use crate::{CrateLint, Module, ModuleKind, ModuleOrUniformRoot};
use crate::{PathResult, PathSource, Segment};
use crate::path_names_to_string;
use crate::diagnostics::{add_typo_suggestion, ImportSuggestion, TypoSuggestion};
use crate::late::{LateResolutionVisitor, RibKind};

use errors::{Applicability, DiagnosticBuilder, DiagnosticId};
use log::debug;
use rustc::hir::def::{self, DefKind, CtorKind};
use rustc::hir::def::Namespace::{self, *};
use rustc::hir::def_id::{CRATE_DEF_INDEX, DefId};
use rustc::hir::PrimTy;
use rustc::session::config::nightly_options;
use rustc::util::nodemap::FxHashSet;
use syntax::ast::{self, Expr, ExprKind, Ident, NodeId, Path, Ty, TyKind};
use syntax::ext::base::MacroKind;
use syntax::symbol::kw;
use syntax::util::lev_distance::find_best_match_for_name;
use syntax_pos::Span;

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

/// A field or associated item from self type suggested in case of resolution failure.
enum AssocSuggestion {
    Field,
    MethodWithSelf,
    AssocItem,
}

fn is_self_type(path: &[Segment], namespace: Namespace) -> bool {
    namespace == TypeNS && path.len() == 1 && path[0].ident.name == kw::SelfUpper
}

fn is_self_value(path: &[Segment], namespace: Namespace) -> bool {
    namespace == ValueNS && path.len() == 1 && path[0].ident.name == kw::SelfLower
}

/// Gets the stringified path for an enum from an `ImportSuggestion` for an enum variant.
fn import_candidate_to_enum_paths(suggestion: &ImportSuggestion) -> (String, String) {
    let variant_path = &suggestion.path;
    let variant_path_string = path_names_to_string(variant_path);

    let path_len = suggestion.path.segments.len();
    let enum_path = ast::Path {
        span: suggestion.path.span,
        segments: suggestion.path.segments[0..path_len - 1].to_vec(),
    };
    let enum_path_string = path_names_to_string(&enum_path);

    (variant_path_string, enum_path_string)
}

impl<'a> LateResolutionVisitor<'a, '_> {
    /// Handles error reporting for `smart_resolve_path_fragment` function.
    /// Creates base error and amends it with one short label and possibly some longer helps/notes.
    pub(crate) fn smart_resolve_report_errors(
        &mut self,
        path: &[Segment],
        span: Span,
        source: PathSource<'_>,
        res: Option<Res>,
    ) -> (DiagnosticBuilder<'a>, Vec<ImportSuggestion>) {
        let ident_span = path.last().map_or(span, |ident| ident.ident.span);
        let ns = source.namespace();
        let is_expected = &|res| source.is_expected(res);
        let is_enum_variant = &|res| {
            if let Res::Def(DefKind::Variant, _) = res { true } else { false }
        };

        // Make the base error.
        let expected = source.descr_expected();
        let path_str = Segment::names_to_string(path);
        let item_str = path.last().unwrap().ident;
        let code = source.error_code(res.is_some());
        let (base_msg, fallback_label, base_span) = if let Some(res) = res {
            (format!("expected {}, found {} `{}`", expected, res.descr(), path_str),
                format!("not a {}", expected),
                span)
        } else {
            let item_span = path.last().unwrap().ident.span;
            let (mod_prefix, mod_str) = if path.len() == 1 {
                (String::new(), "this scope".to_string())
            } else if path.len() == 2 && path[0].ident.name == kw::PathRoot {
                (String::new(), "the crate root".to_string())
            } else {
                let mod_path = &path[..path.len() - 1];
                let mod_prefix = match self.resolve_path(
                    mod_path, Some(TypeNS), false, span, CrateLint::No
                ) {
                    PathResult::Module(ModuleOrUniformRoot::Module(module)) => module.res(),
                    _ => None,
                }.map_or(String::new(), |res| format!("{} ", res.descr()));
                (mod_prefix, format!("`{}`", Segment::names_to_string(mod_path)))
            };
            (format!("cannot find {} `{}` in {}{}", expected, item_str, mod_prefix, mod_str),
                format!("not found in {}", mod_str),
                item_span)
        };

        let code = DiagnosticId::Error(code.into());
        let mut err = self.r.session.struct_span_err_with_code(base_span, &base_msg, code);

