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    use std::fmt;
    use std::task::Poll;

    use crate::core::{Dependency, PackageId, Registry, Summary};
    use crate::sources::source::QueryKind;
    use crate::util::edit_distance::edit_distance;
    use crate::util::{Config, OptVersionReq, VersionExt};
    use anyhow::Error;

    use super::context::Context;
    use super::types::{ConflictMap, ConflictReason};

    /// Error during resolution providing a path of `PackageId`s.
    pub struct ResolveError {
        cause: Error,
        package_path: Vec<PackageId>,
    }

    impl ResolveError {
        pub fn new<E: Into<Error>>(cause: E, package_path: Vec<PackageId>) -> Self {
            Self {
                cause: cause.into(),
                package_path,
            }
        }

        /// Returns a path of packages from the package whose requirements could not be resolved up to
        /// the root.
        pub fn package_path(&self) -> &[PackageId] {
            &self.package_path
        }
    }

    impl std::error::Error for ResolveError {
        fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
            self.cause.source()
        }
    }

    impl fmt::Debug for ResolveError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            self.cause.fmt(f)
        }
    }

    impl fmt::Display for ResolveError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            self.cause.fmt(f)
        }
    }

    pub type ActivateResult<T> = Result<T, ActivateError>;

    #[derive(Debug)]
    pub enum ActivateError {
        Fatal(anyhow::Error),
        Conflict(PackageId, ConflictReason),
    }

    impl From<::anyhow::Error> for ActivateError {
        fn from(t: ::anyhow::Error) -> Self {
            ActivateError::Fatal(t)
        }
    }

    impl From<(PackageId, ConflictReason)> for ActivateError {
        fn from(t: (PackageId, ConflictReason)) -> Self {
            ActivateError::Conflict(t.0, t.1)
        }
    }

    pub(super) fn activation_error(
        cx: &Context,
        registry: &mut dyn Registry,
        parent: &Summary,
        dep: &Dependency,
        conflicting_activations: &ConflictMap,
        candidates: &[Summary],
        config: Option<&Config>,
    ) -> ResolveError {
        let to_resolve_err = |err| {
            ResolveError::new(
                err,
                cx.parents
                    .path_to_bottom(&parent.package_id())
                    .into_iter()
                    .map(|(node, _)| node)
                    .cloned()
                    .collect(),
            )
        };

        if !candidates.is_empty() {
            let mut msg = format!("failed to select a version for `{}`.", dep.package_name());
            msg.push_str("\n    ... required by ");
            msg.push_str(&describe_path_in_context(cx, &parent.package_id()));

            msg.push_str("\nversions that meet the requirements `");
            msg.push_str(&dep.version_req().to_string());
            msg.push_str("` ");

            if let Some(v) = dep.version_req().locked_version() {
                msg.push_str("(locked to ");
                msg.push_str(&v.to_string());
                msg.push_str(") ");
            }

            msg.push_str("are: ");
            msg.push_str(
                &candidates
                    .iter()
                    .map(|v| v.version())
                    .map(|v| v.to_string())
                    .collect::<Vec<_>>()
                    .join(", "),
            );

            let mut conflicting_activations: Vec<_> = conflicting_activations.iter().collect();
            conflicting_activations.sort_unstable();
            // This is reversed to show the newest versions first. I don't know if there is
            // a strong reason to do this, but that is how the code previously worked
            // (see https://github.com/rust-lang/cargo/pull/5037) and I don't feel like changing it.
            conflicting_activations.reverse();
            // Flag used for grouping all semver errors together.
            let mut has_semver = false;

