use GenericParameters::*;

use crate::{path_names_to_string, resolve_error};
use crate::{AliasPossibility, BindingError, CrateLint, LexicalScopeBinding, Module};
use crate::{ModuleOrUniformRoot, NameBinding, NameBindingKind, ParentScope, PathResult};
use crate::{PathSource, ResolutionError, Resolver, Rib, RibKind, Segment, UseError};
use crate::RibKind::*;

use log::debug;
use rustc::{bug, lint, span_bug};
use rustc::hir::def::{self, PartialRes, DefKind, CtorKind, PerNS};
use rustc::hir::def::Namespace::{self, *};
use rustc::hir::def_id::{DefId, CRATE_DEF_INDEX};
use rustc::hir::TraitCandidate;
use rustc::util::nodemap::FxHashMap;
use smallvec::{smallvec, SmallVec};
use syntax::{unwrap_or, walk_list};
use syntax::ast::*;
use syntax::ptr::P;
use syntax::symbol::{kw, sym};
use syntax::util::lev_distance::find_best_match_for_name;
use syntax::visit::{self, Visitor, FnKind};
use syntax_pos::Span;

use std::collections::BTreeSet;
use std::mem::replace;
use std::ops::{Deref, DerefMut};

mod diagnostics;

type Res = def::Res<NodeId>;

/// Map from the name in a pattern to its binding mode.
type BindingMap = FxHashMap<Ident, BindingInfo>;

#[derive(Copy, Clone, Debug)]
struct BindingInfo {
    span: Span,
    binding_mode: BindingMode,
}

#[derive(Copy, Clone)]
enum GenericParameters<'a, 'b> {
    NoGenericParams,
    HasGenericParams(// Type parameters.
                      &'b Generics,

                      // The kind of the rib used for type parameters.
                      RibKind<'a>),
}

#[derive(Copy, Clone, PartialEq, Eq, Debug)]
enum PatternSource {
    Match,
    Let,
    For,
    FnParam,
}

impl PatternSource {
    fn descr(self) -> &'static str {
        match self {
            PatternSource::Match => "match binding",
            PatternSource::Let => "let binding",
            PatternSource::For => "for binding",
            PatternSource::FnParam => "function parameter",
        }
    }
}

struct LateResolutionVisitor<'a, 'b> {
    resolver: &'b mut Resolver<'a>,

    /// The module that represents the current item scope.
    parent_scope: ParentScope<'a>,

    /// The current set of local scopes for types and values.
    /// FIXME #4948: Reuse ribs to avoid allocation.
    ribs: PerNS<Vec<Rib<'a>>>,

    /// The current set of local scopes, for labels.
    label_ribs: Vec<Rib<'a, NodeId>>,

    /// The trait that the current context can refer to.
    current_trait_ref: Option<(Module<'a>, TraitRef)>,

    /// The current trait's associated types' ident, used for diagnostic suggestions.
    current_trait_assoc_types: Vec<Ident>,

    /// The current self type if inside an impl (used for better errors).
    current_self_type: Option<Ty>,

    /// The current self item if inside an ADT (used for better errors).
    current_self_item: Option<NodeId>,

    /// A list of labels as of yet unused. Labels will be removed from this map when
    /// they are used (in a `break` or `continue` statement)
    unused_labels: FxHashMap<NodeId, Span>,

    /// Only used for better errors on `fn(): fn()`.
    current_type_ascription: Vec<Span>,
}

impl<'a> Deref for LateResolutionVisitor<'a, '_> {
    type Target = Resolver<'a>;
    fn deref(&self) -> &Self::Target {
        self.resolver
    }
}

impl<'a> DerefMut for LateResolutionVisitor<'a, '_> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        self.resolver
    }
}

