// Note that because this is hashed its contents are carefully managed. Like
    /// mentioned in the above module docs, we don't want to hash absolute paths or
    /// mtime information.
    ///
    /// Also note that a `LocalFingerprint` is used in `check_filesystem` to detect
    /// when the filesystem contains stale information (based on mtime currently).
    /// The paths here don't change much between compilations but they're used as
    /// inputs when we probe the filesystem looking at information.
    #[derive(Debug, Serialize, Deserialize, Hash)]
    enum LocalFingerprint {
        /// This is a precalculated fingerprint which has an opaque string we just
        /// hash as usual. This variant is primarily used for rustdoc where we
        /// don't have a dep-info file to compare against.
        ///
        /// This is also used for build scripts with no `rerun-if-*` statements, but
        /// that's overall a mistake and causes bugs in Cargo. We shouldn't use this
        /// for build scripts.
        Precalculated(String),

        /// This is used for crate compilations. The `dep_info` file is a relative
        /// path anchored at `target_root(...)` to the dep-info file that Cargo
        /// generates (which is a custom serialization after parsing rustc's own
        /// `dep-info` output).
        ///
        /// The `dep_info` file, when present, also lists a number of other files
        /// for us to look at. If any of those files are newer than this file then
        /// we need to recompile.
        CheckDepInfo { dep_info: PathBuf },

        /// This represents a nonempty set of `rerun-if-changed` annotations printed
        /// out by a build script. The `output` file is a relative file anchored at
        /// `target_root(...)` which is the actual output of the build script. That
        /// output has already been parsed and the paths printed out via
        /// `rerun-if-changed` are listed in `paths`. The `paths` field is relative
        /// to `pkg.root()`
        ///
        /// This is considered up-to-date if all of the `paths` are older than
        /// `output`, otherwise we need to recompile.
        RerunIfChanged {
            output: PathBuf,
            paths: Vec<PathBuf>,
        },

        /// This represents a single `rerun-if-env-changed` annotation printed by a
        /// build script. The exact env var and value are hashed here. There's no
        /// filesystem dependence here, and if the values are changed the hash will
        /// change forcing a recompile.
        RerunIfEnvChanged { var: String, val: Option<String> },
    }

    /// See [`FsStatus::StaleItem`].
    #[derive(Clone, Debug)]
    pub enum StaleItem {
        MissingFile(PathBuf),
        ChangedFile {
            reference: PathBuf,
            reference_mtime: FileTime,
            stale: PathBuf,
            stale_mtime: FileTime,
        },
        ChangedEnv {
            var: String,
            previous: Option<String>,
            current: Option<String>,
        },
    }

    impl LocalFingerprint {
        /// Read the environment variable of the given env `key`, and creates a new
        /// [`LocalFingerprint::RerunIfEnvChanged`] for it.
        ///
        // TODO: This is allowed at this moment. Should figure out if it makes
        // sense if permitting to read env from the config system.
        #[allow(clippy::disallowed_methods)]
        fn from_env<K: AsRef<str>>(key: K) -> LocalFingerprint {
            let key = key.as_ref();
            let var = key.to_owned();
            let val = env::var(key).ok();
            LocalFingerprint::RerunIfEnvChanged { var, val }
        }

        /// Checks dynamically at runtime if this `LocalFingerprint` has a stale
        /// item inside of it.
        ///
        /// The main purpose of this function is to handle two different ways
        /// fingerprints can be invalidated:
        ///
        /// * One is a dependency listed in rustc's dep-info files is invalid. Note
        ///   that these could either be env vars or files. We check both here.
        ///
        /// * Another is the `rerun-if-changed` directive from build scripts. This
        ///   is where we'll find whether files have actually changed
        fn find_stale_item(
            &self,
            mtime_cache: &mut HashMap<PathBuf, FileTime>,
            pkg_root: &Path,
            target_root: &Path,
            cargo_exe: &Path,
            gctx: &GlobalContext,
        ) -> CargoResult<Option<StaleItem>> {
            match self {
                // We need to parse `dep_info`, learn about the crate's dependencies.
                //
                // For each env var we see if our current process's env var still
                // matches, and for each file we see if any of them are newer than
                // the `dep_info` file itself whose mtime represents the start of
                // rustc.
                LocalFingerprint::CheckDepInfo { dep_info } => {
                    let dep_info = target_root.join(dep_info);
                    let Some(info) = parse_dep_info(pkg_root, target_root, &dep_info)? else {
                        return Ok(Some(StaleItem::MissingFile(dep_info)));
                    };
                    for (key, previous) in info.env.iter() {
                        let current = if key == CARGO_ENV {
                            Some(
                                cargo_exe
                                    .to_str()
                                    .ok_or_else(|| {
                                        format_err!(
                                            "cargo exe path {} must be valid UTF-8",
                                            cargo_exe.display()
                                        )
                                    })?
                                    .to_string(),
                            )
                        } else {
                            gctx.get_env(key).ok()
                        };
                        if current == *previous {
                            continue;
                        }
                        return Ok(Some(StaleItem::ChangedEnv {
                            var: key.clone(),
                            previous: previous.clone(),
                            current,
                        }));
                    }
                    Ok(find_stale_file(mtime_cache, &dep_info, info.files.iter()))
                }

                // We need to verify that no paths listed in `paths` are newer than
                // the `output` path itself, or the last time the build script ran.
                LocalFingerprint::RerunIfChanged { output, paths } => Ok(find_stale_file(
                    mtime_cache,
                    &target_root.join(output),
                    paths.iter().map(|p| pkg_root.join(p)),
                )),

                // These have no dependencies on the filesystem, and their values
                // are included natively in the `Fingerprint` hash so nothing
                // tocheck for here.
                LocalFingerprint::RerunIfEnvChanged { .. } => Ok(None),
                LocalFingerprint::Precalculated(..) => Ok(None),
            }
        }

        fn kind(&self) -> &'static str {
            match self {
                LocalFingerprint::Precalculated(..) => "precalculated",
                LocalFingerprint::CheckDepInfo { .. } => "dep-info",
                LocalFingerprint::RerunIfChanged { .. } => "rerun-if-changed",
                LocalFingerprint::RerunIfEnvChanged { .. } => "rerun-if-env-changed",
            }
        }
    }

    impl Fingerprint {
        fn new() -> Fingerprint {
            Fingerprint {
                rustc: 0,
                target: 0,
                profile: 0,
                path: 0,
                features: String::new(),
                declared_features: String::new(),
                deps: Vec::new(),
                local: Mutex::new(Vec::new()),
                memoized_hash: Mutex::new(None),
                rustflags: Vec::new(),
                metadata: 0,
                config: 0,
                compile_kind: 0,
                fs_status: FsStatus::Stale,
                outputs: Vec::new(),
            }
        }

        /// For performance reasons fingerprints will memoize their own hash, but
        /// there's also internal mutability with its `local` field which can
        /// change, for example with build scripts, during a build.
        ///
        /// This method can be used to bust all memoized hashes just before a build
        /// to ensure that after a build completes everything is up-to-date.
        pub fn clear_memoized(&self) {
            *self.memoized_hash.lock().unwrap() = None;
        }

        fn hash_u64(&self) -> u64 {
            if let Some(s) = *self.memoized_hash.lock().unwrap() {
                return s;
            }
            let ret = util::hash_u64(self);
            *self.memoized_hash.lock().unwrap() = Some(ret);
            ret
        }

