Start using 2018 idioms in Cargo

[cargo.git] / src / cargo / ops / cargo_compile.rs

//!
//! Cargo compile currently does the following steps:
//!
//! All configurations are already injected as environment variables via the
//! main cargo command
//!
//! 1. Read the manifest
//! 2. Shell out to `cargo-resolve` with a list of dependencies and sources as
//!    stdin
//!
//!    a. Shell out to `--do update` and `--do list` for each source
//!    b. Resolve dependencies and return a list of name/version/source
//!
//! 3. Shell out to `--do download` for each source
//! 4. Shell out to `--do get` for each source, and build up the list of paths
//!    to pass to rustc -L
//! 5. Call `cargo-rustc` with the results of the resolver zipped together with
//!    the results of the `get`
//!
//!    a. Topologically sort the dependencies
//!    b. Compile each dependency in order, passing in the -L's pointing at each
//!       previously compiled dependency
//!

use std::collections::{HashMap, HashSet};
use std::path::PathBuf;
use std::sync::Arc;

use crate::core::compiler::{
    BuildConfig, BuildContext, Compilation, Context, DefaultExecutor, Executor,
};
use crate::core::compiler::{CompileMode, Kind, Unit};
use crate::core::profiles::{Profiles, UnitFor};
use crate::core::resolver::{Method, Resolve};
use crate::core::{Package, Source, Target};
use crate::core::{PackageId, PackageIdSpec, TargetKind, Workspace};
use crate::ops;
use crate::util::config::Config;
use crate::util::{lev_distance, profile, CargoResult};

/// Contains information about how a package should be compiled.
#[derive(Debug)]
pub struct CompileOptions<'a> {
    pub config: &'a Config,
    /// Configuration information for a rustc build
    pub build_config: BuildConfig,
    /// Extra features to build for the root package
    pub features: Vec<String>,
    /// Flag whether all available features should be built for the root package
    pub all_features: bool,
    /// Flag if the default feature should be built for the root package
    pub no_default_features: bool,
    /// A set of packages to build.
    pub spec: Packages,
    /// Filter to apply to the root package to select which targets will be
    /// built.
    pub filter: CompileFilter,
    /// Extra arguments to be passed to rustdoc (single target only)
    pub target_rustdoc_args: Option<Vec<String>>,
    /// The specified target will be compiled with all the available arguments,
    /// note that this only accounts for the *final* invocation of rustc
    pub target_rustc_args: Option<Vec<String>>,
    /// Extra arguments passed to all selected targets for rustdoc.
    pub local_rustdoc_args: Option<Vec<String>>,
    /// The directory to copy final artifacts to. Note that even if `out_dir` is
    /// set, a copy of artifacts still could be found a `target/(debug\release)`
    /// as usual.
    // Note that, although the cmd-line flag name is `out-dir`, in code we use
    // `export_dir`, to avoid confusion with out dir at `target/debug/deps`.
    pub export_dir: Option<PathBuf>,
}

impl<'a> CompileOptions<'a> {
    pub fn new(config: &'a Config, mode: CompileMode) -> CargoResult<CompileOptions<'a>> {
        Ok(CompileOptions {
            config,
            build_config: BuildConfig::new(config, None, &None, mode)?,
            features: Vec::new(),
            all_features: false,
            no_default_features: false,
            spec: ops::Packages::Packages(Vec::new()),
            filter: CompileFilter::Default {
                required_features_filterable: false,
            },
            target_rustdoc_args: None,
            target_rustc_args: None,
            local_rustdoc_args: None,
            export_dir: None,
        })
    }
}

#[derive(Clone, PartialEq, Eq, Debug)]
pub enum Packages {
    Default,
    All,
    OptOut(Vec<String>),
    Packages(Vec<String>),
}

impl Packages {
    pub fn from_flags(all: bool, exclude: Vec<String>, package: Vec<String>) -> CargoResult<Self> {
        Ok(match (all, exclude.len(), package.len()) {
            (false, 0, 0) => Packages::Default,
            (false, 0, _) => Packages::Packages(package),
            (false, _, _) => bail!("--exclude can only be used together with --all"),
            (true, 0, _) => Packages::All,
            (true, _, _) => Packages::OptOut(exclude),
        })
    }