            for (p, r) in &conflicting_activations {
                match r {
                    ConflictReason::Semver => {
                        has_semver = true;
                    }
                    ConflictReason::Links(link) => {
                        msg.push_str("\n\nthe package `");
                        msg.push_str(&*dep.package_name());
                        msg.push_str("` links to the native library `");
                        msg.push_str(link);
                        msg.push_str("`, but it conflicts with a previous package which links to `");
                        msg.push_str(link);
                        msg.push_str("` as well:\n");
                        msg.push_str(&describe_path_in_context(cx, p));
                        msg.push_str("\nOnly one package in the dependency graph may specify the same links value. This helps ensure that only one copy of a native library is linked in the final binary. ");
                        msg.push_str("Try to adjust your dependencies so that only one package uses the `links = \"");
                        msg.push_str(link);
                        msg.push_str("\"` value. For more information, see https://doc.rust-lang.org/cargo/reference/resolver.html#links.");
                    }
                    ConflictReason::MissingFeatures(features) => {
                        msg.push_str("\n\nthe package `");
                        msg.push_str(&*p.name());
                        msg.push_str("` depends on `");
                        msg.push_str(&*dep.package_name());
                        msg.push_str("`, with features: `");
                        msg.push_str(features);
                        msg.push_str("` but `");
                        msg.push_str(&*dep.package_name());
                        msg.push_str("` does not have these features.\n");
                        // p == parent so the full path is redundant.
                    }
                    ConflictReason::RequiredDependencyAsFeature(features) => {
                        msg.push_str("\n\nthe package `");
                        msg.push_str(&*p.name());
                        msg.push_str("` depends on `");
                        msg.push_str(&*dep.package_name());
                        msg.push_str("`, with features: `");
                        msg.push_str(features);
                        msg.push_str("` but `");
                        msg.push_str(&*dep.package_name());
                        msg.push_str("` does not have these features.\n");
                        msg.push_str(
                            " It has a required dependency with that name, \
                             but only optional dependencies can be used as features.\n",
                        );
                        // p == parent so the full path is redundant.
                    }
                    ConflictReason::NonImplicitDependencyAsFeature(features) => {
                        msg.push_str("\n\nthe package `");
                        msg.push_str(&*p.name());
                        msg.push_str("` depends on `");
                        msg.push_str(&*dep.package_name());
                        msg.push_str("`, with features: `");
                        msg.push_str(features);
                        msg.push_str("` but `");
                        msg.push_str(&*dep.package_name());
                        msg.push_str("` does not have these features.\n");
                        msg.push_str(
                            " It has an optional dependency with that name, \
                             but that dependency uses the \"dep:\" \
                             syntax in the features table, so it does not have an \
                             implicit feature with that name.\n",
                        );
                        // p == parent so the full path is redundant.
                    }
                    ConflictReason::PublicDependency(pkg_id) => {
                        // TODO: This needs to be implemented.
                        unimplemented!("pub dep {:?}", pkg_id);
                    }
                    ConflictReason::PubliclyExports(pkg_id) => {
                        // TODO: This needs to be implemented.
                        unimplemented!("pub exp {:?}", pkg_id);
                    }
                }
            }

            if has_semver {
                // Group these errors together.
                msg.push_str("\n\nall possible versions conflict with previously selected packages.");
                for (p, r) in &conflicting_activations {
                    if let ConflictReason::Semver = r {
                        msg.push_str("\n\n  previously selected ");
                        msg.push_str(&describe_path_in_context(cx, p));
                    }
                }
            }

            msg.push_str("\n\nfailed to select a version for `");
            msg.push_str(&*dep.package_name());
            msg.push_str("` which could resolve this conflict");

            return to_resolve_err(anyhow::format_err!("{}", msg));
        }

        // We didn't actually find any candidates, so we need to
        // give an error message that nothing was found.
        //
        // Maybe the user mistyped the ver_req? Like `dep="2"` when `dep="0.2"`
        // was meant. So we re-query the registry with `dep="*"` so we can
        // list a few versions that were actually found.
        let mut new_dep = dep.clone();
        new_dep.set_version_req(OptVersionReq::Any);

        let candidates = loop {
            match registry.query_vec(&new_dep, QueryKind::Exact) {
                Poll::Ready(Ok(candidates)) => break candidates,
                Poll::Ready(Err(e)) => return to_resolve_err(e),
                Poll::Pending => match registry.block_until_ready() {
                    Ok(()) => continue,
                    Err(e) => return to_resolve_err(e),
                },
            }
        };

        let mut candidates: Vec<_> = candidates.into_iter().map(|s| s.into_summary()).collect();

        candidates.sort_unstable_by(|a, b| b.version().cmp(a.version()));

        let mut msg = if !candidates.is_empty() {
            let versions = {
                let mut versions = candidates
                    .iter()
                    .take(3)
                    .map(|cand| cand.version().to_string())
                    .collect::<Vec<_>>();

                if candidates.len() > 3 {
                    versions.push("...".into());
                }

                versions.join(", ")
            };

            let locked_version = dep
                .version_req()
                .locked_version()
                .map(|v| format!(" (locked to {})", v))
                .unwrap_or_default();

            let mut msg = format!(
                "failed to select a version for the requirement `{} = \"{}\"`{}\n\
                 candidate versions found which didn't match: {}\n\
                 location searched: {}\n",
                dep.package_name(),
                dep.version_req(),
                locked_version,
                versions,
                registry.describe_source(dep.source_id()),
            );
            msg.push_str("required by ");
            msg.push_str(&describe_path_in_context(cx, &parent.package_id()));

            // If we have a pre-release candidate, then that may be what our user is looking for
            if let Some(pre) = candidates.iter().find(|c| c.version().is_prerelease()) {
                msg.push_str("\nif you are looking for the prerelease package it needs to be specified explicitly");
                msg.push_str(&format!(
                    "\n    {} = {{ version = \"{}\" }}",
                    pre.name(),
                    pre.version()
                ));
            }