        /// Compares this fingerprint with an old version which was previously
        /// serialized to filesystem.
        ///
        /// The purpose of this is exclusively to produce a diagnostic message
        /// [`DirtyReason`], indicating why we're recompiling something.
        fn compare(&self, old: &Fingerprint) -> DirtyReason {
            if self.rustc != old.rustc {
                return DirtyReason::RustcChanged;
            }
            if self.features != old.features {
                return DirtyReason::FeaturesChanged {
                    old: old.features.clone(),
                    new: self.features.clone(),
                };
            }
            if self.declared_features != old.declared_features {
                return DirtyReason::DeclaredFeaturesChanged {
                    old: old.declared_features.clone(),
                    new: self.declared_features.clone(),
                };
            }
            if self.target != old.target {
                return DirtyReason::TargetConfigurationChanged;
            }
            if self.path != old.path {
                return DirtyReason::PathToSourceChanged;
            }
            if self.profile != old.profile {
                return DirtyReason::ProfileConfigurationChanged;
            }
            if self.rustflags != old.rustflags {
                return DirtyReason::RustflagsChanged {
                    old: old.rustflags.clone(),
                    new: self.rustflags.clone(),
                };
            }
            if self.metadata != old.metadata {
                return DirtyReason::MetadataChanged;
            }
            if self.config != old.config {
                return DirtyReason::ConfigSettingsChanged;
            }
            if self.compile_kind != old.compile_kind {
                return DirtyReason::CompileKindChanged;
            }
            let my_local = self.local.lock().unwrap();
            let old_local = old.local.lock().unwrap();
            if my_local.len() != old_local.len() {
                return DirtyReason::LocalLengthsChanged;
            }
            for (new, old) in my_local.iter().zip(old_local.iter()) {
                match (new, old) {
                    (LocalFingerprint::Precalculated(a), LocalFingerprint::Precalculated(b)) => {
                        if a != b {
                            return DirtyReason::PrecalculatedComponentsChanged {
                                old: b.to_string(),
                                new: a.to_string(),
                            };
                        }
                    }
                    (
                        LocalFingerprint::CheckDepInfo { dep_info: adep },
                        LocalFingerprint::CheckDepInfo { dep_info: bdep },
                    ) => {
                        if adep != bdep {
                            return DirtyReason::DepInfoOutputChanged {
                                old: bdep.clone(),
                                new: adep.clone(),
                            };
                        }
                    }
                    (
                        LocalFingerprint::RerunIfChanged {
                            output: aout,
                            paths: apaths,
                        },
                        LocalFingerprint::RerunIfChanged {
                            output: bout,
                            paths: bpaths,
                        },
                    ) => {
                        if aout != bout {
                            return DirtyReason::RerunIfChangedOutputFileChanged {
                                old: bout.clone(),
                                new: aout.clone(),
                            };
                        }
                        if apaths != bpaths {
                            return DirtyReason::RerunIfChangedOutputPathsChanged {
                                old: bpaths.clone(),
                                new: apaths.clone(),
                            };
                        }
                    }
                    (
                        LocalFingerprint::RerunIfEnvChanged {
                            var: akey,
                            val: avalue,
                        },
                        LocalFingerprint::RerunIfEnvChanged {
                            var: bkey,
                            val: bvalue,
                        },
                    ) => {
                        if *akey != *bkey {
                            return DirtyReason::EnvVarsChanged {
                                old: bkey.clone(),
                                new: akey.clone(),
                            };
                        }
                        if *avalue != *bvalue {
                            return DirtyReason::EnvVarChanged {
                                name: akey.clone(),
                                old_value: bvalue.clone(),
                                new_value: avalue.clone(),
                            };
                        }
                    }
                    (a, b) => {
                        return DirtyReason::LocalFingerprintTypeChanged {
                            old: b.kind(),
                            new: a.kind(),
                        }
                    }
                }
            }

            if self.deps.len() != old.deps.len() {
                return DirtyReason::NumberOfDependenciesChanged {
                    old: old.deps.len(),
                    new: self.deps.len(),
                };
            }
            for (a, b) in self.deps.iter().zip(old.deps.iter()) {
                if a.name != b.name {
                    return DirtyReason::UnitDependencyNameChanged {
                        old: b.name,
                        new: a.name,
                    };
                }

                if a.fingerprint.hash_u64() != b.fingerprint.hash_u64() {
                    return DirtyReason::UnitDependencyInfoChanged {
                        new_name: a.name,
                        new_fingerprint: a.fingerprint.hash_u64(),
                        old_name: b.name,
                        old_fingerprint: b.fingerprint.hash_u64(),
                    };
                }
            }

            if !self.fs_status.up_to_date() {
                return DirtyReason::FsStatusOutdated(self.fs_status.clone());
            }

            // This typically means some filesystem modifications happened or
            // something transitive was odd. In general we should strive to provide
            // a better error message than this, so if you see this message a lot it
            // likely means this method needs to be updated!
            DirtyReason::NothingObvious
        }

        /// Dynamically inspect the local filesystem to update the `fs_status` field
        /// of this `Fingerprint`.
        ///
        /// This function is used just after a `Fingerprint` is constructed to check
        /// the local state of the filesystem and propagate any dirtiness from
        /// dependencies up to this unit as well. This function assumes that the
        /// unit starts out as [`FsStatus::Stale`] and then it will optionally switch
        /// it to `UpToDate` if it can.
        fn check_filesystem(
            &mut self,
            mtime_cache: &mut HashMap<PathBuf, FileTime>,
            pkg_root: &Path,
            target_root: &Path,
            cargo_exe: &Path,
            gctx: &GlobalContext,
        ) -> CargoResult<()> {
            assert!(!self.fs_status.up_to_date());

            let mut mtimes = HashMap::new();

            // Get the `mtime` of all outputs. Optionally update their mtime
            // afterwards based on the `mtime_on_use` flag. Afterwards we want the
            // minimum mtime as it's the one we'll be comparing to inputs and
            // dependencies.
            for output in self.outputs.iter() {
                let mtime = match paths::mtime(output) {
                    Ok(mtime) => mtime,

                    // This path failed to report its `mtime`. It probably doesn't
                    // exists, so leave ourselves as stale and bail out.
                    Err(e) => {
                        debug!("failed to get mtime of {:?}: {}", output, e);
                        return Ok(());
                    }
                };
                assert!(mtimes.insert(output.clone(), mtime).is_none());
            }

            let opt_max = mtimes.iter().max_by_key(|kv| kv.1);
            let Some((max_path, max_mtime)) = opt_max else {
                // We had no output files. This means we're an overridden build
                // script and we're just always up to date because we aren't
                // watching the filesystem.
                self.fs_status = FsStatus::UpToDate { mtimes };
                return Ok(());
            };
            debug!(
                "max output mtime for {:?} is {:?} {}",
                pkg_root, max_path, max_mtime
            );

            for dep in self.deps.iter() {
                let dep_mtimes = match &dep.fingerprint.fs_status {
                    FsStatus::UpToDate { mtimes } => mtimes,
                    // If our dependency is stale, so are we, so bail out.
                    FsStatus::Stale
                    | FsStatus::StaleItem(_)
                    | FsStatus::StaleDependency { .. }
                    | FsStatus::StaleDepFingerprint { .. } => {
                        self.fs_status = FsStatus::StaleDepFingerprint { name: dep.name };
                        return Ok(());
                    }
                };

