cargo/ops/

common_for_install_and_uninstall.rs

use std::collections::{btree_map, BTreeMap, BTreeSet};
use std::env;
use std::io::prelude::*;
use std::io::SeekFrom;
use std::path::{Path, PathBuf};
use std::rc::Rc;
use std::task::Poll;

use anyhow::{bail, format_err, Context as _};
use cargo_util::paths;
use cargo_util_schemas::core::PartialVersion;
use ops::FilterRule;
use serde::{Deserialize, Serialize};

use crate::core::compiler::{DirtyReason, Freshness};
use crate::core::Target;
use crate::core::{Dependency, FeatureValue, Package, PackageId, SourceId};
use crate::ops::{self, CompileFilter, CompileOptions};
use crate::sources::source::QueryKind;
use crate::sources::source::Source;
use crate::sources::PathSource;
use crate::util::cache_lock::CacheLockMode;
use crate::util::errors::CargoResult;
use crate::util::GlobalContext;
use crate::util::{FileLock, Filesystem};

/// On-disk tracking for which package installed which binary.
///
/// v1 is an older style, v2 is a new style that tracks more information, and
/// is both backwards and forwards compatible. Cargo keeps both files in sync,
/// updating both v1 and v2 at the same time. Additionally, if it detects
/// changes in v1 that are not in v2 (such as when an older version of Cargo
/// is used), it will automatically propagate those changes to v2.
///
/// This maintains a filesystem lock, preventing other instances of Cargo from
/// modifying at the same time. Drop the value to unlock.
///
/// It is intended that v1 should be retained for a while during a longish
/// transition period, and then v1 can be removed.
pub struct InstallTracker {
    v1: CrateListingV1,
    v2: CrateListingV2,
    v1_lock: FileLock,
    v2_lock: FileLock,
}

/// Tracking information for the set of installed packages.
#[derive(Default, Deserialize, Serialize)]
struct CrateListingV2 {
    /// Map of every installed package.
    installs: BTreeMap<PackageId, InstallInfo>,
    /// Forwards compatibility. Unknown keys from future versions of Cargo
    /// will be stored here and retained when the file is saved.
    #[serde(flatten)]
    other: BTreeMap<String, serde_json::Value>,
}

/// Tracking information for the installation of a single package.
///
/// This tracks the settings that were used when the package was installed.
/// Future attempts to install the same package will check these settings to
/// determine if it needs to be rebuilt/reinstalled. If nothing has changed,
/// then Cargo will inform the user that it is "up to date".
///
/// This is only used for the v2 format.
#[derive(Debug, Deserialize, Serialize)]
struct InstallInfo {
    /// Version requested via `--version`.
    /// None if `--version` not specified. Currently not used, possibly may be
    /// used in the future.
    version_req: Option<String>,
    /// Set of binary names installed.
    bins: BTreeSet<String>,
    /// Set of features explicitly enabled.
    features: BTreeSet<String>,
    all_features: bool,
    no_default_features: bool,
    /// Either "debug" or "release".
    profile: String,
    /// The installation target.
    /// Either the host or the value specified in `--target`.
    /// None if unknown (when loading from v1).
    target: Option<String>,
    /// Output of `rustc -V`.
    /// None if unknown (when loading from v1).
    /// Currently not used, possibly may be used in the future.
    rustc: Option<String>,
    /// Forwards compatibility.
    #[serde(flatten)]
    other: BTreeMap<String, serde_json::Value>,
}