    pub fn to_package_id_specs(&self, ws: &Workspace) -> CargoResult<Vec<PackageIdSpec>> {
        let specs = match *self {
            Packages::All => ws
                .members()
                .map(Package::package_id)
                .map(PackageIdSpec::from_package_id)
                .collect(),
            Packages::OptOut(ref opt_out) => ws
                .members()
                .map(Package::package_id)
                .map(PackageIdSpec::from_package_id)
                .filter(|p| opt_out.iter().position(|x| *x == p.name()).is_none())
                .collect(),
            Packages::Packages(ref packages) if packages.is_empty() => {
                vec![PackageIdSpec::from_package_id(ws.current()?.package_id())]
            }
            Packages::Packages(ref packages) => packages
                .iter()
                .map(|p| PackageIdSpec::parse(p))
                .collect::<CargoResult<Vec<_>>>()?,
            Packages::Default => ws
                .default_members()
                .map(Package::package_id)
                .map(PackageIdSpec::from_package_id)
                .collect(),
        };
        if specs.is_empty() {
            if ws.is_virtual() {
                bail!(
                    "manifest path `{}` contains no package: The manifest is virtual, \
                     and the workspace has no members.",
                    ws.root().display()
                )
            }
            bail!("no packages to compile")
        }
        Ok(specs)
    }

    pub fn get_packages<'ws>(&self, ws: &'ws Workspace) -> CargoResult<Vec<&'ws Package>> {
        let packages: Vec<_> = match self {
            Packages::Default => ws.default_members().collect(),
            Packages::All => ws.members().collect(),
            Packages::OptOut(ref opt_out) => ws
                .members()
                .filter(|pkg| !opt_out.iter().any(|name| pkg.name().as_str() == name))
                .collect(),
            Packages::Packages(ref pkgs) => pkgs
                .iter()
                .map(|name| {
                    ws.members()
                        .find(|pkg| pkg.name().as_str() == name)
                        .ok_or_else(|| {
                            format_err!("package `{}` is not a member of the workspace", name)
                        })
                })
                .collect::<CargoResult<Vec<_>>>()?,
        };
        Ok(packages)
    }
}

#[derive(Debug)]
pub enum FilterRule {
    All,
    Just(Vec<String>),
}

#[derive(Debug)]
pub enum CompileFilter {
    Default {
        /// Flag whether targets can be safely skipped when required-features are not satisfied.
        required_features_filterable: bool,
    },
    Only {
        all_targets: bool,
        lib: bool,
        bins: FilterRule,
        examples: FilterRule,
        tests: FilterRule,
        benches: FilterRule,
    },
}

pub fn compile<'a>(
    ws: &Workspace<'a>,
    options: &CompileOptions<'a>,
) -> CargoResult<Compilation<'a>> {
    let exec: Arc<Executor> = Arc::new(DefaultExecutor);
    compile_with_exec(ws, options, &exec)
}

/// Like `compile` but allows specifying a custom `Executor` that will be able to intercept build
/// calls and add custom logic. `compile` uses `DefaultExecutor` which just passes calls through.
pub fn compile_with_exec<'a>(
    ws: &Workspace<'a>,
    options: &CompileOptions<'a>,
    exec: &Arc<Executor>,
) -> CargoResult<Compilation<'a>> {
    ws.emit_warnings()?;
    compile_ws(ws, None, options, exec)
}

pub fn compile_ws<'a>(
    ws: &Workspace<'a>,
    source: Option<Box<Source + 'a>>,
    options: &CompileOptions<'a>,
    exec: &Arc<Executor>,
) -> CargoResult<Compilation<'a>> {
    let CompileOptions {
        config,
        ref build_config,
        ref spec,
        ref features,
        all_features,
        no_default_features,
        ref filter,
        ref target_rustdoc_args,
        ref target_rustc_args,
        ref local_rustdoc_args,
        ref export_dir,
    } = *options;

    let default_arch_kind = if build_config.requested_target.is_some() {
        Kind::Target
    } else {
        Kind::Host
    };

    let specs = spec.to_package_id_specs(ws)?;
    let features = Method::split_features(features);
    let method = Method::Required {
        dev_deps: ws.require_optional_deps() || filter.need_dev_deps(build_config.mode),
        features: &features,
        all_features,
        uses_default_features: !no_default_features,
    };
    let resolve = ops::resolve_ws_with_method(ws, source, method, &specs)?;
    let (packages, resolve_with_overrides) = resolve;

    let to_build_ids = specs
        .iter()
        .map(|s| s.query(resolve_with_overrides.iter()))
        .collect::<CargoResult<Vec<_>>>()?;
    let mut to_builds = packages.get_many(to_build_ids)?;