                // If our dependency edge only requires the rmeta file to be present
                // then we only need to look at that one output file, otherwise we
                // need to consider all output files to see if we're out of date.
                let (dep_path, dep_mtime) = if dep.only_requires_rmeta {
                    dep_mtimes
                        .iter()
                        .find(|(path, _mtime)| {
                            path.extension().and_then(|s| s.to_str()) == Some("rmeta")
                        })
                        .expect("failed to find rmeta")
                } else {
                    match dep_mtimes.iter().max_by_key(|kv| kv.1) {
                        Some(dep_mtime) => dep_mtime,
                        // If our dependencies is up to date and has no filesystem
                        // interactions, then we can move on to the next dependency.
                        None => continue,
                    }
                };
                debug!(
                    "max dep mtime for {:?} is {:?} {}",
                    pkg_root, dep_path, dep_mtime
                );

                // If the dependency is newer than our own output then it was
                // recompiled previously. We transitively become stale ourselves in
                // that case, so bail out.
                //
                // Note that this comparison should probably be `>=`, not `>`, but
                // for a discussion of why it's `>` see the discussion about #5918
                // below in `find_stale`.
                if dep_mtime > max_mtime {
                    info!(
                        "dependency on `{}` is newer than we are {} > {} {:?}",
                        dep.name, dep_mtime, max_mtime, pkg_root
                    );

                    self.fs_status = FsStatus::StaleDependency {
                        name: dep.name,
                        dep_mtime: *dep_mtime,
                        max_mtime: *max_mtime,
                    };

                    return Ok(());
                }
            }

            // If we reached this far then all dependencies are up to date. Check
            // all our `LocalFingerprint` information to see if we have any stale
            // files for this package itself. If we do find something log a helpful
            // message and bail out so we stay stale.
            for local in self.local.get_mut().unwrap().iter() {
                if let Some(item) =
                    local.find_stale_item(mtime_cache, pkg_root, target_root, cargo_exe, gctx)?
                {
                    item.log();
                    self.fs_status = FsStatus::StaleItem(item);
                    return Ok(());
                }
            }

            // Everything was up to date! Record such.
            self.fs_status = FsStatus::UpToDate { mtimes };
            debug!("filesystem up-to-date {:?}", pkg_root);

            Ok(())
        }
    }

    impl hash::Hash for Fingerprint {
        fn hash<H: Hasher>(&self, h: &mut H) {
            let Fingerprint {
                rustc,
                ref features,
                ref declared_features,
                target,
                path,
                profile,
                ref deps,
                ref local,
                metadata,
                config,
                compile_kind,
                ref rustflags,
                ..
            } = *self;
            let local = local.lock().unwrap();
            (
                rustc,
                features,
                declared_features,
                target,
                path,
                profile,
                &*local,
                metadata,
                config,
                compile_kind,
                rustflags,
            )
                .hash(h);

            h.write_usize(deps.len());
            for DepFingerprint {
                pkg_id,
                name,
                public,
                fingerprint,
                only_requires_rmeta: _, // static property, no need to hash
            } in deps
            {
                pkg_id.hash(h);
                name.hash(h);
                public.hash(h);
                // use memoized dep hashes to avoid exponential blowup
                h.write_u64(fingerprint.hash_u64());
            }
        }
    }

    impl DepFingerprint {
        fn new(
            build_runner: &mut BuildRunner<'_, '_>,
            parent: &Unit,
            dep: &UnitDep,
        ) -> CargoResult<DepFingerprint> {
            let fingerprint = calculate(build_runner, &dep.unit)?;
            // We need to be careful about what we hash here. We have a goal of
            // supporting renaming a project directory and not rebuilding
            // everything. To do that, however, we need to make sure that the cwd
            // doesn't make its way into any hashes, and one source of that is the
            // `SourceId` for `path` packages.
            //
            // We already have a requirement that `path` packages all have unique
            // names (sort of for this same reason), so if the package source is a
            // `path` then we just hash the name, but otherwise we hash the full
            // id as it won't change when the directory is renamed.
            let pkg_id = if dep.unit.pkg.package_id().source_id().is_path() {
                util::hash_u64(dep.unit.pkg.package_id().name())
            } else {
                util::hash_u64(dep.unit.pkg.package_id())
            };

            Ok(DepFingerprint {
                pkg_id,
                name: dep.extern_crate_name,
                public: dep.public,
                fingerprint,
                only_requires_rmeta: build_runner.only_requires_rmeta(parent, &dep.unit),
            })
        }
    }

    impl StaleItem {
        /// Use the `log` crate to log a hopefully helpful message in diagnosing
        /// what file is considered stale and why. This is intended to be used in
        /// conjunction with `CARGO_LOG` to determine why Cargo is recompiling
        /// something. Currently there's no user-facing usage of this other than
        /// that.
        fn log(&self) {
            match self {
                StaleItem::MissingFile(path) => {
                    info!("stale: missing {:?}", path);
                }
                StaleItem::ChangedFile {
                    reference,
                    reference_mtime,
                    stale,
                    stale_mtime,
                } => {
                    info!("stale: changed {:?}", stale);
                    info!("          (vs) {:?}", reference);
                    info!("               {:?} < {:?}", reference_mtime, stale_mtime);
                }
                StaleItem::ChangedEnv {
                    var,
                    previous,
                    current,
                } => {
                    info!("stale: changed env {:?}", var);
                    info!("       {:?} != {:?}", previous, current);
                }
            }
        }
    }

    /// Calculates the fingerprint for a [`Unit`].
    ///
    /// This fingerprint is used by Cargo to learn about when information such as:
    ///
    /// * A non-path package changes (changes version, changes revision, etc).
    /// * Any dependency changes
    /// * The compiler changes
    /// * The set of features a package is built with changes
    /// * The profile a target is compiled with changes (e.g., opt-level changes)
    /// * Any other compiler flags change that will affect the result
    ///
    /// Information like file modification time is only calculated for path
    /// dependencies.
    fn calculate(build_runner: &mut BuildRunner<'_, '_>, unit: &Unit) -> CargoResult<Arc<Fingerprint>> {
        // This function is slammed quite a lot, so the result is memoized.
        if let Some(s) = build_runner.fingerprints.get(unit) {
            return Ok(Arc::clone(s));
        }
        let mut fingerprint = if unit.mode.is_run_custom_build() {
            calculate_run_custom_build(build_runner, unit)?
        } else if unit.mode.is_doc_test() {
            panic!("doc tests do not fingerprint");
        } else {
            calculate_normal(build_runner, unit)?
        };