/// Tracking information for the set of installed packages.
#[derive(Default, Deserialize, Serialize)]
pub struct CrateListingV1 {
    /// Map of installed package id to the set of binary names for that package.
    v1: BTreeMap<PackageId, BTreeSet<String>>,
}

impl InstallTracker {
    /// Create an `InstallTracker` from information on disk.
    pub fn load(gctx: &GlobalContext, root: &Filesystem) -> CargoResult<InstallTracker> {
        let v1_lock =
            root.open_rw_exclusive_create(Path::new(".crates.toml"), gctx, "crate metadata")?;
        let v2_lock =
            root.open_rw_exclusive_create(Path::new(".crates2.json"), gctx, "crate metadata")?;

        let v1 = (|| -> CargoResult<_> {
            let mut contents = String::new();
            v1_lock.file().read_to_string(&mut contents)?;
            if contents.is_empty() {
                Ok(CrateListingV1::default())
            } else {
                Ok(toml::from_str(&contents).context("invalid TOML found for metadata")?)
            }
        })()
        .with_context(|| {
            format!(
                "failed to parse crate metadata at `{}`",
                v1_lock.path().to_string_lossy()
            )
        })?;

        let v2 = (|| -> CargoResult<_> {
            let mut contents = String::new();
            v2_lock.file().read_to_string(&mut contents)?;
            let mut v2 = if contents.is_empty() {
                CrateListingV2::default()
            } else {
                serde_json::from_str(&contents).context("invalid JSON found for metadata")?
            };
            v2.sync_v1(&v1);
            Ok(v2)
        })()
        .with_context(|| {
            format!(
                "failed to parse crate metadata at `{}`",
                v2_lock.path().to_string_lossy()
            )
        })?;

        Ok(InstallTracker {
            v1,
            v2,
            v1_lock,
            v2_lock,
        })
    }

    /// Checks if the given package should be built, and checks if executables
    /// already exist in the destination directory.
    ///
    /// Returns a tuple `(freshness, map)`. `freshness` indicates if the
    /// package should be built (`Dirty`) or if it is already up-to-date
    /// (`Fresh`) and should be skipped. The map maps binary names to the
    /// `PackageId` that installed it (which is `None` if not known).
    ///
    /// If there are no duplicates, then it will be considered `Dirty` (i.e.,
    /// it is OK to build/install).
    ///
    /// `force=true` will always be considered `Dirty` (i.e., it will always
    /// be rebuilt/reinstalled).
    ///
    /// Returns an error if there is a duplicate and `--force` is not used.
    pub fn check_upgrade(
        &self,
        dst: &Path,
        pkg: &Package,
        force: bool,
        opts: &CompileOptions,
        target: &str,
        _rustc: &str,
    ) -> CargoResult<(Freshness, BTreeMap<String, Option<PackageId>>)> {
        let exes = exe_names(pkg, &opts.filter);
        // Check if any tracked exe's are already installed.
        let duplicates = self.find_duplicates(dst, &exes);
        if force || duplicates.is_empty() {
            return Ok((Freshness::Dirty(DirtyReason::Forced), duplicates));
        }
        // Check if all duplicates come from packages of the same name. If
        // there are duplicates from other packages, then --force will be
        // required.
        //
        // There may be multiple matching duplicates if different versions of
        // the same package installed different binaries.
        //
        // This does not check the source_id in order to allow the user to
        // switch between different sources. For example, installing from git,
        // and then switching to the official crates.io release or vice-versa.
        // If the source_id were included, then the user would get possibly
        // confusing errors like "package `foo 1.0.0` is already installed"
        // and the change of source may not be obvious why it fails.
        let matching_duplicates: Vec<PackageId> = duplicates
            .values()
            .filter_map(|v| match v {
                Some(dupe_pkg_id) if dupe_pkg_id.name() == pkg.name() => Some(*dupe_pkg_id),
                _ => None,
            })
            .collect();

