use std::cmp::Reverse;

use errors::{Applicability, DiagnosticBuilder};
use log::debug;
use rustc::bug;
use rustc::hir::def::{self, DefKind, NonMacroAttrKind};
use rustc::hir::def::Namespace::{self, *};
use rustc::hir::def_id::{CRATE_DEF_INDEX, DefId};
use rustc::session::Session;
use rustc::ty::{self, DefIdTree};
use rustc::util::nodemap::FxHashSet;
use rustc_feature::BUILTIN_ATTRIBUTES;
use syntax::ast::{self, Ident, Path};
use syntax::source_map::SourceMap;
use syntax::struct_span_err;
use syntax::symbol::{Symbol, kw};
use syntax::util::lev_distance::find_best_match_for_name;
use syntax_pos::hygiene::MacroKind;
use syntax_pos::{BytePos, Span, MultiSpan};

use crate::resolve_imports::{ImportDirective, ImportDirectiveSubclass, ImportResolver};
use crate::path_names_to_string;
use crate::{BindingError, CrateLint, HasGenericParams, LegacyScope, Module, ModuleOrUniformRoot};
use crate::{PathResult, ParentScope, ResolutionError, Resolver, Scope, ScopeSet, Segment};

use rustc_error_codes::*;

type Res = def::Res<ast::NodeId>;

/// A vector of spans and replacements, a message and applicability.
crate type Suggestion = (Vec<(Span, String)>, String, Applicability);

crate struct TypoSuggestion {
    pub candidate: Symbol,
    pub res: Res,
}

impl TypoSuggestion {
    crate fn from_res(candidate: Symbol, res: Res) -> TypoSuggestion {
        TypoSuggestion { candidate, res }
    }
}

/// A free importable items suggested in case of resolution failure.
crate struct ImportSuggestion {
    pub did: Option<DefId>,
    pub path: Path,
}

/// Adjust the impl span so that just the `impl` keyword is taken by removing
/// everything after `<` (`"impl<T> Iterator for A<T> {}" -> "impl"`) and
/// everything after the first whitespace (`"impl Iterator for A" -> "impl"`).
///
/// *Attention*: the method used is very fragile since it essentially duplicates the work of the
/// parser. If you need to use this function or something similar, please consider updating the
/// `source_map` functions and this function to something more robust.
fn reduce_impl_span_to_impl_keyword(cm: &SourceMap, impl_span: Span) -> Span {
    let impl_span = cm.span_until_char(impl_span, '<');
    let impl_span = cm.span_until_whitespace(impl_span);
    impl_span
}

impl<'a> Resolver<'a> {
    crate fn add_module_candidates(
        &mut self,
        module: Module<'a>,
        names: &mut Vec<TypoSuggestion>,
        filter_fn: &impl Fn(Res) -> bool,
    ) {
        for (key, resolution) in self.resolutions(module).borrow().iter() {
            if let Some(binding) = resolution.borrow().binding {
                let res = binding.res();
                if filter_fn(res) {
                    names.push(TypoSuggestion::from_res(key.ident.name, res));
                }
            }
        }
    }

    /// Combines an error with provided span and emits it.
    ///
    /// This takes the error provided, combines it with the span and any additional spans inside the
    /// error and emits it.
    crate fn report_error(&self, span: Span, resolution_error: ResolutionError<'_>) {
        self.into_struct_error(span, resolution_error).emit();
    }

    crate fn into_struct_error(
        &self, span: Span, resolution_error: ResolutionError<'_>
    ) -> DiagnosticBuilder<'_> {
        match resolution_error {
            ResolutionError::GenericParamsFromOuterFunction(outer_res, has_generic_params) => {
                let mut err = struct_span_err!(self.session,
                    span,
                    E0401,
                    "can't use generic parameters from outer function",
                );
                err.span_label(span, format!("use of generic parameter from outer function"));

                let cm = self.session.source_map();
                match outer_res {
                    Res::SelfTy(maybe_trait_defid, maybe_impl_defid) => {
                        if let Some(impl_span) = maybe_impl_defid.and_then(|def_id| {
                            self.definitions.opt_span(def_id)
                        }) {
                            err.span_label(
                                reduce_impl_span_to_impl_keyword(cm, impl_span),
                                "`Self` type implicitly declared here, by this `impl`",
                            );
                        }
                        match (maybe_trait_defid, maybe_impl_defid) {
                            (Some(_), None) => {
                                err.span_label(span, "can't use `Self` here");
                            }
                            (_, Some(_)) => {
                                err.span_label(span, "use a type here instead");
                            }
                            (None, None) => bug!("`impl` without trait nor type?"),
                        }
                        return err;
                    },
                    Res::Def(DefKind::TyParam, def_id) => {
                        if let Some(span) = self.definitions.opt_span(def_id) {
                            err.span_label(span, "type parameter from outer function");
                        }
                    }
                    Res::Def(DefKind::ConstParam, def_id) => {
                        if let Some(span) = self.definitions.opt_span(def_id) {
                            err.span_label(span, "const parameter from outer function");
                        }
                    }
                    _ => {
                        bug!("GenericParamsFromOuterFunction should only be used with Res::SelfTy, \
                            DefKind::TyParam");
                    }
                }