    // The ordering here affects some error messages coming out of cargo, so
    // let's be test and CLI friendly by always printing in the same order if
    // there's an error.
    to_builds.sort_by_key(|p| p.package_id());

    for pkg in to_builds.iter() {
        pkg.manifest().print_teapot(ws.config());

        if build_config.mode.is_any_test()
            && !ws.is_member(pkg)
            && pkg.dependencies().iter().any(|dep| !dep.is_transitive())
        {
            bail!(
                "package `{}` cannot be tested because it requires dev-dependencies \
                 and is not a member of the workspace",
                pkg.name()
            );
        }
    }

    let (extra_args, extra_args_name) = match (target_rustc_args, target_rustdoc_args) {
        (&Some(ref args), _) => (Some(args.clone()), "rustc"),
        (_, &Some(ref args)) => (Some(args.clone()), "rustdoc"),
        _ => (None, ""),
    };

    if extra_args.is_some() && to_builds.len() != 1 {
        panic!(
            "`{}` should not accept multiple `-p` flags",
            extra_args_name
        );
    }

    let profiles = ws.profiles();
    profiles.validate_packages(&mut config.shell(), &packages)?;

    let units = generate_targets(
        ws,
        profiles,
        &to_builds,
        filter,
        default_arch_kind,
        &resolve_with_overrides,
        build_config,
    )?;

    let mut extra_compiler_args = HashMap::new();
    if let Some(args) = extra_args {
        if units.len() != 1 {
            bail!(
                "extra arguments to `{}` can only be passed to one \
                 target, consider filtering\nthe package by passing \
                 e.g. `--lib` or `--bin NAME` to specify a single target",
                extra_args_name
            );
        }
        extra_compiler_args.insert(units[0], args);
    }
    if let Some(args) = local_rustdoc_args {
        for unit in &units {
            if unit.mode.is_doc() {
                extra_compiler_args.insert(*unit, args.clone());
            }
        }
    }

    let ret = {
        let _p = profile::start("compiling");
        let bcx = BuildContext::new(
            ws,
            &resolve_with_overrides,
            &packages,
            config,
            &build_config,
            profiles,
            extra_compiler_args,
        )?;
        let cx = Context::new(config, &bcx)?;
        cx.compile(&units, export_dir.clone(), &exec)?
    };

    Ok(ret)
}

impl FilterRule {
    pub fn new(targets: Vec<String>, all: bool) -> FilterRule {
        if all {
            FilterRule::All
        } else {
            FilterRule::Just(targets)
        }
    }

    fn matches(&self, target: &Target) -> bool {
        match *self {
            FilterRule::All => true,
            FilterRule::Just(ref targets) => targets.iter().any(|x| *x == target.name()),
        }
    }

    fn is_specific(&self) -> bool {
        match *self {
            FilterRule::All => true,
            FilterRule::Just(ref targets) => !targets.is_empty(),
        }
    }

    pub fn try_collect(&self) -> Option<Vec<String>> {
        match *self {
            FilterRule::All => None,
            FilterRule::Just(ref targets) => Some(targets.clone()),
        }
    }
}

impl CompileFilter {
    pub fn new(
        lib_only: bool,
        bins: Vec<String>,
        all_bins: bool,
        tsts: Vec<String>,
        all_tsts: bool,
        exms: Vec<String>,
        all_exms: bool,
        bens: Vec<String>,
        all_bens: bool,
        all_targets: bool,
    ) -> CompileFilter {
        let rule_bins = FilterRule::new(bins, all_bins);
        let rule_tsts = FilterRule::new(tsts, all_tsts);
        let rule_exms = FilterRule::new(exms, all_exms);
        let rule_bens = FilterRule::new(bens, all_bens);

        if all_targets {
            CompileFilter::Only {
                all_targets: true,
                lib: true,
                bins: FilterRule::All,
                examples: FilterRule::All,
                benches: FilterRule::All,
                tests: FilterRule::All,
            }
        } else if lib_only
            || rule_bins.is_specific()
            || rule_tsts.is_specific()
            || rule_exms.is_specific()
            || rule_bens.is_specific()
        {
            CompileFilter::Only {
                all_targets: false,
                lib: lib_only,
                bins: rule_bins,
                examples: rule_exms,
                benches: rule_bens,
                tests: rule_tsts,
            }
        } else {
            CompileFilter::Default {
                required_features_filterable: true,
            }
        }
    }

    pub fn need_dev_deps(&self, mode: CompileMode) -> bool {
        match mode {
            CompileMode::Test | CompileMode::Doctest | CompileMode::Bench => true,
            CompileMode::Build | CompileMode::Doc { .. } | CompileMode::Check { .. } => match *self
            {
                CompileFilter::Default { .. } => false,
                CompileFilter::Only {
                    ref examples,
                    ref tests,
                    ref benches,
                    ..
                } => examples.is_specific() || tests.is_specific() || benches.is_specific(),
            },
            CompileMode::RunCustomBuild => panic!("Invalid mode"),
        }
    }