        // If both sets are the same length, that means all duplicates come
        // from packages with the same name.
        if matching_duplicates.len() == duplicates.len() {
            // Determine if it is dirty or fresh.
            let source_id = pkg.package_id().source_id();
            if source_id.is_path() {
                // `cargo install --path ...` is always rebuilt.
                return Ok((Freshness::Dirty(DirtyReason::Forced), duplicates));
            }
            let is_up_to_date = |dupe_pkg_id| {
                let info = self
                    .v2
                    .installs
                    .get(dupe_pkg_id)
                    .expect("dupes must be in sync");
                let precise_equal = if source_id.is_git() {
                    // Git sources must have the exact same hash to be
                    // considered "fresh".
                    dupe_pkg_id.source_id().has_same_precise_as(source_id)
                } else {
                    true
                };

                dupe_pkg_id.version() == pkg.version()
                    && dupe_pkg_id.source_id() == source_id
                    && precise_equal
                    && info.is_up_to_date(opts, target, &exes)
            };
            if matching_duplicates.iter().all(is_up_to_date) {
                Ok((Freshness::Fresh, duplicates))
            } else {
                Ok((Freshness::Dirty(DirtyReason::Forced), duplicates))
            }
        } else {
            // Format the error message.
            let mut msg = String::new();
            for (bin, p) in duplicates.iter() {
                msg.push_str(&format!("binary `{}` already exists in destination", bin));
                if let Some(p) = p.as_ref() {
                    msg.push_str(&format!(" as part of `{}`\n", p));
                } else {
                    msg.push('\n');
                }
            }
            msg.push_str("Add --force to overwrite");
            bail!("{}", msg);
        }
    }

    /// Check if any executables are already installed.
    ///
    /// Returns a map of duplicates, the key is the executable name and the
    /// value is the `PackageId` that is already installed. The `PackageId` is
    /// None if it is an untracked executable.
    fn find_duplicates(
        &self,
        dst: &Path,
        exes: &BTreeSet<String>,
    ) -> BTreeMap<String, Option<PackageId>> {
        exes.iter()
            .filter_map(|name| {
                if !dst.join(&name).exists() {
                    None
                } else {
                    let p = self.v2.package_for_bin(name);
                    Some((name.clone(), p))
                }
            })
            .collect()
    }

    /// Mark that a package was installed.
    pub fn mark_installed(
        &mut self,
        package: &Package,
        bins: &BTreeSet<String>,
        version_req: Option<String>,
        opts: &CompileOptions,
        target: &str,
        rustc: &str,
    ) {
        self.v2
            .mark_installed(package, bins, version_req, opts, target, rustc);
        self.v1.mark_installed(package, bins);
    }

    /// Save tracking information to disk.
    pub fn save(&self) -> CargoResult<()> {
        self.v1.save(&self.v1_lock).with_context(|| {
            format!(
                "failed to write crate metadata at `{}`",
                self.v1_lock.path().to_string_lossy()
            )
        })?;

        self.v2.save(&self.v2_lock).with_context(|| {
            format!(
                "failed to write crate metadata at `{}`",
                self.v2_lock.path().to_string_lossy()
            )
        })?;
        Ok(())
    }

    /// Iterator of all installed binaries.
    /// Items are `(pkg_id, bins)` where `bins` is the set of binaries that
    /// package installed.
    pub fn all_installed_bins(&self) -> impl Iterator<Item = (&PackageId, &BTreeSet<String>)> {
        self.v1.v1.iter()
    }

    /// Set of binaries installed by a particular package.
    /// Returns None if the package is not installed.
    pub fn installed_bins(&self, pkg_id: PackageId) -> Option<&BTreeSet<String>> {
        self.v1.v1.get(&pkg_id)
    }

    /// Remove a package from the tracker.
    pub fn remove(&mut self, pkg_id: PackageId, bins: &BTreeSet<String>) {
        self.v1.remove(pkg_id, bins);
        self.v2.remove(pkg_id, bins);
    }