                if has_generic_params == HasGenericParams::Yes {
                    // Try to retrieve the span of the function signature and generate a new
                    // message with a local type or const parameter.
                    let sugg_msg = &format!("try using a local generic parameter instead");
                    if let Some((sugg_span, snippet)) = cm.generate_local_type_param_snippet(span) {
                        // Suggest the modification to the user
                        err.span_suggestion(
                            sugg_span,
                            sugg_msg,
                            snippet,
                            Applicability::MachineApplicable,
                        );
                    } else if let Some(sp) = cm.generate_fn_name_span(span) {
                        err.span_label(sp,
                            format!("try adding a local generic parameter in this method instead"));
                    } else {
                        err.help(&format!("try using a local generic parameter instead"));
                    }
                }

                err
            }
            ResolutionError::NameAlreadyUsedInParameterList(name, first_use_span) => {
                let mut err = struct_span_err!(
                    self.session,
                    span,
                    E0403,
                    "the name `{}` is already used for a generic \
                     parameter in this item's generic parameters",
                    name,
                );
                err.span_label(span, "already used");
                err.span_label(first_use_span, format!("first use of `{}`", name));
                err
            }
            ResolutionError::MethodNotMemberOfTrait(method, trait_) => {
                let mut err = struct_span_err!(self.session,
                                            span,
                                            E0407,
                                            "method `{}` is not a member of trait `{}`",
                                            method,
                                            trait_);
                err.span_label(span, format!("not a member of trait `{}`", trait_));
                err
            }
            ResolutionError::TypeNotMemberOfTrait(type_, trait_) => {
                let mut err = struct_span_err!(self.session,
                                span,
                                E0437,
                                "type `{}` is not a member of trait `{}`",
                                type_,
                                trait_);
                err.span_label(span, format!("not a member of trait `{}`", trait_));
                err
            }
            ResolutionError::ConstNotMemberOfTrait(const_, trait_) => {
                let mut err = struct_span_err!(self.session,
                                span,
                                E0438,
                                "const `{}` is not a member of trait `{}`",
                                const_,
                                trait_);
                err.span_label(span, format!("not a member of trait `{}`", trait_));
                err
            }
            ResolutionError::VariableNotBoundInPattern(binding_error) => {
                let BindingError { name, target, origin, could_be_path } = binding_error;

                let target_sp = target.iter().copied().collect::<Vec<_>>();
                let origin_sp = origin.iter().copied().collect::<Vec<_>>();

                let msp = MultiSpan::from_spans(target_sp.clone());
                let mut err = struct_span_err!(
                    self.session,
                    msp,
                    E0408,
                    "variable `{}` is not bound in all patterns", name,
                );
                for sp in target_sp {
                    err.span_label(sp, format!("pattern doesn't bind `{}`", name));
                }
                for sp in origin_sp {
                    err.span_label(sp, "variable not in all patterns");
                }
                if *could_be_path {
                    let help_msg = format!(
                        "if you meant to match on a variant or a `const` item, consider \
                         making the path in the pattern qualified: `?::{}`",
                         name,
                     );
                    err.span_help(span, &help_msg);
                }
                err
            }
            ResolutionError::VariableBoundWithDifferentMode(variable_name,
                                                            first_binding_span) => {
                let mut err = struct_span_err!(self.session,
                                span,
                                E0409,
                                "variable `{}` is bound in inconsistent \
                                ways within the same match arm",
                                variable_name);
                err.span_label(span, "bound in different ways");
                err.span_label(first_binding_span, "first binding");
                err
            }
            ResolutionError::IdentifierBoundMoreThanOnceInParameterList(identifier) => {
                let mut err = struct_span_err!(self.session,
                                span,
                                E0415,
                                "identifier `{}` is bound more than once in this parameter list",
                                identifier);
                err.span_label(span, "used as parameter more than once");
                err
            }
            ResolutionError::IdentifierBoundMoreThanOnceInSamePattern(identifier) => {
                let mut err = struct_span_err!(self.session,
                                span,
                                E0416,
                                "identifier `{}` is bound more than once in the same pattern",
                                identifier);
                err.span_label(span, "used in a pattern more than once");
                err
            }
            ResolutionError::UndeclaredLabel(name, lev_candidate) => {
                let mut err = struct_span_err!(self.session,
                                            span,
                                            E0426,
                                            "use of undeclared label `{}`",
                                            name);
                if let Some(lev_candidate) = lev_candidate {
                    err.span_suggestion(
                        span,
                        "a label with a similar name exists in this scope",
                        lev_candidate.to_string(),
                        Applicability::MaybeIncorrect,
                    );
                } else {
                    err.span_label(span, format!("undeclared label `{}`", name));
                }
                err
            }
            ResolutionError::SelfImportsOnlyAllowedWithin => {
                struct_span_err!(self.session,
                                span,
                                E0429,
                                "{}",
                                "`self` imports are only allowed within a { } list")
            }
            ResolutionError::SelfImportCanOnlyAppearOnceInTheList => {
                let mut err = struct_span_err!(self.session, span, E0430,
                                            "`self` import can only appear once in an import list");
                err.span_label(span, "can only appear once in an import list");
                err
            }
            ResolutionError::SelfImportOnlyInImportListWithNonEmptyPrefix => {
                let mut err = struct_span_err!(self.session, span, E0431,
                                            "`self` import can only appear in an import list with \
                                                a non-empty prefix");
                err.span_label(span, "can only appear in an import list with a non-empty prefix");
                err
            }
            ResolutionError::FailedToResolve { label, suggestion } => {
                let mut err = struct_span_err!(self.session, span, E0433,
                                            "failed to resolve: {}", &label);
                err.span_label(span, label);