        // Emit help message for fake-self from other languages (e.g., `this` in Javascript).
        if ["this", "my"].contains(&&*item_str.as_str())
            && self.self_value_is_available(path[0].ident.span, span) {
            err.span_suggestion(
                span,
                "did you mean",
                "self".to_string(),
                Applicability::MaybeIncorrect,
            );
        }

        // Emit special messages for unresolved `Self` and `self`.
        if is_self_type(path, ns) {
            syntax::diagnostic_used!(E0411);
            err.code(DiagnosticId::Error("E0411".into()));
            err.span_label(span, format!("`Self` is only available in impls, traits, \
                                          and type definitions"));
            return (err, Vec::new());
        }
        if is_self_value(path, ns) {
            debug!("smart_resolve_path_fragment: E0424, source={:?}", source);

            syntax::diagnostic_used!(E0424);
            err.code(DiagnosticId::Error("E0424".into()));
            err.span_label(span, match source {
                PathSource::Pat => {
                    format!("`self` value is a keyword \
                             and may not be bound to \
                             variables or shadowed")
                }
                _ => {
                    format!("`self` value is a keyword \
                             only available in methods \
                             with `self` parameter")
                }
            });
            return (err, Vec::new());
        }

        // Try to lookup name in more relaxed fashion for better error reporting.
        let ident = path.last().unwrap().ident;
        let candidates = self.r.lookup_import_candidates(ident, ns, is_expected)
            .drain(..)
            .filter(|ImportSuggestion { did, .. }| {
                match (did, res.and_then(|res| res.opt_def_id())) {
                    (Some(suggestion_did), Some(actual_did)) => *suggestion_did != actual_did,
                    _ => true,
                }
            })
            .collect::<Vec<_>>();
        let crate_def_id = DefId::local(CRATE_DEF_INDEX);
        if candidates.is_empty() && is_expected(Res::Def(DefKind::Enum, crate_def_id)) {
            let enum_candidates =
                self.r.lookup_import_candidates(ident, ns, is_enum_variant);
            let mut enum_candidates = enum_candidates.iter()
                .map(|suggestion| {
                    import_candidate_to_enum_paths(&suggestion)
                }).collect::<Vec<_>>();
            enum_candidates.sort();

            if !enum_candidates.is_empty() {
                // Contextualize for E0412 "cannot find type", but don't belabor the point
                // (that it's a variant) for E0573 "expected type, found variant".
                let preamble = if res.is_none() {
                    let others = match enum_candidates.len() {
                        1 => String::new(),
                        2 => " and 1 other".to_owned(),
                        n => format!(" and {} others", n)
                    };
                    format!("there is an enum variant `{}`{}; ",
                            enum_candidates[0].0, others)
                } else {
                    String::new()
                };
                let msg = format!("{}try using the variant's enum", preamble);

                err.span_suggestions(
                    span,
                    &msg,
                    enum_candidates.into_iter()
                        .map(|(_variant_path, enum_ty_path)| enum_ty_path)
                        // Variants re-exported in prelude doesn't mean `prelude::v1` is the
                        // type name!
                        // FIXME: is there a more principled way to do this that
                        // would work for other re-exports?
                        .filter(|enum_ty_path| enum_ty_path != "std::prelude::v1")
                        // Also write `Option` rather than `std::prelude::v1::Option`.
                        .map(|enum_ty_path| {
                            // FIXME #56861: DRY-er prelude filtering.
                            enum_ty_path.trim_start_matches("std::prelude::v1::").to_owned()
                        }),
                    Applicability::MachineApplicable,
                );
            }
        }
        if path.len() == 1 && self.self_type_is_available(span) {
            if let Some(candidate) = self.lookup_assoc_candidate(ident, ns, is_expected) {
                let self_is_available = self.self_value_is_available(path[0].ident.span, span);
                match candidate {
                    AssocSuggestion::Field => {
                        if self_is_available {
                            err.span_suggestion(
                                span,
                                "you might have meant to use the available field",
                                format!("self.{}", path_str),
                                Applicability::MachineApplicable,
                            );
                        } else {
                            err.span_label(
                                span,
                                "a field by this name exists in `Self`",
                            );
                        }
                    }
                    AssocSuggestion::MethodWithSelf if self_is_available => {
                        err.span_suggestion(
                            span,
                            "try",
                            format!("self.{}", path_str),
                            Applicability::MachineApplicable,
                        );
                    }
                    AssocSuggestion::MethodWithSelf | AssocSuggestion::AssocItem => {
                        err.span_suggestion(
                            span,
                            "try",
                            format!("Self::{}", path_str),
                            Applicability::MachineApplicable,
                        );
                    }
                }
                return (err, candidates);
            }
        }