/// Walks the whole crate in DFS order, visiting each item, resolving names as it goes.
impl<'a, 'tcx> Visitor<'tcx> for LateResolutionVisitor<'a, '_> {
    fn visit_item(&mut self, item: &'tcx Item) {
        self.resolve_item(item);
    }
    fn visit_arm(&mut self, arm: &'tcx Arm) {
        self.resolve_arm(arm);
    }
    fn visit_block(&mut self, block: &'tcx Block) {
        self.resolve_block(block);
    }
    fn visit_anon_const(&mut self, constant: &'tcx AnonConst) {
        debug!("visit_anon_const {:?}", constant);
        self.with_constant_rib(|this| {
            visit::walk_anon_const(this, constant);
        });
    }
    fn visit_expr(&mut self, expr: &'tcx Expr) {
        self.resolve_expr(expr, None);
    }
    fn visit_local(&mut self, local: &'tcx Local) {
        self.resolve_local(local);
    }
    fn visit_ty(&mut self, ty: &'tcx Ty) {
        match ty.node {
            TyKind::Path(ref qself, ref path) => {
                self.smart_resolve_path(ty.id, qself.as_ref(), path, PathSource::Type);
            }
            TyKind::ImplicitSelf => {
                let self_ty = Ident::with_empty_ctxt(kw::SelfUpper);
                let res = self.resolve_ident_in_lexical_scope(self_ty, TypeNS, Some(ty.id), ty.span)
                              .map_or(Res::Err, |d| d.res());
                self.record_partial_res(ty.id, PartialRes::new(res));
            }
            _ => (),
        }
        visit::walk_ty(self, ty);
    }
    fn visit_poly_trait_ref(&mut self,
                            tref: &'tcx PolyTraitRef,
                            m: &'tcx TraitBoundModifier) {
        self.smart_resolve_path(tref.trait_ref.ref_id, None,
                                &tref.trait_ref.path, PathSource::Trait(AliasPossibility::Maybe));
        visit::walk_poly_trait_ref(self, tref, m);
    }
    fn visit_foreign_item(&mut self, foreign_item: &'tcx ForeignItem) {
        let generic_params = match foreign_item.node {
            ForeignItemKind::Fn(_, ref generics) => {
                HasGenericParams(generics, ItemRibKind)
            }
            ForeignItemKind::Static(..) => NoGenericParams,
            ForeignItemKind::Ty => NoGenericParams,
            ForeignItemKind::Macro(..) => NoGenericParams,
        };
        self.with_generic_param_rib(generic_params, |this| {
            visit::walk_foreign_item(this, foreign_item);
        });
    }
    fn visit_fn(&mut self,
                function_kind: FnKind<'tcx>,
                declaration: &'tcx FnDecl,
                _: Span,
                _: NodeId)
    {
        debug!("(resolving function) entering function");
        let rib_kind = match function_kind {
            FnKind::ItemFn(..) => FnItemRibKind,
            FnKind::Method(..) | FnKind::Closure(_) => NormalRibKind,
        };

        // Create a value rib for the function.
        self.ribs[ValueNS].push(Rib::new(rib_kind));

        // Create a label rib for the function.
        self.label_ribs.push(Rib::new(rib_kind));

        // Add each argument to the rib.
        let mut bindings_list = FxHashMap::default();
        for argument in &declaration.inputs {
            self.resolve_pattern(&argument.pat, PatternSource::FnParam, &mut bindings_list);

            self.visit_ty(&argument.ty);

            debug!("(resolving function) recorded argument");
        }
        visit::walk_fn_ret_ty(self, &declaration.output);

        // Resolve the function body, potentially inside the body of an async closure
        match function_kind {
            FnKind::ItemFn(.., body) |
            FnKind::Method(.., body) => {
                self.visit_block(body);
            }
            FnKind::Closure(body) => {
                self.visit_expr(body);
            }
        };

        debug!("(resolving function) leaving function");

        self.label_ribs.pop();
        self.ribs[ValueNS].pop();
    }

    fn visit_generics(&mut self, generics: &'tcx Generics) {
        // For type parameter defaults, we have to ban access
        // to following type parameters, as the InternalSubsts can only
        // provide previous type parameters as they're built. We
        // put all the parameters on the ban list and then remove
        // them one by one as they are processed and become available.
        let mut default_ban_rib = Rib::new(ForwardTyParamBanRibKind);
        let mut found_default = false;
        default_ban_rib.bindings.extend(generics.params.iter()
            .filter_map(|param| match param.kind {
                GenericParamKind::Const { .. } |
                GenericParamKind::Lifetime { .. } => None,
                GenericParamKind::Type { ref default, .. } => {
                    found_default |= default.is_some();
                    if found_default {
                        Some((Ident::with_empty_ctxt(param.ident.name), Res::Err))
                    } else {
                        None
                    }
                }
            }));

