#
cargo/core/compiler/
output_depinfo.rs

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    //! dep-info files for external build system integration.
    //! See [`output_depinfo`] for more.

    use cargo_util::paths::normalize_path;
    use std::collections::{BTreeSet, HashSet};
    use std::io::{BufWriter, Write};
    use std::path::{Path, PathBuf};

    use super::{fingerprint, BuildRunner, FileFlavor, Unit};
    use crate::util::{internal, CargoResult};
    use cargo_util::paths;
    use tracing::debug;

    /// Bacially just normalizes a given path and converts it to a string.
    fn render_filename<P: AsRef<Path>>(path: P, basedir: Option<&str>) -> CargoResult<String> {
        fn wrap_path(path: &Path) -> CargoResult<String> {
            path.to_str()
                .ok_or_else(|| internal(format!("path `{:?}` not utf-8", path)))
                .map(|f| f.replace(" ", "\\ "))
        }

        let path = path.as_ref();
        if let Some(basedir) = basedir {
            let norm_path = normalize_path(path);
            let norm_basedir = normalize_path(basedir.as_ref());
            match norm_path.strip_prefix(norm_basedir) {
                Ok(relpath) => wrap_path(relpath),
                _ => wrap_path(path),
            }
        } else {
            wrap_path(path)
        }
    }

    /// Collects all dependencies of the `unit` for the output dep info file.
    ///
    /// Dependencies will be stored in `deps`, including:
    ///
    /// * dependencies from [fingerprint dep-info]
    /// * paths from `rerun-if-changed` build script instruction
    /// * ...and traverse transitive dependencies recursively
    ///
    /// [fingerprint dep-info]: super::fingerprint#fingerprint-dep-info-files
    fn add_deps_for_unit(
        deps: &mut BTreeSet<PathBuf>,
        build_runner: &mut BuildRunner<'_, '_>,
        unit: &Unit,
        visited: &mut HashSet<Unit>,
    ) -> CargoResult<()> {
        if !visited.insert(unit.clone()) {
            return Ok(());
        }

        // units representing the execution of a build script don't actually
        // generate a dep info file, so we just keep on going below
        if !unit.mode.is_run_custom_build() {
            // Add dependencies from rustc dep-info output (stored in fingerprint directory)
            let dep_info_loc = fingerprint::dep_info_loc(build_runner, unit);
            if let Some(paths) = fingerprint::parse_dep_info(
                unit.pkg.root(),
                build_runner.files().host_root(),
                &dep_info_loc,
            )? {
                for path in paths.files.into_keys() {
                    deps.insert(path);
                }
            } else {
                debug!(
                    "can't find dep_info for {:?} {}",
                    unit.pkg.package_id(),
                    unit.target
                );
                return Err(internal("dep_info missing"));
            }
        }

        // Add rerun-if-changed dependencies
        if let Some(metadata) = build_runner.find_build_script_metadata(unit) {
            if let Some(output) = build_runner
                .build_script_outputs
                .lock()
                .unwrap()
                .get(metadata)
            {
                for path in &output.rerun_if_changed {
                    // The paths we have saved from the unit are of arbitrary relativeness and may be
                    // relative to the crate root of the dependency.
                    let path = unit.pkg.root().join(path);
                    deps.insert(path);
                }
            }
        }

        // Recursively traverse all transitive dependencies
        let unit_deps = Vec::from(build_runner.unit_deps(unit)); // Create vec due to mutable borrow.
        for dep in unit_deps {
            if dep.unit.is_local() {
                add_deps_for_unit(deps, build_runner, &dep.unit, visited)?;
            }
        }
        Ok(())
    }

    /// Save a `.d` dep-info file for the given unit. This is the third kind of
    /// dep-info mentioned in [`fingerprint`] module.
    ///
    /// Argument `unit` is expected to be the root unit, which will be uplifted.
    ///
    /// Cargo emits its own dep-info files in the output directory. This is
    /// only done for every "uplifted" artifact. These are intended to be used
    /// with external build systems so that they can detect if Cargo needs to be
    /// re-executed.
    ///
    /// It includes all the entries from the `rustc` dep-info file, and extends it
    /// with any `rerun-if-changed` entries from build scripts. It also includes
    /// sources from any path dependencies. Registry dependencies are not included
    /// under the assumption that changes to them can be detected via changes to
    /// `Cargo.lock`.
    ///
    /// [`fingerprint`]: super::fingerprint#dep-info-files
    pub fn output_depinfo(build_runner: &mut BuildRunner<'_, '_>, unit: &Unit) -> CargoResult<()> {
        let bcx = build_runner.bcx;
        let mut deps = BTreeSet::new();
        let mut visited = HashSet::new();
        let success = add_deps_for_unit(&mut deps, build_runner, unit, &mut visited).is_ok();
        let basedir_string;
        let basedir = match bcx.gctx.build_config()?.dep_info_basedir.clone() {
            Some(value) => {
                basedir_string = value
                    .resolve_path(bcx.gctx)
                    .as_os_str()
                    .to_str()
                    .ok_or_else(|| anyhow::format_err!("build.dep-info-basedir path not utf-8"))?
                    .to_string();
                Some(basedir_string.as_str())
            }
            None => None,
        };
        let deps = deps
            .iter()
            .map(|f| render_filename(f, basedir))
            .collect::<CargoResult<Vec<_>>>()?;

        for output in build_runner
            .outputs(unit)?
            .iter()
            .filter(|o| !matches!(o.flavor, FileFlavor::DebugInfo | FileFlavor::Auxiliary))
        {
            if let Some(ref link_dst) = output.hardlink {
                let output_path = link_dst.with_extension("d");
                if success {
                    let target_fn = render_filename(link_dst, basedir)?;

                    // If nothing changed don't recreate the file which could alter
                    // its mtime
                    if let Ok(previous) = fingerprint::parse_rustc_dep_info(&output_path) {
                        if previous
                            .files
                            .iter()
                            .map(|(path, _checksum)| path)
                            .eq(deps.iter().map(Path::new))
                        {
                            continue;
                        }
                    }

                    // Otherwise write it all out
                    let mut outfile = BufWriter::new(paths::create(output_path)?);
                    write!(outfile, "{}:", target_fn)?;
                    for dep in &deps {
                        write!(outfile, " {}", dep)?;
                    }
                    writeln!(outfile)?;

                // dep-info generation failed, so delete output file. This will
                // usually cause the build system to always rerun the build
                // rule, which is correct if inefficient.
                } else if output_path.exists() {
                    paths::remove_file(output_path)?;
                }
            }
        }
        Ok(())
    }