    /// Remove a bin after it successfully had been removed in disk and then save the tracker at last.
    pub fn remove_bin_then_save(
        &mut self,
        pkg_id: PackageId,
        bin: &str,
        bin_path: &PathBuf,
    ) -> CargoResult<()> {
        paths::remove_file(bin_path)?;
        self.v1.remove_bin(pkg_id, bin);
        self.v2.remove_bin(pkg_id, bin);
        self.save()?;
        Ok(())
    }
}

impl CrateListingV1 {
    fn mark_installed(&mut self, pkg: &Package, bins: &BTreeSet<String>) {
        // Remove bins from any other packages.
        for other_bins in self.v1.values_mut() {
            for bin in bins {
                other_bins.remove(bin);
            }
        }
        // Remove entries where `bins` is empty.
        let to_remove = self
            .v1
            .iter()
            .filter_map(|(&p, set)| if set.is_empty() { Some(p) } else { None })
            .collect::<Vec<_>>();
        for p in to_remove.iter() {
            self.v1.remove(p);
        }
        // Add these bins.
        self.v1
            .entry(pkg.package_id())
            .or_insert_with(BTreeSet::new)
            .append(&mut bins.clone());
    }

    fn remove(&mut self, pkg_id: PackageId, bins: &BTreeSet<String>) {
        let mut installed = match self.v1.entry(pkg_id) {
            btree_map::Entry::Occupied(e) => e,
            btree_map::Entry::Vacant(..) => panic!("v1 unexpected missing `{}`", pkg_id),
        };

        for bin in bins {
            installed.get_mut().remove(bin);
        }
        if installed.get().is_empty() {
            installed.remove();
        }
    }

    fn remove_bin(&mut self, pkg_id: PackageId, bin: &str) {
        let mut installed = match self.v1.entry(pkg_id) {
            btree_map::Entry::Occupied(e) => e,
            btree_map::Entry::Vacant(..) => panic!("v1 unexpected missing `{}`", pkg_id),
        };
        installed.get_mut().remove(bin);
        if installed.get().is_empty() {
            installed.remove();
        }
    }

    fn save(&self, lock: &FileLock) -> CargoResult<()> {
        let mut file = lock.file();
        file.seek(SeekFrom::Start(0))?;
        file.set_len(0)?;
        let data = toml::to_string_pretty(self)?;
        file.write_all(data.as_bytes())?;
        Ok(())
    }
}

impl CrateListingV2 {
    /// Incorporate any changes from v1 into self.
    /// This handles the initial upgrade to v2, *and* handles the case
    /// where v2 is in use, and a v1 update is made, then v2 is used again.
    /// i.e., `cargo +new install foo ; cargo +old install bar ; cargo +new install bar`
    /// For now, v1 is the source of truth, so its values are trusted over v2.
    fn sync_v1(&mut self, v1: &CrateListingV1) {
        // Make the `bins` entries the same.
        for (pkg_id, bins) in &v1.v1 {
            self.installs
                .entry(*pkg_id)
                .and_modify(|info| info.bins = bins.clone())
                .or_insert_with(|| InstallInfo::from_v1(bins));
        }
        // Remove any packages that aren't present in v1.
        let to_remove: Vec<_> = self
            .installs
            .keys()
            .filter(|pkg_id| !v1.v1.contains_key(pkg_id))
            .cloned()
            .collect();
        for pkg_id in to_remove {
            self.installs.remove(&pkg_id);
        }
    }

    fn package_for_bin(&self, bin_name: &str) -> Option<PackageId> {
        self.installs
            .iter()
            .find(|(_, info)| info.bins.contains(bin_name))
            .map(|(pkg_id, _)| *pkg_id)
    }