    // this selects targets for "cargo run". for logic to select targets for
    // other subcommands, see generate_targets and filter_default_targets
    pub fn target_run(&self, target: &Target) -> bool {
        match *self {
            CompileFilter::Default { .. } => true,
            CompileFilter::Only {
                lib,
                ref bins,
                ref examples,
                ref tests,
                ref benches,
                ..
            } => {
                let rule = match *target.kind() {
                    TargetKind::Bin => bins,
                    TargetKind::Test => tests,
                    TargetKind::Bench => benches,
                    TargetKind::ExampleBin | TargetKind::ExampleLib(..) => examples,
                    TargetKind::Lib(..) => return lib,
                    TargetKind::CustomBuild => return false,
                };
                rule.matches(target)
            }
        }
    }

    pub fn is_specific(&self) -> bool {
        match *self {
            CompileFilter::Default { .. } => false,
            CompileFilter::Only { .. } => true,
        }
    }
}

/// A proposed target.
///
/// Proposed targets are later filtered into actual Units based on whether or
/// not the target requires its features to be present.
#[derive(Debug)]
struct Proposal<'a> {
    pkg: &'a Package,
    target: &'a Target,
    /// Indicates whether or not all required features *must* be present. If
    /// false, and the features are not available, then it will be silently
    /// skipped. Generally, targets specified by name (`--bin foo`) are
    /// required, all others can be silently skipped if features are missing.
    requires_features: bool,
    mode: CompileMode,
}

/// Generates all the base targets for the packages the user has requested to
/// compile. Dependencies for these targets are computed later in
/// `unit_dependencies`.
fn generate_targets<'a>(
    ws: &Workspace,
    profiles: &Profiles,
    packages: &[&'a Package],
    filter: &CompileFilter,
    default_arch_kind: Kind,
    resolve: &Resolve,
    build_config: &BuildConfig,
) -> CargoResult<Vec<Unit<'a>>> {
    // Helper for creating a Unit struct.
    let new_unit = |pkg: &'a Package, target: &'a Target, target_mode: CompileMode| {
        let unit_for = if build_config.mode.is_any_test() {
            // NOTE: The UnitFor here is subtle.  If you have a profile
            // with `panic` set, the `panic` flag is cleared for
            // tests/benchmarks and their dependencies.  If this
            // was `normal`, then the lib would get compiled three
            // times (once with panic, once without, and once with
            // --test).
            //
            // This would cause a problem for Doc tests, which would fail
            // because `rustdoc` would attempt to link with both libraries
            // at the same time. Also, it's probably not important (or
            // even desirable?) for rustdoc to link with a lib with
            // `panic` set.
            //
            // As a consequence, Examples and Binaries get compiled
            // without `panic` set.  This probably isn't a bad deal.
            //
            // Forcing the lib to be compiled three times during `cargo
            // test` is probably also not desirable.
            UnitFor::new_test()
        } else if target.for_host() {
            // proc-macro/plugin should not have `panic` set.
            UnitFor::new_compiler()
        } else {
            UnitFor::new_normal()
        };
        // Custom build units are added in `build_unit_dependencies`.
        assert!(!target.is_custom_build());
        let target_mode = match target_mode {
            CompileMode::Test => {
                if target.is_example() && !filter.is_specific() && !target.tested() {
                    // Examples are included as regular binaries to verify
                    // that they compile.
                    CompileMode::Build
                } else {
                    CompileMode::Test
                }
            }
            CompileMode::Build => match *target.kind() {
                TargetKind::Test => CompileMode::Test,
                TargetKind::Bench => CompileMode::Bench,
                _ => CompileMode::Build,
            },
            // CompileMode::Bench is only used to inform filter_default_targets
            // which command is being used (`cargo bench`). Afterwards, tests
            // and benches are treated identically. Switching the mode allows
            // de-duplication of units that are essentially identical.  For
            // example, `cargo build --all-targets --release` creates the units
            // (lib profile:bench, mode:test) and (lib profile:bench, mode:bench)
            // and since these are the same, we want them to be de-duped in
            // `unit_dependencies`.
            CompileMode::Bench => CompileMode::Test,
            _ => target_mode,
        };
        // Plugins or proc-macro should be built for the host.
        let kind = if target.for_host() {
            Kind::Host
        } else {
            default_arch_kind
        };
        let profile = profiles.get_profile(
            pkg.package_id(),
            ws.is_member(pkg),
            unit_for,
            target_mode,
            build_config.release,
        );
        Unit {
            pkg,
            target,
            profile,
            kind,
            mode: target_mode,
        }
    };