                if let Some((suggestions, msg, applicability)) = suggestion {
                    err.multipart_suggestion(&msg, suggestions, applicability);
                }

                err
            }
            ResolutionError::CannotCaptureDynamicEnvironmentInFnItem => {
                let mut err = struct_span_err!(self.session,
                                            span,
                                            E0434,
                                            "{}",
                                            "can't capture dynamic environment in a fn item");
                err.help("use the `|| { ... }` closure form instead");
                err
            }
            ResolutionError::AttemptToUseNonConstantValueInConstant => {
                let mut err = struct_span_err!(self.session, span, E0435,
                                            "attempt to use a non-constant value in a constant");
                err.span_label(span, "non-constant value");
                err
            }
            ResolutionError::BindingShadowsSomethingUnacceptable(what_binding, name, binding) => {
                let res = binding.res();
                let shadows_what = res.descr();
                let mut err = struct_span_err!(self.session, span, E0530, "{}s cannot shadow {}s",
                                            what_binding, shadows_what);
                err.span_label(span, format!("cannot be named the same as {} {}",
                                            res.article(), shadows_what));
                let participle = if binding.is_import() { "imported" } else { "defined" };
                let msg = format!("the {} `{}` is {} here", shadows_what, name, participle);
                err.span_label(binding.span, msg);
                err
            }
            ResolutionError::ForwardDeclaredTyParam => {
                let mut err = struct_span_err!(self.session, span, E0128,
                                            "type parameters with a default cannot use \
                                                forward declared identifiers");
                err.span_label(
                    span, "defaulted type parameters cannot be forward declared".to_string());
                err
            }
            ResolutionError::SelfInTyParamDefault => {
                let mut err = struct_span_err!(
                    self.session,
                    span,
                    E0735,
                    "type parameters cannot use `Self` in their defaults"
                );
                err.span_label(
                    span, "`Self` in type parameter default".to_string());
                err
            }
        }
    }

    /// Lookup typo candidate in scope for a macro or import.
    fn early_lookup_typo_candidate(
        &mut self,
        scope_set: ScopeSet,
        parent_scope: &ParentScope<'a>,
        ident: Ident,
        filter_fn: &impl Fn(Res) -> bool,
    ) -> Option<TypoSuggestion> {
        let mut suggestions = Vec::new();
        self.visit_scopes(scope_set, parent_scope, ident, |this, scope, use_prelude, _| {
            match scope {
                Scope::DeriveHelpers(expn_id) => {
                    let res = Res::NonMacroAttr(NonMacroAttrKind::DeriveHelper);
                    if filter_fn(res) {
                        suggestions.extend(this.helper_attrs.get(&expn_id)
                                               .into_iter().flatten().map(|ident| {
                            TypoSuggestion::from_res(ident.name, res)
                        }));
                    }
                }
                Scope::DeriveHelpersCompat => {
                    let res = Res::NonMacroAttr(NonMacroAttrKind::DeriveHelper);
                    if filter_fn(res) {
                        for derive in parent_scope.derives {
                            let parent_scope =
                                &ParentScope { derives: &[], ..*parent_scope };
                            if let Ok((Some(ext), _)) = this.resolve_macro_path(
                                derive, Some(MacroKind::Derive), parent_scope, false, false
                            ) {
                                suggestions.extend(ext.helper_attrs.iter().map(|name| {
                                    TypoSuggestion::from_res(*name, res)
                                }));
                            }
                        }
                    }
                }
                Scope::MacroRules(legacy_scope) => {
                    if let LegacyScope::Binding(legacy_binding) = legacy_scope {
                        let res = legacy_binding.binding.res();
                        if filter_fn(res) {
                            suggestions.push(
                                TypoSuggestion::from_res(legacy_binding.ident.name, res)
                            )
                        }
                    }
                }
                Scope::CrateRoot => {
                    let root_ident = Ident::new(kw::PathRoot, ident.span);
                    let root_module = this.resolve_crate_root(root_ident);
                    this.add_module_candidates(root_module, &mut suggestions, filter_fn);
                }
                Scope::Module(module) => {
                    this.add_module_candidates(module, &mut suggestions, filter_fn);
                }
                Scope::RegisteredAttrs => {
                    let res = Res::NonMacroAttr(NonMacroAttrKind::Registered);
                    if filter_fn(res) {
                        suggestions.extend(this.registered_attrs.iter().map(|ident| {
                            TypoSuggestion::from_res(ident.name, res)
                        }));
                    }
                }
                Scope::MacroUsePrelude => {
                    suggestions.extend(this.macro_use_prelude.iter().filter_map(|(name, binding)| {
                        let res = binding.res();
                        if filter_fn(res) {
                            Some(TypoSuggestion::from_res(*name, res))
                        } else {
                            None
                        }
                    }));
                }
                Scope::BuiltinAttrs => {
                    let res = Res::NonMacroAttr(NonMacroAttrKind::Builtin);
                    if filter_fn(res) {
                        suggestions.extend(BUILTIN_ATTRIBUTES.iter().map(|(name, ..)| {
                            TypoSuggestion::from_res(*name, res)
                        }));
                    }
                }
                Scope::ExternPrelude => {
                    suggestions.extend(this.extern_prelude.iter().filter_map(|(ident, _)| {
                        let res = Res::Def(DefKind::Mod, DefId::local(CRATE_DEF_INDEX));
                        if filter_fn(res) {
                            Some(TypoSuggestion::from_res(ident.name, res))
                        } else {
                            None
                        }
                    }));
                }
                Scope::ToolPrelude => {
                    let res = Res::NonMacroAttr(NonMacroAttrKind::Tool);
                    suggestions.extend(this.registered_tools.iter().map(|ident| {
                        TypoSuggestion::from_res(ident.name, res)
                    }));
                }
                Scope::StdLibPrelude => {
                    if let Some(prelude) = this.prelude {
                        let mut tmp_suggestions = Vec::new();
                        this.add_module_candidates(prelude, &mut tmp_suggestions, filter_fn);
                        suggestions.extend(tmp_suggestions.into_iter().filter(|s| {
                            use_prelude || this.is_builtin_macro(s.res)
                        }));
                    }
                }
                Scope::BuiltinTypes => {
                    let primitive_types = &this.primitive_type_table.primitive_types;
                    suggestions.extend(
                        primitive_types.iter().flat_map(|(name, prim_ty)| {
                            let res = Res::PrimTy(*prim_ty);
                            if filter_fn(res) {
                                Some(TypoSuggestion::from_res(*name, res))
                            } else {
                                None
                            }
                        })
                    )
                }
            }