    fn mark_installed(
        &mut self,
        pkg: &Package,
        bins: &BTreeSet<String>,
        version_req: Option<String>,
        opts: &CompileOptions,
        target: &str,
        rustc: &str,
    ) {
        // Remove bins from any other packages.
        for info in &mut self.installs.values_mut() {
            for bin in bins {
                info.bins.remove(bin);
            }
        }
        // Remove entries where `bins` is empty.
        let to_remove = self
            .installs
            .iter()
            .filter_map(|(&p, info)| if info.bins.is_empty() { Some(p) } else { None })
            .collect::<Vec<_>>();
        for p in to_remove.iter() {
            self.installs.remove(p);
        }
        // Add these bins.
        if let Some(info) = self.installs.get_mut(&pkg.package_id()) {
            info.bins.append(&mut bins.clone());
            info.version_req = version_req;
            info.features = feature_set(&opts.cli_features.features);
            info.all_features = opts.cli_features.all_features;
            info.no_default_features = !opts.cli_features.uses_default_features;
            info.profile = opts.build_config.requested_profile.to_string();
            info.target = Some(target.to_string());
            info.rustc = Some(rustc.to_string());
        } else {
            self.installs.insert(
                pkg.package_id(),
                InstallInfo {
                    version_req,
                    bins: bins.clone(),
                    features: feature_set(&opts.cli_features.features),
                    all_features: opts.cli_features.all_features,
                    no_default_features: !opts.cli_features.uses_default_features,
                    profile: opts.build_config.requested_profile.to_string(),
                    target: Some(target.to_string()),
                    rustc: Some(rustc.to_string()),
                    other: BTreeMap::new(),
                },
            );
        }
    }

    fn remove(&mut self, pkg_id: PackageId, bins: &BTreeSet<String>) {
        let mut info_entry = match self.installs.entry(pkg_id) {
            btree_map::Entry::Occupied(e) => e,
            btree_map::Entry::Vacant(..) => panic!("v2 unexpected missing `{}`", pkg_id),
        };

        for bin in bins {
            info_entry.get_mut().bins.remove(bin);
        }
        if info_entry.get().bins.is_empty() {
            info_entry.remove();
        }
    }

    fn remove_bin(&mut self, pkg_id: PackageId, bin: &str) {
        let mut info_entry = match self.installs.entry(pkg_id) {
            btree_map::Entry::Occupied(e) => e,
            btree_map::Entry::Vacant(..) => panic!("v1 unexpected missing `{}`", pkg_id),
        };
        info_entry.get_mut().bins.remove(bin);
        if info_entry.get().bins.is_empty() {
            info_entry.remove();
        }
    }

    fn save(&self, lock: &FileLock) -> CargoResult<()> {
        let mut file = lock.file();
        file.seek(SeekFrom::Start(0))?;
        file.set_len(0)?;
        let data = serde_json::to_string(self)?;
        file.write_all(data.as_bytes())?;
        Ok(())
    }
}

impl InstallInfo {
    fn from_v1(set: &BTreeSet<String>) -> InstallInfo {
        InstallInfo {
            version_req: None,
            bins: set.clone(),
            features: BTreeSet::new(),
            all_features: false,
            no_default_features: false,
            profile: "release".to_string(),
            target: None,
            rustc: None,
            other: BTreeMap::new(),
        }
    }

    /// Determine if this installation is "up to date", or if it needs to be reinstalled.
    ///
    /// This does not do Package/Source/Version checking.
    fn is_up_to_date(&self, opts: &CompileOptions, target: &str, exes: &BTreeSet<String>) -> bool {
        self.features == feature_set(&opts.cli_features.features)
            && self.all_features == opts.cli_features.all_features
            && self.no_default_features != opts.cli_features.uses_default_features
            && self.profile.as_str() == opts.build_config.requested_profile.as_str()
            && (self.target.is_none() || self.target.as_deref() == Some(target))
            && &self.bins == exes
    }
}

/// Determines the root directory where installation is done.
pub fn resolve_root(flag: Option<&str>, gctx: &GlobalContext) -> CargoResult<Filesystem> {
    let config_root = gctx.get_path("install.root")?;
    Ok(flag
        .map(PathBuf::from)
        .or_else(|| gctx.get_env_os("CARGO_INSTALL_ROOT").map(PathBuf::from))
        .or_else(move || config_root.map(|v| v.val))
        .map(Filesystem::new)
        .unwrap_or_else(|| gctx.home().clone()))
}

/// Determines the `PathSource` from a `SourceId`.
pub fn path_source(source_id: SourceId, gctx: &GlobalContext) -> CargoResult<PathSource<'_>> {
    let path = source_id
        .url()
        .to_file_path()
        .map_err(|()| format_err!("path sources must have a valid path"))?;
    Ok(PathSource::new(&path, source_id, gctx))
}