        // Try Levenshtein algorithm.
        let levenshtein_worked = add_typo_suggestion(
            &mut err, self.lookup_typo_candidate(path, ns, is_expected, span), ident_span
        );

        // Try context-dependent help if relaxed lookup didn't work.
        if let Some(res) = res {
            if self.smart_resolve_context_dependent_help(&mut err,
                                                         span,
                                                         source,
                                                         res,
                                                         &path_str,
                                                         &fallback_label) {
                return (err, candidates);
            }
        }

        // Fallback label.
        if !levenshtein_worked {
            err.span_label(base_span, fallback_label);
            self.type_ascription_suggestion(&mut err, base_span);
        }
        (err, candidates)
    }

    fn followed_by_brace(&self, span: Span) -> (bool, Option<(Span, String)>) {
        // HACK(estebank): find a better way to figure out that this was a
        // parser issue where a struct literal is being used on an expression
        // where a brace being opened means a block is being started. Look
        // ahead for the next text to see if `span` is followed by a `{`.
        let sm = self.r.session.source_map();
        let mut sp = span;
        loop {
            sp = sm.next_point(sp);
            match sm.span_to_snippet(sp) {
                Ok(ref snippet) => {
                    if snippet.chars().any(|c| { !c.is_whitespace() }) {
                        break;
                    }
                }
                _ => break,
            }
        }
        let followed_by_brace = match sm.span_to_snippet(sp) {
            Ok(ref snippet) if snippet == "{" => true,
            _ => false,
        };
        // In case this could be a struct literal that needs to be surrounded
        // by parenthesis, find the appropriate span.
        let mut i = 0;
        let mut closing_brace = None;
        loop {
            sp = sm.next_point(sp);
            match sm.span_to_snippet(sp) {
                Ok(ref snippet) => {
                    if snippet == "}" {
                        let sp = span.to(sp);
                        if let Ok(snippet) = sm.span_to_snippet(sp) {
                            closing_brace = Some((sp, snippet));
                        }
                        break;
                    }
                }
                _ => break,
            }
            i += 1;
            // The bigger the span, the more likely we're incorrect --
            // bound it to 100 chars long.
            if i > 100 {
                break;
            }
        }
        return (followed_by_brace, closing_brace)
    }

    /// Provides context-dependent help for errors reported by the `smart_resolve_path_fragment`
    /// function.
    /// Returns `true` if able to provide context-dependent help.
    fn smart_resolve_context_dependent_help(
        &mut self,
        err: &mut DiagnosticBuilder<'a>,
        span: Span,
        source: PathSource<'_>,
        res: Res,
        path_str: &str,
        fallback_label: &str,
    ) -> bool {
        let ns = source.namespace();
        let is_expected = &|res| source.is_expected(res);

        let path_sep = |err: &mut DiagnosticBuilder<'_>, expr: &Expr| match expr.node {
            ExprKind::Field(_, ident) => {
                err.span_suggestion(
                    expr.span,
                    "use the path separator to refer to an item",
                    format!("{}::{}", path_str, ident),
                    Applicability::MaybeIncorrect,
                );
                true
            }
            ExprKind::MethodCall(ref segment, ..) => {
                let span = expr.span.with_hi(segment.ident.span.hi());
                err.span_suggestion(
                    span,
                    "use the path separator to refer to an item",
                    format!("{}::{}", path_str, segment.ident),
                    Applicability::MaybeIncorrect,
                );
                true
            }
            _ => false,
        };

        let mut bad_struct_syntax_suggestion = || {
            let (followed_by_brace, closing_brace) = self.followed_by_brace(span);
            let mut suggested = false;
            match source {
                PathSource::Expr(Some(parent)) => {
                    suggested = path_sep(err, &parent);
                }
                PathSource::Expr(None) if followed_by_brace == true => {
                    if let Some((sp, snippet)) = closing_brace {
                        err.span_suggestion(
                            sp,
                            "surround the struct literal with parenthesis",
                            format!("({})", snippet),
                            Applicability::MaybeIncorrect,
                        );
                    } else {
                        err.span_label(
                            span,  // Note the parenthesis surrounding the suggestion below
                            format!("did you mean `({} {{ /* fields */ }})`?", path_str),
                        );
                    }
                    suggested = true;
                },
                _ => {}
            }
            if !suggested {
                err.span_label(
                    span,
                    format!("did you mean `{} {{ /* fields */ }}`?", path_str),
                );
            }
        };