            None::<()>
        });

        // Make sure error reporting is deterministic.
        suggestions.sort_by_cached_key(|suggestion| suggestion.candidate.as_str());

        match find_best_match_for_name(
            suggestions.iter().map(|suggestion| &suggestion.candidate),
            &ident.as_str(),
            None,
        ) {
            Some(found) if found != ident.name => suggestions
                .into_iter()
                .find(|suggestion| suggestion.candidate == found),
            _ => None,
        }
    }

    fn lookup_import_candidates_from_module<FilterFn>(&mut self,
                                          lookup_ident: Ident,
                                          namespace: Namespace,
                                          start_module: Module<'a>,
                                          crate_name: Ident,
                                          filter_fn: FilterFn)
                                          -> Vec<ImportSuggestion>
        where FilterFn: Fn(Res) -> bool
    {
        let mut candidates = Vec::new();
        let mut seen_modules = FxHashSet::default();
        let not_local_module = crate_name.name != kw::Crate;
        let mut worklist = vec![(start_module, Vec::<ast::PathSegment>::new(), not_local_module)];

        while let Some((in_module,
                        path_segments,
                        in_module_is_extern)) = worklist.pop() {
            // We have to visit module children in deterministic order to avoid
            // instabilities in reported imports (#43552).
            in_module.for_each_child(self, |this, ident, ns, name_binding| {
                // avoid imports entirely
                if name_binding.is_import() && !name_binding.is_extern_crate() { return; }
                // avoid non-importable candidates as well
                if !name_binding.is_importable() { return; }

                // collect results based on the filter function
                if ident.name == lookup_ident.name && ns == namespace {
                    let res = name_binding.res();
                    if filter_fn(res) {
                        // create the path
                        let mut segms = path_segments.clone();
                        if lookup_ident.span.rust_2018() {
                            // crate-local absolute paths start with `crate::` in edition 2018
                            // FIXME: may also be stabilized for Rust 2015 (Issues #45477, #44660)
                            segms.insert(
                                0, ast::PathSegment::from_ident(crate_name)
                            );
                        }

                        segms.push(ast::PathSegment::from_ident(ident));
                        let path = Path {
                            span: name_binding.span,
                            segments: segms,
                        };
                        // the entity is accessible in the following cases:
                        // 1. if it's defined in the same crate, it's always
                        // accessible (since private entities can be made public)
                        // 2. if it's defined in another crate, it's accessible
                        // only if both the module is public and the entity is
                        // declared as public (due to pruning, we don't explore
                        // outside crate private modules => no need to check this)
                        if !in_module_is_extern || name_binding.vis == ty::Visibility::Public {
                            let did = match res {
                                Res::Def(DefKind::Ctor(..), did) => this.parent(did),
                                _ => res.opt_def_id(),
                            };
                            candidates.push(ImportSuggestion { did, path });
                        }
                    }
                }