        // After we built the initial `Fingerprint` be sure to update the
        // `fs_status` field of it.
        let target_root = target_root(build_runner);
        let cargo_exe = build_runner.bcx.gctx.cargo_exe()?;
        fingerprint.check_filesystem(
            &mut build_runner.mtime_cache,
            unit.pkg.root(),
            &target_root,
            cargo_exe,
            build_runner.bcx.gctx,
        )?;

        let fingerprint = Arc::new(fingerprint);
        build_runner
            .fingerprints
            .insert(unit.clone(), Arc::clone(&fingerprint));
        Ok(fingerprint)
    }

    /// Calculate a fingerprint for a "normal" unit, or anything that's not a build
    /// script. This is an internal helper of [`calculate`], don't call directly.
    fn calculate_normal(
        build_runner: &mut BuildRunner<'_, '_>,
        unit: &Unit,
    ) -> CargoResult<Fingerprint> {
        let deps = {
            // Recursively calculate the fingerprint for all of our dependencies.
            //
            // Skip fingerprints of binaries because they don't actually induce a
            // recompile, they're just dependencies in the sense that they need to be
            // built. The only exception here are artifact dependencies,
            // which is an actual dependency that needs a recompile.
            //
            // Create Vec since mutable build_runner is needed in closure.
            let deps = Vec::from(build_runner.unit_deps(unit));
            let mut deps = deps
                .into_iter()
                .filter(|dep| !dep.unit.target.is_bin() || dep.unit.artifact.is_true())
                .map(|dep| DepFingerprint::new(build_runner, unit, &dep))
                .collect::<CargoResult<Vec<_>>>()?;
            deps.sort_by(|a, b| a.pkg_id.cmp(&b.pkg_id));
            deps
        };

        // Afterwards calculate our own fingerprint information.
        let target_root = target_root(build_runner);
        let local = if unit.mode.is_doc() || unit.mode.is_doc_scrape() {
            // rustdoc does not have dep-info files.
            let fingerprint = pkg_fingerprint(build_runner.bcx, &unit.pkg).with_context(|| {
                format!(
                    "failed to determine package fingerprint for documenting {}",
                    unit.pkg
                )
            })?;
            vec![LocalFingerprint::Precalculated(fingerprint)]
        } else {
            let dep_info = dep_info_loc(build_runner, unit);
            let dep_info = dep_info.strip_prefix(&target_root).unwrap().to_path_buf();
            vec![LocalFingerprint::CheckDepInfo { dep_info }]
        };

        // Figure out what the outputs of our unit is, and we'll be storing them
        // into the fingerprint as well.
        let outputs = build_runner
            .outputs(unit)?
            .iter()
            .filter(|output| !matches!(output.flavor, FileFlavor::DebugInfo | FileFlavor::Auxiliary))
            .map(|output| output.path.clone())
            .collect();

        // Fill out a bunch more information that we'll be tracking typically
        // hashed to take up less space on disk as we just need to know when things
        // change.
        let extra_flags = if unit.mode.is_doc() || unit.mode.is_doc_scrape() {
            build_runner.bcx.rustdocflags_args(unit)
        } else {
            build_runner.bcx.rustflags_args(unit)
        }
        .to_vec();

        // Include all the args from `[lints.rust.unexpected_cfgs.check-cfg]`
        //
        // HACK(#13975): duplicating the lookup logic here until `--check-cfg` is supported
        // on Cargo's MSRV and we can centralize the logic in `lints_to_rustflags`
        let mut lint_check_cfg = Vec::new();
        if let Ok(Some(lints)) = unit.pkg.manifest().resolved_toml().resolved_lints() {
            if let Some(rust_lints) = lints.get("rust") {
                if let Some(unexpected_cfgs) = rust_lints.get("unexpected_cfgs") {
                    if let Some(config) = unexpected_cfgs.config() {
                        if let Some(check_cfg) = config.get("check-cfg") {
                            if let Ok(check_cfgs) =
                                toml::Value::try_into::<Vec<String>>(check_cfg.clone())
                            {
                                lint_check_cfg = check_cfgs;
                            }
                        }
                    }
                }
            }
        }

        let profile_hash = util::hash_u64((
            &unit.profile,
            unit.mode,
            build_runner.bcx.extra_args_for(unit),
            build_runner.lto[unit],
            unit.pkg.manifest().lint_rustflags(),
            lint_check_cfg,
        ));
        // Include metadata since it is exposed as environment variables.
        let m = unit.pkg.manifest().metadata();
        let metadata = util::hash_u64((&m.authors, &m.description, &m.homepage, &m.repository));
        let mut config = StableHasher::new();
        if let Some(linker) = build_runner.compilation.target_linker(unit.kind) {
            linker.hash(&mut config);
        }
        if unit.mode.is_doc() && build_runner.bcx.gctx.cli_unstable().rustdoc_map {
            if let Ok(map) = build_runner.bcx.gctx.doc_extern_map() {
                map.hash(&mut config);
            }
        }
        if let Some(allow_features) = &build_runner.bcx.gctx.cli_unstable().allow_features {
            allow_features.hash(&mut config);
        }
        let compile_kind = unit.kind.fingerprint_hash();
        let mut declared_features = unit.pkg.summary().features().keys().collect::<Vec<_>>();
        declared_features.sort(); // to avoid useless rebuild if the user orders it's features
                                  // differently
        Ok(Fingerprint {
            rustc: util::hash_u64(&build_runner.bcx.rustc().verbose_version),
            target: util::hash_u64(&unit.target),
            profile: profile_hash,
            // Note that .0 is hashed here, not .1 which is the cwd. That doesn't
            // actually affect the output artifact so there's no need to hash it.
            path: util::hash_u64(path_args(build_runner.bcx.ws, unit).0),
            features: format!("{:?}", unit.features),
            declared_features: format!("{declared_features:?}"),
            deps,
            local: Mutex::new(local),
            memoized_hash: Mutex::new(None),
            metadata,
            config: config.finish(),
            compile_kind,
            rustflags: extra_flags,
            fs_status: FsStatus::Stale,
            outputs,
        })
    }

    /// Calculate a fingerprint for an "execute a build script" unit.  This is an
    /// internal helper of [`calculate`], don't call directly.
    fn calculate_run_custom_build(
        build_runner: &mut BuildRunner<'_, '_>,
        unit: &Unit,
    ) -> CargoResult<Fingerprint> {
        assert!(unit.mode.is_run_custom_build());
        // Using the `BuildDeps` information we'll have previously parsed and
        // inserted into `build_explicit_deps` built an initial snapshot of the
        // `LocalFingerprint` list for this build script. If we previously executed
        // the build script this means we'll be watching files and env vars.
        // Otherwise if we haven't previously executed it we'll just start watching
        // the whole crate.
        let (gen_local, overridden) = build_script_local_fingerprints(build_runner, unit);
        let deps = &build_runner.build_explicit_deps[unit];
        let local = (gen_local)(
            deps,
            Some(&|| {
                const IO_ERR_MESSAGE: &str = "\
    An I/O error happened. Please make sure you can access the file.