        match (res, source) {
            (Res::Def(DefKind::Macro(MacroKind::Bang), _), _) => {
                err.span_suggestion(
                    span,
                    "use `!` to invoke the macro",
                    format!("{}!", path_str),
                    Applicability::MaybeIncorrect,
                );
                if path_str == "try" && span.rust_2015() {
                    err.note("if you want the `try` keyword, you need to be in the 2018 edition");
                }
            }
            (Res::Def(DefKind::TyAlias, _), PathSource::Trait(_)) => {
                err.span_label(span, "type aliases cannot be used as traits");
                if nightly_options::is_nightly_build() {
                    err.note("did you mean to use a trait alias?");
                }
            }
            (Res::Def(DefKind::Mod, _), PathSource::Expr(Some(parent))) => {
                if !path_sep(err, &parent) {
                    return false;
                }
            }
            (Res::Def(DefKind::Enum, def_id), PathSource::TupleStruct)
                | (Res::Def(DefKind::Enum, def_id), PathSource::Expr(..))  => {
                if let Some(variants) = self.collect_enum_variants(def_id) {
                    if !variants.is_empty() {
                        let msg = if variants.len() == 1 {
                            "try using the enum's variant"
                        } else {
                            "try using one of the enum's variants"
                        };

                        err.span_suggestions(
                            span,
                            msg,
                            variants.iter().map(path_names_to_string),
                            Applicability::MaybeIncorrect,
                        );
                    }
                } else {
                    err.note("did you mean to use one of the enum's variants?");
                }
            },
            (Res::Def(DefKind::Struct, def_id), _) if ns == ValueNS => {
                if let Some((ctor_def, ctor_vis))
                        = self.r.struct_constructors.get(&def_id).cloned() {
                    let accessible_ctor =
                        self.r.is_accessible_from(ctor_vis, self.parent_scope.module);
                    if is_expected(ctor_def) && !accessible_ctor {
                        err.span_label(
                            span,
                            format!("constructor is not visible here due to private fields"),
                        );
                    }
                } else {
                    bad_struct_syntax_suggestion();
                }
            }
            (Res::Def(DefKind::Union, _), _) |
            (Res::Def(DefKind::Variant, _), _) |
            (Res::Def(DefKind::Ctor(_, CtorKind::Fictive), _), _) if ns == ValueNS => {
                bad_struct_syntax_suggestion();
            }
            (Res::SelfTy(..), _) if ns == ValueNS => {
                err.span_label(span, fallback_label);
                err.note("can't use `Self` as a constructor, you must use the implemented struct");
            }
            (Res::Def(DefKind::TyAlias, _), _)
            | (Res::Def(DefKind::AssocTy, _), _) if ns == ValueNS => {
                err.note("can't use a type alias as a constructor");
            }
            _ => return false,
        }
        true
    }

    fn lookup_assoc_candidate<FilterFn>(&mut self,
                                        ident: Ident,
                                        ns: Namespace,
                                        filter_fn: FilterFn)
                                        -> Option<AssocSuggestion>
        where FilterFn: Fn(Res) -> bool
    {
        fn extract_node_id(t: &Ty) -> Option<NodeId> {
            match t.node {
                TyKind::Path(None, _) => Some(t.id),
                TyKind::Rptr(_, ref mut_ty) => extract_node_id(&mut_ty.ty),
                // This doesn't handle the remaining `Ty` variants as they are not
                // that commonly the self_type, it might be interesting to provide
                // support for those in future.
                _ => None,
            }
        }