                // collect submodules to explore
                if let Some(module) = name_binding.module() {
                    // form the path
                    let mut path_segments = path_segments.clone();
                    path_segments.push(ast::PathSegment::from_ident(ident));

                    let is_extern_crate_that_also_appears_in_prelude =
                        name_binding.is_extern_crate() &&
                        lookup_ident.span.rust_2018();

                    let is_visible_to_user =
                        !in_module_is_extern || name_binding.vis == ty::Visibility::Public;

                    if !is_extern_crate_that_also_appears_in_prelude && is_visible_to_user {
                        // add the module to the lookup
                        let is_extern = in_module_is_extern || name_binding.is_extern_crate();
                        if seen_modules.insert(module.def_id().unwrap()) {
                            worklist.push((module, path_segments, is_extern));
                        }
                    }
                }
            })
        }

        candidates
    }

    /// When name resolution fails, this method can be used to look up candidate
    /// entities with the expected name. It allows filtering them using the
    /// supplied predicate (which should be used to only accept the types of
    /// definitions expected, e.g., traits). The lookup spans across all crates.
    ///
    /// N.B., the method does not look into imports, but this is not a problem,
    /// since we report the definitions (thus, the de-aliased imports).
    crate fn lookup_import_candidates<FilterFn>(
        &mut self, lookup_ident: Ident, namespace: Namespace, filter_fn: FilterFn
    ) -> Vec<ImportSuggestion>
        where FilterFn: Fn(Res) -> bool
    {
        let mut suggestions = self.lookup_import_candidates_from_module(
            lookup_ident, namespace, self.graph_root, Ident::with_dummy_span(kw::Crate), &filter_fn
        );

        if lookup_ident.span.rust_2018() {
            let extern_prelude_names = self.extern_prelude.clone();
            for (ident, _) in extern_prelude_names.into_iter() {
                if ident.span.from_expansion() {
                    // Idents are adjusted to the root context before being
                    // resolved in the extern prelude, so reporting this to the
                    // user is no help. This skips the injected
                    // `extern crate std` in the 2018 edition, which would
                    // otherwise cause duplicate suggestions.
                    continue;
                }
                if let Some(crate_id) = self.crate_loader.maybe_process_path_extern(ident.name,
                                                                                    ident.span) {
                    let crate_root = self.get_module(DefId {
                        krate: crate_id,
                        index: CRATE_DEF_INDEX,
                    });
                    suggestions.extend(self.lookup_import_candidates_from_module(
                        lookup_ident, namespace, crate_root, ident, &filter_fn));
                }
            }
        }

        suggestions
    }

    crate fn unresolved_macro_suggestions(
        &mut self,
        err: &mut DiagnosticBuilder<'a>,
        macro_kind: MacroKind,
        parent_scope: &ParentScope<'a>,
        ident: Ident,
    ) {
        let is_expected = &|res: Res| res.macro_kind() == Some(macro_kind);
        let suggestion = self.early_lookup_typo_candidate(
            ScopeSet::Macro(macro_kind), parent_scope, ident, is_expected
        );
        self.add_typo_suggestion(err, suggestion, ident.span);

        if macro_kind == MacroKind::Derive &&
           (ident.as_str() == "Send" || ident.as_str() == "Sync") {
            let msg = format!("unsafe traits like `{}` should be implemented explicitly", ident);
            err.span_note(ident.span, &msg);
        }
        if self.macro_names.contains(&ident.modern()) {
            err.help("have you added the `#[macro_use]` on the module/import?");
        }
    }

    crate fn add_typo_suggestion(
        &self,
        err: &mut DiagnosticBuilder<'_>,
        suggestion: Option<TypoSuggestion>,
        span: Span,
    ) -> bool {
        if let Some(suggestion) = suggestion {
            let msg = format!(
                "{} {} with a similar name exists", suggestion.res.article(), suggestion.res.descr()
            );
            err.span_suggestion(
                span, &msg, suggestion.candidate.to_string(), Applicability::MaybeIncorrect
            );
            let def_span = suggestion.res.opt_def_id()
                .and_then(|def_id| self.definitions.opt_span(def_id));
            if let Some(span) = def_span {
                err.span_label(span, &format!(
                    "similarly named {} `{}` defined here",
                    suggestion.res.descr(),
                    suggestion.candidate.as_str(),
                ));
            }
            return true;
        }
        false
    }
}

impl<'a, 'b> ImportResolver<'a, 'b> {
    /// Adds suggestions for a path that cannot be resolved.
    pub(crate) fn make_path_suggestion(
        &mut self,
        span: Span,
        mut path: Vec<Segment>,
        parent_scope: &ParentScope<'b>,
    ) -> Option<(Vec<Segment>, Vec<String>)> {
        debug!("make_path_suggestion: span={:?} path={:?}", span, path);

        match (path.get(0), path.get(1)) {
            // `{{root}}::ident::...` on both editions.
            // On 2015 `{{root}}` is usually added implicitly.
            (Some(fst), Some(snd)) if fst.ident.name == kw::PathRoot &&
                                      !snd.ident.is_path_segment_keyword() => {}
            // `ident::...` on 2018.
            (Some(fst), _) if fst.ident.span.rust_2018() &&
                              !fst.ident.is_path_segment_keyword() => {
                // Insert a placeholder that's later replaced by `self`/`super`/etc.
                path.insert(0, Segment::from_ident(Ident::invalid()));
            }
            _ => return None,
        }

        self.make_missing_self_suggestion(span, path.clone(), parent_scope)
            .or_else(|| self.make_missing_crate_suggestion(span, path.clone(), parent_scope))
            .or_else(|| self.make_missing_super_suggestion(span, path.clone(), parent_scope))
            .or_else(|| self.make_external_crate_suggestion(span, path, parent_scope))
    }