    By default, if your project contains a build script, cargo scans all files in
    it to determine whether a rebuild is needed. If you don't expect to access the
    file, specify `rerun-if-changed` in your build script.
    See https://doc.rust-lang.org/cargo/reference/build-scripts.html#rerun-if-changed for more information.";
                pkg_fingerprint(build_runner.bcx, &unit.pkg).map_err(|err| {
                    let mut message = format!("failed to determine package fingerprint for build script for {}", unit.pkg);
                    if err.root_cause().is::<io::Error>() {
                        message = format!("{}\n{}", message, IO_ERR_MESSAGE)
                    }
                    err.context(message)
                })
            }),
        )?
        .unwrap();
        let output = deps.build_script_output.clone();

        // Include any dependencies of our execution, which is typically just the
        // compilation of the build script itself. (if the build script changes we
        // should be rerun!). Note though that if we're an overridden build script
        // we have no dependencies so no need to recurse in that case.
        let deps = if overridden {
            // Overridden build scripts don't need to track deps.
            vec![]
        } else {
            // Create Vec since mutable build_runner is needed in closure.
            let deps = Vec::from(build_runner.unit_deps(unit));
            deps.into_iter()
                .map(|dep| DepFingerprint::new(build_runner, unit, &dep))
                .collect::<CargoResult<Vec<_>>>()?
        };

        let rustflags = build_runner.bcx.rustflags_args(unit).to_vec();

        Ok(Fingerprint {
            local: Mutex::new(local),
            rustc: util::hash_u64(&build_runner.bcx.rustc().verbose_version),
            deps,
            outputs: if overridden { Vec::new() } else { vec![output] },
            rustflags,

            // Most of the other info is blank here as we don't really include it
            // in the execution of the build script, but... this may be a latent
            // bug in Cargo.
            ..Fingerprint::new()
        })
    }

    /// Get ready to compute the [`LocalFingerprint`] values
    /// for a [`RunCustomBuild`] unit.
    ///
    /// This function has, what's on the surface, a seriously wonky interface.
    /// You'll call this function and it'll return a closure and a boolean. The
    /// boolean is pretty simple in that it indicates whether the `unit` has been
    /// overridden via `.cargo/config.toml`. The closure is much more complicated.
    ///
    /// This closure is intended to capture any local state necessary to compute
    /// the `LocalFingerprint` values for this unit. It is `Send` and `'static` to
    /// be sent to other threads as well (such as when we're executing build
    /// scripts). That deduplication is the rationale for the closure at least.
    ///
    /// The arguments to the closure are a bit weirder, though, and I'll apologize
    /// in advance for the weirdness too. The first argument to the closure is a
    /// `&BuildDeps`. This is the parsed version of a build script, and when Cargo
    /// starts up this is cached from previous runs of a build script.  After a
    /// build script executes the output file is reparsed and passed in here.
    ///
    /// The second argument is the weirdest, it's *optionally* a closure to
    /// call [`pkg_fingerprint`]. The `pkg_fingerprint` requires access to
    /// "source map" located in `Context`. That's very non-`'static` and
    /// non-`Send`, so it can't be used on other threads, such as when we invoke
    /// this after a build script has finished. The `Option` allows us to for sure
    /// calculate it on the main thread at the beginning, and then swallow the bug
    /// for now where a worker thread after a build script has finished doesn't
    /// have access. Ideally there would be no second argument or it would be more
    /// "first class" and not an `Option` but something that can be sent between
    /// threads. In any case, it's a bug for now.
    ///
    /// This isn't the greatest of interfaces, and if there's suggestions to
    /// improve please do so!
    ///
    /// FIXME(#6779) - see all the words above
    ///
    /// [`RunCustomBuild`]: crate::core::compiler::CompileMode::RunCustomBuild
    fn build_script_local_fingerprints(
        build_runner: &mut BuildRunner<'_, '_>,
        unit: &Unit,
    ) -> (
        Box<
            dyn FnOnce(
                    &BuildDeps,
                    Option<&dyn Fn() -> CargoResult<String>>,
                ) -> CargoResult<Option<Vec<LocalFingerprint>>>
                + Send,
        >,
        bool,
    ) {
        assert!(unit.mode.is_run_custom_build());
        // First up, if this build script is entirely overridden, then we just
        // return the hash of what we overrode it with. This is the easy case!
        if let Some(fingerprint) = build_script_override_fingerprint(build_runner, unit) {
            debug!("override local fingerprints deps {}", unit.pkg);
            return (
                Box::new(
                    move |_: &BuildDeps, _: Option<&dyn Fn() -> CargoResult<String>>| {
                        Ok(Some(vec![fingerprint]))
                    },
                ),
                true, // this is an overridden build script
            );
        }

        // ... Otherwise this is a "real" build script and we need to return a real
        // closure. Our returned closure classifies the build script based on
        // whether it prints `rerun-if-*`. If it *doesn't* print this it's where the
        // magical second argument comes into play, which fingerprints a whole
        // package. Remember that the fact that this is an `Option` is a bug, but a
        // longstanding bug, in Cargo. Recent refactorings just made it painfully
        // obvious.
        let pkg_root = unit.pkg.root().to_path_buf();
        let target_dir = target_root(build_runner);
        let calculate =
            move |deps: &BuildDeps, pkg_fingerprint: Option<&dyn Fn() -> CargoResult<String>>| {
                if deps.rerun_if_changed.is_empty() && deps.rerun_if_env_changed.is_empty() {
                    match pkg_fingerprint {
                        // FIXME: this is somewhat buggy with respect to docker and
                        // weird filesystems. The `Precalculated` variant
                        // constructed below will, for `path` dependencies, contain
                        // a stringified version of the mtime for the local crate.
                        // This violates one of the things we describe in this
                        // module's doc comment, never hashing mtimes. We should
                        // figure out a better scheme where a package fingerprint
                        // may be a string (like for a registry) or a list of files
                        // (like for a path dependency). Those list of files would
                        // be stored here rather than the mtime of them.
                        Some(f) => {
                            let s = f()?;
                            debug!(
                                "old local fingerprints deps {:?} precalculated={:?}",
                                pkg_root, s
                            );
                            return Ok(Some(vec![LocalFingerprint::Precalculated(s)]));
                        }
                        None => return Ok(None),
                    }
                }

                // Ok so now we're in "new mode" where we can have files listed as
                // dependencies as well as env vars listed as dependencies. Process
                // them all here.
                Ok(Some(local_fingerprints_deps(deps, &target_dir, &pkg_root)))
            };

        // Note that `false` == "not overridden"
        (Box::new(calculate), false)
    }

    /// Create a [`LocalFingerprint`] for an overridden build script.
    /// Returns None if it is not overridden.
    fn build_script_override_fingerprint(
        build_runner: &mut BuildRunner<'_, '_>,
        unit: &Unit,
    ) -> Option<LocalFingerprint> {
        // Build script output is only populated at this stage when it is
        // overridden.
        let build_script_outputs = build_runner.build_script_outputs.lock().unwrap();
        let metadata = build_runner.get_run_build_script_metadata(unit);
        // Returns None if it is not overridden.
        let output = build_script_outputs.get(metadata)?;
        let s = format!(
            "overridden build state with hash: {}",
            util::hash_u64(output)
        );
        Some(LocalFingerprint::Precalculated(s))
    }