        // Fields are generally expected in the same contexts as locals.
        if filter_fn(Res::Local(ast::DUMMY_NODE_ID)) {
            if let Some(node_id) = self.current_self_type.as_ref().and_then(extract_node_id) {
                // Look for a field with the same name in the current self_type.
                if let Some(resolution) = self.r.partial_res_map.get(&node_id) {
                    match resolution.base_res() {
                        Res::Def(DefKind::Struct, did) | Res::Def(DefKind::Union, did)
                                if resolution.unresolved_segments() == 0 => {
                            if let Some(field_names) = self.r.field_names.get(&did) {
                                if field_names.iter().any(|&field_name| ident.name == field_name) {
                                    return Some(AssocSuggestion::Field);
                                }
                            }
                        }
                        _ => {}
                    }
                }
            }
        }

        for assoc_type_ident in &self.current_trait_assoc_types {
            if *assoc_type_ident == ident {
                return Some(AssocSuggestion::AssocItem);
            }
        }

        // Look for associated items in the current trait.
        if let Some((module, _)) = self.current_trait_ref {
            if let Ok(binding) = self.r.resolve_ident_in_module(
                    ModuleOrUniformRoot::Module(module),
                    ident,
                    ns,
                    &self.parent_scope,
                    false,
                    module.span,
                ) {
                let res = binding.res();
                if filter_fn(res) {
                    return Some(if self.r.has_self.contains(&res.def_id()) {
                        AssocSuggestion::MethodWithSelf
                    } else {
                        AssocSuggestion::AssocItem
                    });
                }
            }
        }

        None
    }

    fn lookup_typo_candidate(
        &mut self,
        path: &[Segment],
        ns: Namespace,
        filter_fn: &impl Fn(Res) -> bool,
        span: Span,
    ) -> Option<TypoSuggestion> {
        let mut names = Vec::new();
        if path.len() == 1 {
            // Search in lexical scope.
            // Walk backwards up the ribs in scope and collect candidates.
            for rib in self.ribs[ns].iter().rev() {
                // Locals and type parameters
                for (ident, &res) in &rib.bindings {
                    if filter_fn(res) {
                        names.push(TypoSuggestion::from_res(ident.name, res));
                    }
                }
                // Items in scope
                if let RibKind::ModuleRibKind(module) = rib.kind {
                    // Items from this module
                    self.r.add_module_candidates(module, &mut names, &filter_fn);

                    if let ModuleKind::Block(..) = module.kind {
                        // We can see through blocks
                    } else {
                        // Items from the prelude
                        if !module.no_implicit_prelude {
                            let extern_prelude = self.r.extern_prelude.clone();
                            names.extend(extern_prelude.iter().flat_map(|(ident, _)| {
                                self.r.crate_loader
                                    .maybe_process_path_extern(ident.name, ident.span)
                                    .and_then(|crate_id| {
                                        let crate_mod = Res::Def(
                                            DefKind::Mod,
                                            DefId {
                                                krate: crate_id,
                                                index: CRATE_DEF_INDEX,
                                            },
                                        );

                                        if filter_fn(crate_mod) {
                                            Some(TypoSuggestion::from_res(ident.name, crate_mod))
                                        } else {
                                            None
                                        }
                                    })
                            }));

                            if let Some(prelude) = self.r.prelude {
                                self.r.add_module_candidates(prelude, &mut names, &filter_fn);
                            }
                        }
                        break;
                    }
                }
            }
            // Add primitive types to the mix
            if filter_fn(Res::PrimTy(PrimTy::Bool)) {
                names.extend(
                    self.r.primitive_type_table.primitive_types.iter().map(|(name, prim_ty)| {
                        TypoSuggestion::from_res(*name, Res::PrimTy(*prim_ty))
                    })
                )
            }
        } else {
            // Search in module.
            let mod_path = &path[..path.len() - 1];
            if let PathResult::Module(module) = self.resolve_path(
                mod_path, Some(TypeNS), false, span, CrateLint::No
            ) {
                if let ModuleOrUniformRoot::Module(module) = module {
                    self.r.add_module_candidates(module, &mut names, &filter_fn);
                }
            }
        }

        let name = path[path.len() - 1].ident.name;
        // Make sure error reporting is deterministic.
        names.sort_by_cached_key(|suggestion| suggestion.candidate.as_str());