    /// Suggest a missing `self::` if that resolves to an correct module.
    ///
    /// ```
    ///    |
    /// LL | use foo::Bar;
    ///    |     ^^^ did you mean `self::foo`?
    /// ```
    fn make_missing_self_suggestion(
        &mut self,
        span: Span,
        mut path: Vec<Segment>,
        parent_scope: &ParentScope<'b>,
    ) -> Option<(Vec<Segment>, Vec<String>)> {
        // Replace first ident with `self` and check if that is valid.
        path[0].ident.name = kw::SelfLower;
        let result = self.r.resolve_path(&path, None, parent_scope, false, span, CrateLint::No);
        debug!("make_missing_self_suggestion: path={:?} result={:?}", path, result);
        if let PathResult::Module(..) = result {
            Some((path, Vec::new()))
        } else {
            None
        }
    }

    /// Suggests a missing `crate::` if that resolves to an correct module.
    ///
    /// ```
    ///    |
    /// LL | use foo::Bar;
    ///    |     ^^^ did you mean `crate::foo`?
    /// ```
    fn make_missing_crate_suggestion(
        &mut self,
        span: Span,
        mut path: Vec<Segment>,
        parent_scope: &ParentScope<'b>,
    ) -> Option<(Vec<Segment>, Vec<String>)> {
        // Replace first ident with `crate` and check if that is valid.
        path[0].ident.name = kw::Crate;
        let result = self.r.resolve_path(&path, None, parent_scope, false, span, CrateLint::No);
        debug!("make_missing_crate_suggestion:  path={:?} result={:?}", path, result);
        if let PathResult::Module(..) = result {
            Some((
                path,
                vec![
                    "`use` statements changed in Rust 2018; read more at \
                     <https://doc.rust-lang.org/edition-guide/rust-2018/module-system/path-\
                     clarity.html>".to_string()
                ],
            ))
        } else {
            None
        }
    }

    /// Suggests a missing `super::` if that resolves to an correct module.
    ///
    /// ```
    ///    |
    /// LL | use foo::Bar;
    ///    |     ^^^ did you mean `super::foo`?
    /// ```
    fn make_missing_super_suggestion(
        &mut self,
        span: Span,
        mut path: Vec<Segment>,
        parent_scope: &ParentScope<'b>,
    ) -> Option<(Vec<Segment>, Vec<String>)> {
        // Replace first ident with `crate` and check if that is valid.
        path[0].ident.name = kw::Super;
        let result = self.r.resolve_path(&path, None, parent_scope, false, span, CrateLint::No);
        debug!("make_missing_super_suggestion:  path={:?} result={:?}", path, result);
        if let PathResult::Module(..) = result {
            Some((path, Vec::new()))
        } else {
            None
        }
    }

    /// Suggests a missing external crate name if that resolves to an correct module.
    ///
    /// ```
    ///    |
    /// LL | use foobar::Baz;
    ///    |     ^^^^^^ did you mean `baz::foobar`?
    /// ```
    ///
    /// Used when importing a submodule of an external crate but missing that crate's
    /// name as the first part of path.
    fn make_external_crate_suggestion(
        &mut self,
        span: Span,
        mut path: Vec<Segment>,
        parent_scope: &ParentScope<'b>,
    ) -> Option<(Vec<Segment>, Vec<String>)> {
        if path[1].ident.span.rust_2015() {
            return None;
        }

        // Sort extern crate names in reverse order to get
        // 1) some consistent ordering for emitted dignostics, and
        // 2) `std` suggestions before `core` suggestions.
        let mut extern_crate_names =
            self.r.extern_prelude.iter().map(|(ident, _)| ident.name).collect::<Vec<_>>();
        extern_crate_names.sort_by_key(|name| Reverse(name.as_str()));

        for name in extern_crate_names.into_iter() {
            // Replace first ident with a crate name and check if that is valid.
            path[0].ident.name = name;
            let result = self.r.resolve_path(&path, None, parent_scope, false, span, CrateLint::No);
            debug!("make_external_crate_suggestion: name={:?} path={:?} result={:?}",
                    name, path, result);
            if let PathResult::Module(..) = result {
                return Some((path, Vec::new()));
            }
        }

        None
    }

    /// Suggests importing a macro from the root of the crate rather than a module within
    /// the crate.
    ///
    /// ```
    /// help: a macro with this name exists at the root of the crate
    ///    |
    /// LL | use issue_59764::makro;
    ///    |     ^^^^^^^^^^^^^^^^^^
    ///    |
    ///    = note: this could be because a macro annotated with `#[macro_export]` will be exported
    ///            at the root of the crate instead of the module where it is defined
    /// ```
    pub(crate) fn check_for_module_export_macro(
        &mut self,
        directive: &'b ImportDirective<'b>,
        module: ModuleOrUniformRoot<'b>,
        ident: Ident,
    ) -> Option<(Option<Suggestion>, Vec<String>)> {
        let mut crate_module = if let ModuleOrUniformRoot::Module(module) = module {
            module
        } else {
            return None;
        };