    /// Compute the [`LocalFingerprint`] values for a [`RunCustomBuild`] unit for
    /// non-overridden new-style build scripts only. This is only used when `deps`
    /// is already known to have a nonempty `rerun-if-*` somewhere.
    ///
    /// [`RunCustomBuild`]: crate::core::compiler::CompileMode::RunCustomBuild
    fn local_fingerprints_deps(
        deps: &BuildDeps,
        target_root: &Path,
        pkg_root: &Path,
    ) -> Vec<LocalFingerprint> {
        debug!("new local fingerprints deps {:?}", pkg_root);
        let mut local = Vec::new();

        if !deps.rerun_if_changed.is_empty() {
            // Note that like the module comment above says we are careful to never
            // store an absolute path in `LocalFingerprint`, so ensure that we strip
            // absolute prefixes from them.
            let output = deps
                .build_script_output
                .strip_prefix(target_root)
                .unwrap()
                .to_path_buf();
            let paths = deps
                .rerun_if_changed
                .iter()
                .map(|p| p.strip_prefix(pkg_root).unwrap_or(p).to_path_buf())
                .collect();
            local.push(LocalFingerprint::RerunIfChanged { output, paths });
        }

        local.extend(
            deps.rerun_if_env_changed
                .iter()
                .map(LocalFingerprint::from_env),
        );

        local
    }

    /// Writes the short fingerprint hash value to `<loc>`
    /// and logs detailed JSON information to `<loc>.json`.
    fn write_fingerprint(loc: &Path, fingerprint: &Fingerprint) -> CargoResult<()> {
        debug_assert_ne!(fingerprint.rustc, 0);
        // fingerprint::new().rustc == 0, make sure it doesn't make it to the file system.
        // This is mostly so outside tools can reliably find out what rust version this file is for,
        // as we can use the full hash.
        let hash = fingerprint.hash_u64();
        debug!("write fingerprint ({:x}) : {}", hash, loc.display());
        paths::write(loc, util::to_hex(hash).as_bytes())?;

        let json = serde_json::to_string(fingerprint).unwrap();
        if cfg!(debug_assertions) {
            let f: Fingerprint = serde_json::from_str(&json).unwrap();
            assert_eq!(f.hash_u64(), hash);
        }
        paths::write(&loc.with_extension("json"), json.as_bytes())?;
        Ok(())
    }

    /// Prepare for work when a package starts to build
    pub fn prepare_init(build_runner: &mut BuildRunner<'_, '_>, unit: &Unit) -> CargoResult<()> {
        let new1 = build_runner.files().fingerprint_dir(unit);

        // Doc tests have no output, thus no fingerprint.
        if !new1.exists() && !unit.mode.is_doc_test() {
            paths::create_dir_all(&new1)?;
        }

        Ok(())
    }

    /// Returns the location that the dep-info file will show up at
    /// for the [`Unit`] specified.
    pub fn dep_info_loc(build_runner: &mut BuildRunner<'_, '_>, unit: &Unit) -> PathBuf {
        build_runner.files().fingerprint_file_path(unit, "dep-")
    }

    /// Returns an absolute path that target directory.
    /// All paths are rewritten to be relative to this.
    fn target_root(build_runner: &BuildRunner<'_, '_>) -> PathBuf {
        build_runner.bcx.ws.target_dir().into_path_unlocked()
    }

    /// Reads the value from the old fingerprint hash file and compare.
    ///
    /// If dirty, it then restores the detailed information
    /// from the fingerprint JSON file, and provides an rich dirty reason.
    fn compare_old_fingerprint(
        unit: &Unit,
        old_hash_path: &Path,
        new_fingerprint: &Fingerprint,
        mtime_on_use: bool,
        forced: bool,
    ) -> Option<DirtyReason> {
        if mtime_on_use {
            // update the mtime so other cleaners know we used it
            let t = FileTime::from_system_time(SystemTime::now());
            debug!("mtime-on-use forcing {:?} to {}", old_hash_path, t);
            paths::set_file_time_no_err(old_hash_path, t);
        }

        let compare = _compare_old_fingerprint(old_hash_path, new_fingerprint);

        match compare.as_ref() {
            Ok(None) => {}
            Ok(Some(reason)) => {
                info!(
                    "fingerprint dirty for {}/{:?}/{:?}",
                    unit.pkg, unit.mode, unit.target,
                );
                info!("    dirty: {reason:?}");
            }
            Err(e) => {
                info!(
                    "fingerprint error for {}/{:?}/{:?}",
                    unit.pkg, unit.mode, unit.target,
                );
                info!("    err: {e:?}");
            }
        }

        match compare {
            Ok(None) if forced => Some(DirtyReason::Forced),
            Ok(reason) => reason,
            Err(_) => Some(DirtyReason::FreshBuild),
        }
    }

    fn _compare_old_fingerprint(
        old_hash_path: &Path,
        new_fingerprint: &Fingerprint,
    ) -> CargoResult<Option<DirtyReason>> {
        let old_fingerprint_short = paths::read(old_hash_path)?;

        let new_hash = new_fingerprint.hash_u64();

        if util::to_hex(new_hash) == old_fingerprint_short && new_fingerprint.fs_status.up_to_date() {
            return Ok(None);
        }

        let old_fingerprint_json = paths::read(&old_hash_path.with_extension("json"))?;
        let old_fingerprint: Fingerprint = serde_json::from_str(&old_fingerprint_json)
            .with_context(|| internal("failed to deserialize json"))?;
        // Fingerprint can be empty after a failed rebuild (see comment in prepare_target).
        if !old_fingerprint_short.is_empty() {
            debug_assert_eq!(
                util::to_hex(old_fingerprint.hash_u64()),
                old_fingerprint_short
            );
        }

        Ok(Some(new_fingerprint.compare(&old_fingerprint)))
    }

    /// Parses Cargo's internal [`EncodedDepInfo`] structure that was previously
    /// serialized to disk.
    ///
    /// Note that this is not rustc's `*.d` files.
    ///
    /// Also note that rustc's `*.d` files are translated to Cargo-specific
    /// `EncodedDepInfo` files after compilations have finished in
    /// [`translate_dep_info`].
    ///
    /// Returns `None` if the file is corrupt or couldn't be read from disk. This
    /// indicates that the crate should likely be rebuilt.
    pub fn parse_dep_info(
        pkg_root: &Path,
        target_root: &Path,
        dep_info: &Path,
    ) -> CargoResult<Option<RustcDepInfo>> {
        let Ok(data) = paths::read_bytes(dep_info) else {
            return Ok(None);
        };
        let Some(info) = EncodedDepInfo::parse(&data) else {
            tracing::warn!("failed to parse cargo's dep-info at {:?}", dep_info);
            return Ok(None);
        };
        let mut ret = RustcDepInfo::default();
        ret.env = info.env;
        ret.files.extend(info.files.into_iter().map(|(ty, path)| {
            match ty {
                DepInfoPathType::PackageRootRelative => pkg_root.join(path),
                // N.B. path might be absolute here in which case the join will have no effect
                DepInfoPathType::TargetRootRelative => target_root.join(path),
            }
        }));
        Ok(Some(ret))
    }