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

    /// Only used in a specific case of type ascription suggestions
    fn get_colon_suggestion_span(&self, start: Span) -> Span {
        let cm = self.r.session.source_map();
        start.to(cm.next_point(start))
    }

    fn type_ascription_suggestion(
        &self,
        err: &mut DiagnosticBuilder<'_>,
        base_span: Span,
    ) {
        debug!("type_ascription_suggetion {:?}", base_span);
        let cm = self.r.session.source_map();
        let base_snippet = cm.span_to_snippet(base_span);
        debug!("self.current_type_ascription {:?}", self.current_type_ascription);
        if let Some(sp) = self.current_type_ascription.last() {
            let mut sp = *sp;
            loop {
                // Try to find the `:`; bail on first non-':' / non-whitespace.
                sp = cm.next_point(sp);
                if let Ok(snippet) = cm.span_to_snippet(sp.to(cm.next_point(sp))) {
                    let line_sp = cm.lookup_char_pos(sp.hi()).line;
                    let line_base_sp = cm.lookup_char_pos(base_span.lo()).line;
                    if snippet == ":" {
                        let mut show_label = true;
                        if line_sp != line_base_sp {
                            err.span_suggestion_short(
                                sp,
                                "did you mean to use `;` here instead?",
                                ";".to_string(),
                                Applicability::MaybeIncorrect,
                            );
                        } else {
                            let colon_sp = self.get_colon_suggestion_span(sp);
                            let after_colon_sp = self.get_colon_suggestion_span(
                                colon_sp.shrink_to_hi(),
                            );
                            if !cm.span_to_snippet(after_colon_sp).map(|s| s == " ")
                                .unwrap_or(false)
                            {
                                err.span_suggestion(
                                    colon_sp,
                                    "maybe you meant to write a path separator here",
                                    "::".to_string(),
                                    Applicability::MaybeIncorrect,
                                );
                                show_label = false;
                            }
                            if let Ok(base_snippet) = base_snippet {
                                let mut sp = after_colon_sp;
                                for _ in 0..100 {
                                    // Try to find an assignment
                                    sp = cm.next_point(sp);
                                    let snippet = cm.span_to_snippet(sp.to(cm.next_point(sp)));
                                    match snippet {
                                        Ok(ref x) if x.as_str() == "=" => {
                                            err.span_suggestion(
                                                base_span,
                                                "maybe you meant to write an assignment here",
                                                format!("let {}", base_snippet),
                                                Applicability::MaybeIncorrect,
                                            );
                                            show_label = false;
                                            break;
                                        }
                                        Ok(ref x) if x.as_str() == "\n" => break,
                                        Err(_) => break,
                                        Ok(_) => {}
                                    }
                                }
                            }
                        }
                        if show_label {
                            err.span_label(base_span,
                                           "expecting a type here because of type ascription");
                        }
                        break;
                    } else if !snippet.trim().is_empty() {
                        debug!("tried to find type ascription `:` token, couldn't find it");
                        break;
                    }
                } else {
                    break;
                }
            }
        }
    }

    fn find_module(&mut self, def_id: DefId) -> Option<(Module<'a>, ImportSuggestion)> {
        let mut result = None;
        let mut seen_modules = FxHashSet::default();
        let mut worklist = vec![(self.r.graph_root, Vec::new())];

        while let Some((in_module, path_segments)) = worklist.pop() {
            // abort if the module is already found
            if result.is_some() { break; }

            in_module.for_each_child_stable(self.r, |_, ident, _, name_binding| {
                // abort if the module is already found or if name_binding is private external
                if result.is_some() || !name_binding.vis.is_visible_locally() {
                    return
                }
                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 module_def_id = module.def_id().unwrap();
                    if module_def_id == def_id {
                        let path = Path {
                            span: name_binding.span,
                            segments: path_segments,
                        };
                        result = Some((module, ImportSuggestion { did: Some(def_id), path }));
                    } else {
                        // add the module to the lookup
                        if seen_modules.insert(module_def_id) {
                            worklist.push((module, path_segments));
                        }
                    }
                }
            });
        }

        result
    }

    fn collect_enum_variants(&mut self, def_id: DefId) -> Option<Vec<Path>> {
        self.find_module(def_id).map(|(enum_module, enum_import_suggestion)| {
            let mut variants = Vec::new();
            enum_module.for_each_child_stable(self.r, |_, ident, _, name_binding| {
                if let Res::Def(DefKind::Variant, _) = name_binding.res() {
                    let mut segms = enum_import_suggestion.path.segments.clone();
                    segms.push(ast::PathSegment::from_ident(ident));
                    variants.push(Path {
                        span: name_binding.span,
                        segments: segms,
                    });
                }
            });
            variants
        })
    }
}