        while let Some(parent) = crate_module.parent {
            crate_module = parent;
        }

        if ModuleOrUniformRoot::same_def(ModuleOrUniformRoot::Module(crate_module), module) {
            // Don't make a suggestion if the import was already from the root of the
            // crate.
            return None;
        }

        let resolutions = self.r.resolutions(crate_module).borrow();
        let resolution = resolutions.get(&self.r.new_key(ident, MacroNS))?;
        let binding = resolution.borrow().binding()?;
        if let Res::Def(DefKind::Macro(MacroKind::Bang), _) = binding.res() {
            let module_name = crate_module.kind.name().unwrap();
            let import = match directive.subclass {
                ImportDirectiveSubclass::SingleImport { source, target, .. } if source != target =>
                    format!("{} as {}", source, target),
                _ => format!("{}", ident),
            };

            let mut corrections: Vec<(Span, String)> = Vec::new();
            if !directive.is_nested() {
                // Assume this is the easy case of `use issue_59764::foo::makro;` and just remove
                // intermediate segments.
                corrections.push((directive.span, format!("{}::{}", module_name, import)));
            } else {
                // Find the binding span (and any trailing commas and spaces).
                //   ie. `use a::b::{c, d, e};`
                //                      ^^^
                let (found_closing_brace, binding_span) = find_span_of_binding_until_next_binding(
                    self.r.session, directive.span, directive.use_span,
                );
                debug!("check_for_module_export_macro: found_closing_brace={:?} binding_span={:?}",
                       found_closing_brace, binding_span);

                let mut removal_span = binding_span;
                if found_closing_brace {
                    // If the binding span ended with a closing brace, as in the below example:
                    //   ie. `use a::b::{c, d};`
                    //                      ^
                    // Then expand the span of characters to remove to include the previous
                    // binding's trailing comma.
                    //   ie. `use a::b::{c, d};`
                    //                    ^^^
                    if let Some(previous_span) = extend_span_to_previous_binding(
                        self.r.session, binding_span,
                    ) {
                        debug!("check_for_module_export_macro: previous_span={:?}", previous_span);
                        removal_span = removal_span.with_lo(previous_span.lo());
                    }
                }
                debug!("check_for_module_export_macro: removal_span={:?}", removal_span);

                // Remove the `removal_span`.
                corrections.push((removal_span, "".to_string()));

                // Find the span after the crate name and if it has nested imports immediatately
                // after the crate name already.
                //   ie. `use a::b::{c, d};`
                //               ^^^^^^^^^
                //   or  `use a::{b, c, d}};`
                //               ^^^^^^^^^^^
                let (has_nested, after_crate_name) = find_span_immediately_after_crate_name(
                    self.r.session, module_name, directive.use_span,
                );
                debug!("check_for_module_export_macro: has_nested={:?} after_crate_name={:?}",
                       has_nested, after_crate_name);

                let source_map = self.r.session.source_map();

                // Add the import to the start, with a `{` if required.
                let start_point = source_map.start_point(after_crate_name);
                if let Ok(start_snippet) = source_map.span_to_snippet(start_point) {
                    corrections.push((
                        start_point,
                        if has_nested {
                            // In this case, `start_snippet` must equal '{'.
                            format!("{}{}, ", start_snippet, import)
                        } else {
                            // In this case, add a `{`, then the moved import, then whatever
                            // was there before.
                            format!("{{{}, {}", import, start_snippet)
                        }
                    ));
                }

                // Add a `};` to the end if nested, matching the `{` added at the start.
                if !has_nested {
                    corrections.push((source_map.end_point(after_crate_name),
                                     "};".to_string()));
                }
            }

            let suggestion = Some((
                corrections,
                String::from("a macro with this name exists at the root of the crate"),
                Applicability::MaybeIncorrect,
            ));
            let note = vec![
                "this could be because a macro annotated with `#[macro_export]` will be exported \
                 at the root of the crate instead of the module where it is defined".to_string(),
            ];
            Some((suggestion, note))
        } else {
            None
        }
    }
}

/// Given a `binding_span` of a binding within a use statement:
///
/// ```
/// use foo::{a, b, c};
///              ^
/// ```
///
/// then return the span until the next binding or the end of the statement:
///
/// ```
/// use foo::{a, b, c};
///              ^^^
/// ```
pub(crate) fn find_span_of_binding_until_next_binding(
    sess: &Session,
    binding_span: Span,
    use_span: Span,
) -> (bool, Span) {
    let source_map = sess.source_map();

    // Find the span of everything after the binding.
    //   ie. `a, e};` or `a};`
    let binding_until_end = binding_span.with_hi(use_span.hi());

    // Find everything after the binding but not including the binding.
    //   ie. `, e};` or `};`
    let after_binding_until_end = binding_until_end.with_lo(binding_span.hi());

    // Keep characters in the span until we encounter something that isn't a comma or
    // whitespace.
    //   ie. `, ` or ``.
    //
    // Also note whether a closing brace character was encountered. If there
    // was, then later go backwards to remove any trailing commas that are left.
    let mut found_closing_brace = false;
    let after_binding_until_next_binding = source_map.span_take_while(
        after_binding_until_end,
        |&ch| {
            if ch == '}' { found_closing_brace = true; }
            ch == ' ' || ch == ','
        }
    );