/// Gets a Package based on command-line requirements.
pub fn select_dep_pkg<T>(
    source: &mut T,
    dep: Dependency,
    gctx: &GlobalContext,
    needs_update: bool,
    current_rust_version: Option<&PartialVersion>,
) -> CargoResult<Package>
where
    T: Source,
{
    // This operation may involve updating some sources or making a few queries
    // which may involve frobbing caches, as a result make sure we synchronize
    // with other global Cargos
    let _lock = gctx.acquire_package_cache_lock(CacheLockMode::DownloadExclusive)?;

    if needs_update {
        source.invalidate_cache();
    }

    let deps = loop {
        match source.query_vec(&dep, QueryKind::Exact)? {
            Poll::Ready(deps) => break deps,
            Poll::Pending => source.block_until_ready()?,
        }
    };
    match deps
        .iter()
        .map(|s| s.as_summary())
        .max_by_key(|p| p.package_id())
    {
        Some(summary) => {
            if let (Some(current), Some(msrv)) = (current_rust_version, summary.rust_version()) {
                if !msrv.is_compatible_with(current) {
                    let name = summary.name();
                    let ver = summary.version();
                    let extra = if dep.source_id().is_registry() {
                        // Match any version, not just the selected
                        let msrv_dep =
                            Dependency::parse(dep.package_name(), None, dep.source_id())?;
                        let msrv_deps = loop {
                            match source.query_vec(&msrv_dep, QueryKind::Exact)? {
                                Poll::Ready(deps) => break deps,
                                Poll::Pending => source.block_until_ready()?,
                            }
                        };
                        if let Some(alt) = msrv_deps
                            .iter()
                            .map(|s| s.as_summary())
                            .filter(|summary| {
                                summary
                                    .rust_version()
                                    .map(|msrv| msrv.is_compatible_with(current))
                                    .unwrap_or(true)
                            })
                            .max_by_key(|s| s.package_id())
                        {
                            if let Some(rust_version) = alt.rust_version() {
                                format!(
                                    "\n`{name} {}` supports rustc {rust_version}",
                                    alt.version()
                                )
                            } else {
                                format!(
                                    "\n`{name} {}` has an unspecified minimum rustc version",
                                    alt.version()
                                )
                            }
                        } else {
                            String::new()
                        }
                    } else {
                        String::new()
                    };
                    bail!("\
cannot install package `{name} {ver}`, it requires rustc {msrv} or newer, while the currently active rustc version is {current}{extra}"
)
                }
            }
            let pkg = Box::new(source).download_now(summary.package_id(), gctx)?;
            Ok(pkg)
        }
        None => {
            let is_yanked: bool = if dep.version_req().is_exact() {
                let version: String = dep.version_req().to_string();
                if let Ok(pkg_id) =
                    PackageId::try_new(dep.package_name(), &version[1..], source.source_id())
                {
                    source.invalidate_cache();
                    loop {
                        match source.is_yanked(pkg_id) {
                            Poll::Ready(Ok(is_yanked)) => break is_yanked,
                            Poll::Ready(Err(_)) => break false,
                            Poll::Pending => source.block_until_ready()?,
                        }
                    }
                } else {
                    false
                }
            } else {
                false
            };
            if is_yanked {
                bail!(
                    "cannot install package `{}`, it has been yanked from {}",
                    dep.package_name(),
                    source.source_id()
                )
            } else {
                bail!(
                    "could not find `{}` in {} with version `{}`",
                    dep.package_name(),
                    source.source_id(),
                    dep.version_req(),
                )
            }
        }
    }
}