    /// Calculates the fingerprint of a unit thats contains no dep-info files.
    fn pkg_fingerprint(bcx: &BuildContext<'_, '_>, pkg: &Package) -> CargoResult<String> {
        let source_id = pkg.package_id().source_id();
        let sources = bcx.packages.sources();

        let source = sources
            .get(source_id)
            .ok_or_else(|| internal("missing package source"))?;
        source.fingerprint(pkg)
    }

    /// The `reference` file is considered as "stale" if any file from `paths` has a newer mtime.
    fn find_stale_file<I>(
        mtime_cache: &mut HashMap<PathBuf, FileTime>,
        reference: &Path,
        paths: I,
    ) -> Option<StaleItem>
    where
        I: IntoIterator,
        I::Item: AsRef<Path>,
    {
        let Ok(reference_mtime) = paths::mtime(reference) else {
            return Some(StaleItem::MissingFile(reference.to_path_buf()));
        };

        let skipable_dirs = if let Ok(cargo_home) = home::cargo_home() {
            let skipable_dirs: Vec<_> = ["git", "registry"]
                .into_iter()
                .map(|subfolder| cargo_home.join(subfolder))
                .collect();
            Some(skipable_dirs)
        } else {
            None
        };

        for path in paths {
            let path = path.as_ref();

            // Assuming anything in cargo_home/{git, registry} is immutable
            // (see also #9455 about marking the src directory readonly) which avoids rebuilds when CI
            // caches $CARGO_HOME/registry/{index, cache} and $CARGO_HOME/git/db across runs, keeping
            // the content the same but changing the mtime.
            if let Some(ref skipable_dirs) = skipable_dirs {
                if skipable_dirs.iter().any(|dir| path.starts_with(dir)) {
                    continue;
                }
            }
            let path_mtime = match mtime_cache.entry(path.to_path_buf()) {
                Entry::Occupied(o) => *o.get(),
                Entry::Vacant(v) => {
                    let Ok(mtime) = paths::mtime_recursive(path) else {
                        return Some(StaleItem::MissingFile(path.to_path_buf()));
                    };
                    *v.insert(mtime)
                }
            };

            // TODO: fix #5918.
            // Note that equal mtimes should be considered "stale". For filesystems with
            // not much timestamp precision like 1s this is would be a conservative approximation
            // to handle the case where a file is modified within the same second after
            // a build starts. We want to make sure that incremental rebuilds pick that up!
            //
            // For filesystems with nanosecond precision it's been seen in the wild that
            // its "nanosecond precision" isn't really nanosecond-accurate. It turns out that
            // kernels may cache the current time so files created at different times actually
            // list the same nanosecond precision. Some digging on #5919 picked up that the
            // kernel caches the current time between timer ticks, which could mean that if
            // a file is updated at most 10ms after a build starts then Cargo may not
            // pick up the build changes.
            //
            // All in all, an equality check here would be a conservative assumption that,
            // if equal, files were changed just after a previous build finished.
            // Unfortunately this became problematic when (in #6484) cargo switch to more accurately
            // measuring the start time of builds.
            if path_mtime <= reference_mtime {
                continue;
            }

            return Some(StaleItem::ChangedFile {
                reference: reference.to_path_buf(),
                reference_mtime,
                stale: path.to_path_buf(),
                stale_mtime: path_mtime,
            });
        }

        debug!(
            "all paths up-to-date relative to {:?} mtime={}",
            reference, reference_mtime
        );
        None
    }

    /// Tells the associated path in [`EncodedDepInfo::files`] is relative to package root,
    /// target root, or absolute.
    enum DepInfoPathType {
        /// src/, e.g. src/lib.rs
        PackageRootRelative,
        /// target/debug/deps/lib...
        /// or an absolute path /.../sysroot/...
        TargetRootRelative,
    }

    /// Parses the dep-info file coming out of rustc into a Cargo-specific format.
    ///
    /// This function will parse `rustc_dep_info` as a makefile-style dep info to
    /// learn about the all files which a crate depends on. This is then
    /// re-serialized into the `cargo_dep_info` path in a Cargo-specific format.
    ///
    /// The `pkg_root` argument here is the absolute path to the directory
    /// containing `Cargo.toml` for this crate that was compiled. The paths listed
    /// in the rustc dep-info file may or may not be absolute but we'll want to
    /// consider all of them relative to the `root` specified.
    ///
    /// The `rustc_cwd` argument is the absolute path to the cwd of the compiler
    /// when it was invoked.
    ///
    /// If the `allow_package` argument is true, then package-relative paths are
    /// included. If it is false, then package-relative paths are skipped and
    /// ignored (typically used for registry or git dependencies where we assume
    /// the source never changes, and we don't want the cost of running `stat` on
    /// all those files). See the module-level docs for the note about
    /// `-Zbinary-dep-depinfo` for more details on why this is done.
    ///
    /// The serialized Cargo format will contain a list of files, all of which are
    /// relative if they're under `root`. or absolute if they're elsewhere.
    pub fn translate_dep_info(
        rustc_dep_info: &Path,
        cargo_dep_info: &Path,
        rustc_cwd: &Path,
        pkg_root: &Path,
        target_root: &Path,
        rustc_cmd: &ProcessBuilder,
        allow_package: bool,
    ) -> CargoResult<()> {
        let depinfo = parse_rustc_dep_info(rustc_dep_info)?;

        let target_root = try_canonicalize(target_root)?;
        let pkg_root = try_canonicalize(pkg_root)?;
        let mut on_disk_info = EncodedDepInfo::default();
        on_disk_info.env = depinfo.env;

        // This is a bit of a tricky statement, but here we're *removing* the
        // dependency on environment variables that were defined specifically for
        // the command itself. Environment variables returned by `get_envs` includes
        // environment variables like:
        //
        // * `OUT_DIR` if applicable
        // * env vars added by a build script, if any
        //
        // The general idea here is that the dep info file tells us what, when
        // changed, should cause us to rebuild the crate. These environment
        // variables are synthesized by Cargo and/or the build script, and the
        // intention is that their values are tracked elsewhere for whether the
        // crate needs to be rebuilt.
        //
        // For example a build script says when it needs to be rerun and otherwise
        // it's assumed to produce the same output, so we're guaranteed that env
        // vars defined by the build script will always be the same unless the build
        // script itself reruns, in which case the crate will rerun anyway.
        //
        // For things like `OUT_DIR` it's a bit sketchy for now. Most of the time
        // that's used for code generation but this is technically buggy where if
        // you write a binary that does `println!("{}", env!("OUT_DIR"))` we won't
        // recompile that if you move the target directory. Hopefully that's not too
        // bad of an issue for now...
        //
        // This also includes `CARGO` since if the code is explicitly wanting to
        // know that path, it should be rebuilt if it changes. The CARGO path is
        // not tracked elsewhere in the fingerprint.
        on_disk_info
            .env
            .retain(|(key, _)| !rustc_cmd.get_envs().contains_key(key) || key == CARGO_ENV);