    // Create a list of proposed targets.
    let mut proposals: Vec<Proposal> = Vec::new();

    match *filter {
        CompileFilter::Default {
            required_features_filterable,
        } => {
            for pkg in packages {
                let default = filter_default_targets(pkg.targets(), build_config.mode);
                proposals.extend(default.into_iter().map(|target| Proposal {
                    pkg,
                    target,
                    requires_features: !required_features_filterable,
                    mode: build_config.mode,
                }));
                if build_config.mode == CompileMode::Test {
                    if let Some(t) = pkg
                        .targets()
                        .iter()
                        .find(|t| t.is_lib() && t.doctested() && t.doctestable())
                    {
                        proposals.push(Proposal {
                            pkg,
                            target: t,
                            requires_features: false,
                            mode: CompileMode::Doctest,
                        });
                    }
                }
            }
        }
        CompileFilter::Only {
            all_targets,
            lib,
            ref bins,
            ref examples,
            ref tests,
            ref benches,
        } => {
            if lib {
                let mut libs = Vec::new();
                for pkg in packages {
                    for target in pkg.targets().iter().filter(|t| t.is_lib()) {
                        if build_config.mode == CompileMode::Doctest && !target.doctestable() {
                            ws.config()
                                .shell()
                                .warn(format!(
                                "doc tests are not supported for crate type(s) `{}` in package `{}`",
                                target.rustc_crate_types().join(", "),
                                pkg.name()
                            ))?;
                        } else {
                            libs.push(Proposal {
                                pkg,
                                target,
                                requires_features: false,
                                mode: build_config.mode,
                            });
                        }
                    }
                }
                if !all_targets && libs.is_empty() {
                    let names = packages.iter().map(|pkg| pkg.name()).collect::<Vec<_>>();
                    if names.len() == 1 {
                        bail!("no library targets found in package `{}`", names[0]);
                    } else {
                        bail!("no library targets found in packages: {}", names.join(", "));
                    }
                }
                proposals.extend(libs);
            }

            // If --tests was specified, add all targets that would be
            // generated by `cargo test`.
            let test_filter = match *tests {
                FilterRule::All => Target::tested,
                FilterRule::Just(_) => Target::is_test,
            };
            let test_mode = match build_config.mode {
                CompileMode::Build => CompileMode::Test,
                CompileMode::Check { .. } => CompileMode::Check { test: true },
                _ => build_config.mode,
            };
            // If --benches was specified, add all targets that would be
            // generated by `cargo bench`.
            let bench_filter = match *benches {
                FilterRule::All => Target::benched,
                FilterRule::Just(_) => Target::is_bench,
            };
            let bench_mode = match build_config.mode {
                CompileMode::Build => CompileMode::Bench,
                CompileMode::Check { .. } => CompileMode::Check { test: true },
                _ => build_config.mode,
            };

            proposals.extend(list_rule_targets(
                packages,
                bins,
                "bin",
                Target::is_bin,
                build_config.mode,
            )?);
            proposals.extend(list_rule_targets(
                packages,
                examples,
                "example",
                Target::is_example,
                build_config.mode,
            )?);
            proposals.extend(list_rule_targets(
                packages,
                tests,
                "test",
                test_filter,
                test_mode,
            )?);
            proposals.extend(list_rule_targets(
                packages,
                benches,
                "bench",
                bench_filter,
                bench_mode,
            )?);
        }
    }