pub fn select_pkg<T, F>(
    source: &mut T,
    dep: Option<Dependency>,
    mut list_all: F,
    gctx: &GlobalContext,
    current_rust_version: Option<&PartialVersion>,
) -> CargoResult<Package>
where
    T: Source,
    F: FnMut(&mut T) -> CargoResult<Vec<Package>>,
{
    // This operation may involve updating some sources or making a few queries
    // which may involve frobbing caches, as a result make sure we synchronize
    // with other global Cargos
    let _lock = gctx.acquire_package_cache_lock(CacheLockMode::DownloadExclusive)?;

    source.invalidate_cache();

    return if let Some(dep) = dep {
        select_dep_pkg(source, dep, gctx, false, current_rust_version)
    } else {
        let candidates = list_all(source)?;
        let binaries = candidates
            .iter()
            .filter(|cand| cand.targets().iter().filter(|t| t.is_bin()).count() > 0);
        let examples = candidates
            .iter()
            .filter(|cand| cand.targets().iter().filter(|t| t.is_example()).count() > 0);
        let git_url = source.source_id().url().to_string();
        let pkg = match one(binaries, |v| multi_err("binaries", &git_url, v))? {
            Some(p) => p,
            None => match one(examples, |v| multi_err("examples", &git_url, v))? {
                Some(p) => p,
                None => bail!(
                    "no packages found with binaries or \
                         examples"
                ),
            },
        };
        Ok(pkg.clone())
    };

    fn multi_err(kind: &str, git_url: &str, mut pkgs: Vec<&Package>) -> String {
        pkgs.sort_unstable_by_key(|a| a.name());
        let first_pkg = pkgs[0];
        format!(
            "multiple packages with {} found: {}. When installing a git repository, \
            cargo will always search the entire repo for any Cargo.toml.\n\
            Please specify a package, e.g. `cargo install --git {} {}`.",
            kind,
            pkgs.iter()
                .map(|p| p.name().as_str())
                .collect::<Vec<_>>()
                .join(", "),
            git_url,
            first_pkg.name()
        )
    }
}

/// Get one element from the iterator.
/// Returns None if none left.
/// Returns error if there is more than one item in the iterator.
fn one<I, F>(mut i: I, f: F) -> CargoResult<Option<I::Item>>
where
    I: Iterator,
    F: FnOnce(Vec<I::Item>) -> String,
{
    match (i.next(), i.next()) {
        (Some(i1), Some(i2)) => {
            let mut v = vec![i1, i2];
            v.extend(i);
            Err(format_err!("{}", f(v)))
        }
        (Some(i), None) => Ok(Some(i)),
        (None, _) => Ok(None),
    }
}

/// Helper to convert features to a `BTreeSet`.
fn feature_set(features: &Rc<BTreeSet<FeatureValue>>) -> BTreeSet<String> {
    features.iter().map(|s| s.to_string()).collect()
}

/// Helper to get the executable names from a filter.
pub fn exe_names(pkg: &Package, filter: &ops::CompileFilter) -> BTreeSet<String> {
    let to_exe = |name| format!("{}{}", name, env::consts::EXE_SUFFIX);
    match filter {
        CompileFilter::Default { .. } => pkg
            .targets()
            .iter()
            .filter(|t| t.is_bin())
            .map(|t| to_exe(t.name()))
            .collect(),
        CompileFilter::Only {
            all_targets: true, ..
        } => pkg
            .targets()
            .iter()
            .filter(|target| target.is_executable())
            .map(|target| to_exe(target.name()))
            .collect(),
        CompileFilter::Only {
            ref bins,
            ref examples,
            ..
        } => {
            let collect = |rule: &_, f: fn(&Target) -> _| match rule {
                FilterRule::All => pkg
                    .targets()
                    .iter()
                    .filter(|t| f(t))
                    .map(|t| t.name().into())
                    .collect(),
                FilterRule::Just(targets) => targets.clone(),
            };
            let all_bins = collect(bins, Target::is_bin);
            let all_examples = collect(examples, Target::is_exe_example);

            all_bins
                .iter()
                .chain(all_examples.iter())
                .map(|name| to_exe(name))
                .collect()
        }
    }
}