            // If we have a path dependency with a locked version, then this may
            // indicate that we updated a sub-package and forgot to run `cargo
            // update`. In this case try to print a helpful error!
            if dep.source_id().is_path() && dep.version_req().is_locked() {
                msg.push_str(
                    "\nconsider running `cargo update` to update \
                              a path dependency's locked version",
                );
            }

            if registry.is_replaced(dep.source_id()) {
                msg.push_str("\nperhaps a crate was updated and forgotten to be re-vendored?");
            }

            msg
        } else {
            // Maybe the user mistyped the name? Like `dep-thing` when `Dep_Thing`
            // was meant. So we try asking the registry for a `fuzzy` search for suggestions.
            let candidates = loop {
                match registry.query_vec(&new_dep, QueryKind::Fuzzy) {
                    Poll::Ready(Ok(candidates)) => break candidates,
                    Poll::Ready(Err(e)) => return to_resolve_err(e),
                    Poll::Pending => match registry.block_until_ready() {
                        Ok(()) => continue,
                        Err(e) => return to_resolve_err(e),
                    },
                }
            };

            let mut candidates: Vec<_> = candidates.into_iter().map(|s| s.into_summary()).collect();

            candidates.sort_unstable_by_key(|a| a.name());
            candidates.dedup_by(|a, b| a.name() == b.name());
            let mut candidates: Vec<_> = candidates
                .iter()
                .filter_map(|n| Some((edit_distance(&*new_dep.package_name(), &*n.name(), 3)?, n)))
                .collect();
            candidates.sort_by_key(|o| o.0);
            let mut msg: String;
            if candidates.is_empty() {
                msg = format!("no matching package named `{}` found\n", dep.package_name());
            } else {
                msg = format!(
                    "no matching package found\nsearched package name: `{}`\n",
                    dep.package_name()
                );
                let mut names = candidates
                    .iter()
                    .take(3)
                    .map(|c| c.1.name().as_str())
                    .collect::<Vec<_>>();

                if candidates.len() > 3 {
                    names.push("...");
                }
                // Vertically align first suggestion with missing crate name
                // so a typo jumps out at you.
                msg.push_str("perhaps you meant:      ");
                msg.push_str(&names.iter().enumerate().fold(
                    String::default(),
                    |acc, (i, el)| match i {
                        0 => acc + el,
                        i if names.len() - 1 == i && candidates.len() <= 3 => acc + " or " + el,
                        _ => acc + ", " + el,
                    },
                ));
                msg.push('\n');
            }
            msg.push_str(&format!("location searched: {}\n", dep.source_id()));
            msg.push_str("required by ");
            msg.push_str(&describe_path_in_context(cx, &parent.package_id()));

            msg
        };

        if let Some(config) = config {
            if config.offline() {
                msg.push_str(
                    "\nAs a reminder, you're using offline mode (--offline) \
                     which can sometimes cause surprising resolution failures, \
                     if this error is too confusing you may wish to retry \
                     without the offline flag.",
                );
            }
        }

        to_resolve_err(anyhow::format_err!("{}", msg))
    }

    /// Returns String representation of dependency chain for a particular `pkgid`
    /// within given context.
    pub(super) fn describe_path_in_context(cx: &Context, id: &PackageId) -> String {
        let iter = cx
            .parents
            .path_to_bottom(id)
            .into_iter()
            .map(|(p, d)| (p, d.and_then(|d| d.iter().next())));
        describe_path(iter)
    }

    /// Returns String representation of dependency chain for a particular `pkgid`.
    ///
    /// Note that all elements of `path` iterator should have `Some` dependency
    /// except the first one. It would look like:
    ///
    /// (pkg0, None)
    /// -> (pkg1, dep from pkg1 satisfied by pkg0)
    /// -> (pkg2, dep from pkg2 satisfied by pkg1)
    /// -> ...
    pub(crate) fn describe_path<'a>(
        mut path: impl Iterator<Item = (&'a PackageId, Option<&'a Dependency>)>,
    ) -> String {
        use std::fmt::Write;

        if let Some(p) = path.next() {
            let mut dep_path_desc = format!("package `{}`", p.0);
            for (pkg, dep) in path {
                let dep = dep.unwrap();
                let source_kind = if dep.source_id().is_path() {
                    "path "
                } else if dep.source_id().is_git() {
                    "git "
                } else {
                    ""
                };
                let requirement = if source_kind.is_empty() {
                    format!("{} = \"{}\"", dep.name_in_toml(), dep.version_req())
                } else {
                    dep.name_in_toml().to_string()
                };
                let locked_version = dep
                    .version_req()
                    .locked_version()
                    .map(|v| format!("(locked to {}) ", v))
                    .unwrap_or_default();

                write!(
                    dep_path_desc,
                    "\n    ... which satisfies {}dependency `{}` {}of package `{}`",
                    source_kind, requirement, locked_version, pkg
                )
                .unwrap();
            }

            return dep_path_desc;
        }

        String::new()
    }