    // Combine the two spans.
    //   ie. `a, ` or `a`.
    //
    // Removing these would leave `issue_52891::{d, e};` or `issue_52891::{d, e, };`
    let span = binding_span.with_hi(after_binding_until_next_binding.hi());

    (found_closing_brace, span)
}

/// Given a `binding_span`, return the span through to the comma or opening brace of the previous
/// binding.
///
/// ```
/// use foo::a::{a, b, c};
///               ^^--- binding span
///               |
///               returned span
///
/// use foo::{a, b, c};
///           --- binding span
/// ```
pub(crate) fn extend_span_to_previous_binding(
    sess: &Session,
    binding_span: Span,
) -> Option<Span> {
    let source_map = sess.source_map();

    // `prev_source` will contain all of the source that came before the span.
    // Then split based on a command and take the first (ie. closest to our span)
    // snippet. In the example, this is a space.
    let prev_source = source_map.span_to_prev_source(binding_span).ok()?;

    let prev_comma = prev_source.rsplit(',').collect::<Vec<_>>();
    let prev_starting_brace = prev_source.rsplit('{').collect::<Vec<_>>();
    if prev_comma.len() <= 1 || prev_starting_brace.len() <= 1 {
        return None;
    }

    let prev_comma = prev_comma.first().unwrap();
    let prev_starting_brace = prev_starting_brace.first().unwrap();

    // If the amount of source code before the comma is greater than
    // the amount of source code before the starting brace then we've only
    // got one item in the nested item (eg. `issue_52891::{self}`).
    if prev_comma.len() > prev_starting_brace.len() {
        return None;
    }

    Some(binding_span.with_lo(BytePos(
        // Take away the number of bytes for the characters we've found and an
        // extra for the comma.
        binding_span.lo().0 - (prev_comma.as_bytes().len() as u32) - 1
    )))
}

/// Given a `use_span` of a binding within a use statement, returns the highlighted span and if
/// it is a nested use tree.
///
/// ```
/// use foo::a::{b, c};
///          ^^^^^^^^^^ // false
///
/// use foo::{a, b, c};
///          ^^^^^^^^^^ // true
///
/// use foo::{a, b::{c, d}};
///          ^^^^^^^^^^^^^^^ // true
/// ```
fn find_span_immediately_after_crate_name(
    sess: &Session,
    module_name: Symbol,
    use_span: Span,
) -> (bool, Span) {
    debug!("find_span_immediately_after_crate_name: module_name={:?} use_span={:?}",
           module_name, use_span);
    let source_map = sess.source_map();

    // Using `use issue_59764::foo::{baz, makro};` as an example throughout..
    let mut num_colons = 0;
    // Find second colon.. `use issue_59764:`
    let until_second_colon = source_map.span_take_while(use_span, |c| {
        if *c == ':' { num_colons += 1; }
        match c {
            ':' if num_colons == 2 => false,
            _ => true,
        }
    });
    // Find everything after the second colon.. `foo::{baz, makro};`
    let from_second_colon = use_span.with_lo(until_second_colon.hi() + BytePos(1));

    let mut found_a_non_whitespace_character = false;
    // Find the first non-whitespace character in `from_second_colon`.. `f`
    let after_second_colon = source_map.span_take_while(from_second_colon, |c| {
        if found_a_non_whitespace_character { return false; }
        if !c.is_whitespace() { found_a_non_whitespace_character = true; }
        true
    });

    // Find the first `{` in from_second_colon.. `foo::{`
    let next_left_bracket = source_map.span_through_char(from_second_colon, '{');

    (next_left_bracket == after_second_colon, from_second_colon)
}

/// When an entity with a given name is not available in scope, we search for
/// entities with that name in all crates. This method allows outputting the
/// results of this search in a programmer-friendly way
crate fn show_candidates(
    err: &mut DiagnosticBuilder<'_>,
    // This is `None` if all placement locations are inside expansions
    span: Option<Span>,
    candidates: &[ImportSuggestion],
    better: bool,
    found_use: bool,
) {
    // we want consistent results across executions, but candidates are produced
    // by iterating through a hash map, so make sure they are ordered:
    let mut path_strings: Vec<_> =
        candidates.into_iter().map(|c| path_names_to_string(&c.path)).collect();
    path_strings.sort();

    let better = if better { "better " } else { "" };
    let msg_diff = match path_strings.len() {
        1 => " is found in another module, you can import it",
        _ => "s are found in other modules, you can import them",
    };
    let msg = format!("possible {}candidate{} into scope", better, msg_diff);

    if let Some(span) = span {
        for candidate in &mut path_strings {
            // produce an additional newline to separate the new use statement
            // from the directly following item.
            let additional_newline = if found_use {
                ""
            } else {
                "\n"
            };
            *candidate = format!("use {};\n{}", candidate, additional_newline);
        }

        err.span_suggestions(
            span,
            &msg,
            path_strings.into_iter(),
            Applicability::Unspecified,
        );
    } else {
        let mut msg = msg;
        msg.push(':');
        for candidate in path_strings {
            msg.push('\n');
            msg.push_str(&candidate);
        }
    }
}