    // Only include targets that are libraries or have all required
    // features available.
    let mut features_map = HashMap::new();
    let mut units = HashSet::new();
    for Proposal {
        pkg,
        target,
        requires_features,
        mode,
    } in proposals
    {
        let unavailable_features = match target.required_features() {
            Some(rf) => {
                let features = features_map
                    .entry(pkg)
                    .or_insert_with(|| resolve_all_features(resolve, pkg.package_id()));
                rf.iter().filter(|f| !features.contains(*f)).collect()
            }
            None => Vec::new(),
        };
        if target.is_lib() || unavailable_features.is_empty() {
            let unit = new_unit(pkg, target, mode);
            units.insert(unit);
        } else if requires_features {
            let required_features = target.required_features().unwrap();
            let quoted_required_features: Vec<String> = required_features
                .iter()
                .map(|s| format!("`{}`", s))
                .collect();
            bail!(
                "target `{}` in package `{}` requires the features: {}\n\
                 Consider enabling them by passing e.g. `--features=\"{}\"`",
                target.name(),
                pkg.name(),
                quoted_required_features.join(", "),
                required_features.join(" ")
            );
        }
        // else, silently skip target.
    }
    Ok(units.into_iter().collect())
}

fn resolve_all_features(
    resolve_with_overrides: &Resolve,
    package_id: PackageId,
) -> HashSet<String> {
    let mut features = resolve_with_overrides.features(package_id).clone();

    // Include features enabled for use by dependencies so targets can also use them with the
    // required-features field when deciding whether to be built or skipped.
    for (dep, _) in resolve_with_overrides.deps(package_id) {
        for feature in resolve_with_overrides.features(dep) {
            features.insert(dep.name().to_string() + "/" + feature);
        }
    }

    features
}

/// Given a list of all targets for a package, filters out only the targets
/// that are automatically included when the user doesn't specify any targets.
fn filter_default_targets(targets: &[Target], mode: CompileMode) -> Vec<&Target> {
    match mode {
        CompileMode::Bench => targets.iter().filter(|t| t.benched()).collect(),
        CompileMode::Test => targets
            .iter()
            .filter(|t| t.tested() || t.is_example())
            .collect(),
        CompileMode::Build | CompileMode::Check { .. } => targets
            .iter()
            .filter(|t| t.is_bin() || t.is_lib())
            .collect(),
        CompileMode::Doc { .. } => {
            // `doc` does lib and bins (bin with same name as lib is skipped).
            targets
                .iter()
                .filter(|t| {
                    t.documented()
                        && (!t.is_bin()
                            || !targets.iter().any(|l| l.is_lib() && l.name() == t.name()))
                })
                .collect()
        }
        CompileMode::Doctest | CompileMode::RunCustomBuild => panic!("Invalid mode {:?}", mode),
    }
}

/// Returns a list of targets based on command-line target selection flags.
/// The return value is a list of `(Package, Target, bool, CompileMode)`
/// tuples.  The `bool` value indicates whether or not all required features
/// *must* be present.
fn list_rule_targets<'a>(
    packages: &[&'a Package],
    rule: &FilterRule,
    target_desc: &'static str,
    is_expected_kind: fn(&Target) -> bool,
    mode: CompileMode,
) -> CargoResult<Vec<Proposal<'a>>> {
    let mut result = Vec::new();
    match *rule {
        FilterRule::All => {
            for pkg in packages {
                for target in pkg.targets() {
                    if is_expected_kind(target) {
                        result.push(Proposal {
                            pkg,
                            target,
                            requires_features: false,
                            mode,
                        });
                    }
                }
            }
        }
        FilterRule::Just(ref names) => {
            for name in names {
                result.extend(find_named_targets(
                    packages,
                    name,
                    target_desc,
                    is_expected_kind,
                    mode,
                )?);
            }
        }
    }
    Ok(result)
}

/// Find the targets for a specifically named target.
fn find_named_targets<'a>(
    packages: &[&'a Package],
    target_name: &str,
    target_desc: &'static str,
    is_expected_kind: fn(&Target) -> bool,
    mode: CompileMode,
) -> CargoResult<Vec<Proposal<'a>>> {
    let mut result = Vec::new();
    for pkg in packages {
        for target in pkg.targets() {
            if target.name() == target_name && is_expected_kind(target) {
                result.push(Proposal {
                    pkg,
                    target,
                    requires_features: true,
                    mode,
                });
            }
        }
    }
    if result.is_empty() {
        let suggestion = packages
            .iter()
            .flat_map(|pkg| {
                pkg.targets()
                    .iter()
                    .filter(|target| is_expected_kind(target))
            })
            .map(|target| (lev_distance(target_name, target.name()), target))
            .filter(|&(d, _)| d < 4)
            .min_by_key(|t| t.0)
            .map(|t| t.1);
        match suggestion {
            Some(s) => bail!(
                "no {} target named `{}`\n\nDid you mean `{}`?",
                target_desc,
                target_name,
                s.name()
            ),
            None => bail!("no {} target named `{}`", target_desc, target_name),
        }
    }
    Ok(result)
}