        for file in depinfo.files {
            // The path may be absolute or relative, canonical or not. Make sure
            // it is canonicalized so we are comparing the same kinds of paths.
            let abs_file = rustc_cwd.join(file);
            // If canonicalization fails, just use the abs path. There is currently
            // a bug where --remap-path-prefix is affecting .d files, causing them
            // to point to non-existent paths.
            let canon_file = try_canonicalize(&abs_file).unwrap_or_else(|_| abs_file.clone());

            let (ty, path) = if let Ok(stripped) = canon_file.strip_prefix(&target_root) {
                (DepInfoPathType::TargetRootRelative, stripped)
            } else if let Ok(stripped) = canon_file.strip_prefix(&pkg_root) {
                if !allow_package {
                    continue;
                }
                (DepInfoPathType::PackageRootRelative, stripped)
            } else {
                // It's definitely not target root relative, but this is an absolute path (since it was
                // joined to rustc_cwd) and as such re-joining it later to the target root will have no
                // effect.
                (DepInfoPathType::TargetRootRelative, &*abs_file)
            };
            on_disk_info.files.push((ty, path.to_owned()));
        }
        paths::write(cargo_dep_info, on_disk_info.serialize()?)?;
        Ok(())
    }

    /// The representation of the `.d` dep-info file generated by rustc
    #[derive(Default)]
    pub struct RustcDepInfo {
        /// The list of files that the main target in the dep-info file depends on.
        pub files: Vec<PathBuf>,
        /// The list of environment variables we found that the rustc compilation
        /// depends on.
        ///
        /// The first element of the pair is the name of the env var and the second
        /// item is the value. `Some` means that the env var was set, and `None`
        /// means that the env var wasn't actually set and the compilation depends
        /// on it not being set.
        pub env: Vec<(String, Option<String>)>,
    }

    /// Same as [`RustcDepInfo`] except avoids absolute paths as much as possible to
    /// allow moving around the target directory.
    ///
    /// This is also stored in an optimized format to make parsing it fast because
    /// Cargo will read it for crates on all future compilations.
    #[derive(Default)]
    struct EncodedDepInfo {
        files: Vec<(DepInfoPathType, PathBuf)>,
        env: Vec<(String, Option<String>)>,
    }

    impl EncodedDepInfo {
        fn parse(mut bytes: &[u8]) -> Option<EncodedDepInfo> {
            let bytes = &mut bytes;
            let nfiles = read_usize(bytes)?;
            let mut files = Vec::with_capacity(nfiles as usize);
            for _ in 0..nfiles {
                let ty = match read_u8(bytes)? {
                    0 => DepInfoPathType::PackageRootRelative,
                    1 => DepInfoPathType::TargetRootRelative,
                    _ => return None,
                };
                let bytes = read_bytes(bytes)?;
                files.push((ty, paths::bytes2path(bytes).ok()?));
            }

            let nenv = read_usize(bytes)?;
            let mut env = Vec::with_capacity(nenv as usize);
            for _ in 0..nenv {
                let key = str::from_utf8(read_bytes(bytes)?).ok()?.to_string();
                let val = match read_u8(bytes)? {
                    0 => None,
                    1 => Some(str::from_utf8(read_bytes(bytes)?).ok()?.to_string()),
                    _ => return None,
                };
                env.push((key, val));
            }
            return Some(EncodedDepInfo { files, env });

            fn read_usize(bytes: &mut &[u8]) -> Option<usize> {
                let ret = bytes.get(..4)?;
                *bytes = &bytes[4..];
                Some(u32::from_le_bytes(ret.try_into().unwrap()) as usize)
            }

            fn read_u8(bytes: &mut &[u8]) -> Option<u8> {
                let ret = *bytes.get(0)?;
                *bytes = &bytes[1..];
                Some(ret)
            }

            fn read_bytes<'a>(bytes: &mut &'a [u8]) -> Option<&'a [u8]> {
                let n = read_usize(bytes)? as usize;
                let ret = bytes.get(..n)?;
                *bytes = &bytes[n..];
                Some(ret)
            }
        }

        fn serialize(&self) -> CargoResult<Vec<u8>> {
            let mut ret = Vec::new();
            let dst = &mut ret;
            write_usize(dst, self.files.len());
            for (ty, file) in self.files.iter() {
                match ty {
                    DepInfoPathType::PackageRootRelative => dst.push(0),
                    DepInfoPathType::TargetRootRelative => dst.push(1),
                }
                write_bytes(dst, paths::path2bytes(file)?);
            }

            write_usize(dst, self.env.len());
            for (key, val) in self.env.iter() {
                write_bytes(dst, key);
                match val {
                    None => dst.push(0),
                    Some(val) => {
                        dst.push(1);
                        write_bytes(dst, val);
                    }
                }
            }
            return Ok(ret);

            fn write_bytes(dst: &mut Vec<u8>, val: impl AsRef<[u8]>) {
                let val = val.as_ref();
                write_usize(dst, val.len());
                dst.extend_from_slice(val);
            }

            fn write_usize(dst: &mut Vec<u8>, val: usize) {
                dst.extend(&u32::to_le_bytes(val as u32));
            }
        }
    }

    /// Parse the `.d` dep-info file generated by rustc.
    pub fn parse_rustc_dep_info(rustc_dep_info: &Path) -> CargoResult<RustcDepInfo> {
        let contents = paths::read(rustc_dep_info)?;
        let mut ret = RustcDepInfo::default();
        let mut found_deps = false;

        for line in contents.lines() {
            if let Some(rest) = line.strip_prefix("# env-dep:") {
                let mut parts = rest.splitn(2, '=');
                let Some(env_var) = parts.next() else {
                    continue;
                };
                let env_val = match parts.next() {
                    Some(s) => Some(unescape_env(s)?),
                    None => None,
                };
                ret.env.push((unescape_env(env_var)?, env_val));
            } else if let Some(pos) = line.find(": ") {
                if found_deps {
                    continue;
                }
                found_deps = true;
                let mut deps = line[pos + 2..].split_whitespace();

                while let Some(s) = deps.next() {
                    let mut file = s.to_string();
                    while file.ends_with('\\') {
                        file.pop();
                        file.push(' ');
                        file.push_str(deps.next().ok_or_else(|| {
                            internal("malformed dep-info format, trailing \\".to_string())
                        })?);
                    }
                    ret.files.push(file.into());
                }
            }
        }
        return Ok(ret);

        // rustc tries to fit env var names and values all on a single line, which
        // means it needs to escape `\r` and `\n`. The escape syntax used is "\n"
        // which means that `\` also needs to be escaped.
        fn unescape_env(s: &str) -> CargoResult<String> {
            let mut ret = String::with_capacity(s.len());
            let mut chars = s.chars();
            while let Some(c) = chars.next() {
                if c != '\\' {
                    ret.push(c);
                    continue;
                }
                match chars.next() {
                    Some('\\') => ret.push('\\'),
                    Some('n') => ret.push('\n'),
                    Some('r') => ret.push('\r'),
                    Some(c) => bail!("unknown escape character `{}`", c),
                    None => bail!("unterminated escape character"),
                }
            }
            Ok(ret)
        }
    }