        // We also ban access to type parameters for use as the types of const parameters.
        let mut const_ty_param_ban_rib = Rib::new(TyParamAsConstParamTy);
        const_ty_param_ban_rib.bindings.extend(generics.params.iter()
            .filter(|param| {
                if let GenericParamKind::Type { .. } = param.kind {
                    true
                } else {
                    false
                }
            })
            .map(|param| (Ident::with_empty_ctxt(param.ident.name), Res::Err)));

        for param in &generics.params {
            match param.kind {
                GenericParamKind::Lifetime { .. } => self.visit_generic_param(param),
                GenericParamKind::Type { ref default, .. } => {
                    for bound in &param.bounds {
                        self.visit_param_bound(bound);
                    }

                    if let Some(ref ty) = default {
                        self.ribs[TypeNS].push(default_ban_rib);
                        self.visit_ty(ty);
                        default_ban_rib = self.ribs[TypeNS].pop().unwrap();
                    }

                    // Allow all following defaults to refer to this type parameter.
                    default_ban_rib.bindings.remove(&Ident::with_empty_ctxt(param.ident.name));
                }
                GenericParamKind::Const { ref ty } => {
                    self.ribs[TypeNS].push(const_ty_param_ban_rib);

                    for bound in &param.bounds {
                        self.visit_param_bound(bound);
                    }

                    self.visit_ty(ty);

                    const_ty_param_ban_rib = self.ribs[TypeNS].pop().unwrap();
                }
            }
        }
        for p in &generics.where_clause.predicates {
            self.visit_where_predicate(p);
        }
    }
}

impl<'a, 'b> LateResolutionVisitor<'a, '_> {
    fn new(resolver: &'b mut Resolver<'a>) -> LateResolutionVisitor<'a, 'b> {
        // During late resolution we only track the module component of the parent scope,
        // although it may be useful to track other components as well for diagnostics.
        let parent_scope = resolver.dummy_parent_scope();
        let graph_root = resolver.graph_root;
        LateResolutionVisitor {
            resolver,
            parent_scope,
            ribs: PerNS {
                value_ns: vec![Rib::new(ModuleRibKind(graph_root))],
                type_ns: vec![Rib::new(ModuleRibKind(graph_root))],
                macro_ns: vec![Rib::new(ModuleRibKind(graph_root))],
            },
            label_ribs: Vec::new(),
            current_trait_ref: None,
            current_trait_assoc_types: Vec::new(),
            current_self_type: None,
            current_self_item: None,
            unused_labels: Default::default(),
            current_type_ascription: Vec::new(),
        }
    }

    fn resolve_ident_in_lexical_scope(&mut self,
                                      ident: Ident,
                                      ns: Namespace,
                                      record_used_id: Option<NodeId>,
                                      path_span: Span)
                                      -> Option<LexicalScopeBinding<'a>> {
        self.resolver.resolve_ident_in_lexical_scope(
            ident, ns, &self.parent_scope, record_used_id, path_span, &self.ribs[ns]
        )
    }

    fn resolve_path(
        &mut self,
        path: &[Segment],
        opt_ns: Option<Namespace>, // `None` indicates a module path in import
        record_used: bool,
        path_span: Span,
        crate_lint: CrateLint,
    ) -> PathResult<'a> {
        self.resolver.resolve_path_with_ribs(
            path, opt_ns, &self.parent_scope, record_used, path_span, crate_lint, &self.ribs
        )
    }

    // AST resolution
    //
    // We maintain a list of value ribs and type ribs.
    //
    // Simultaneously, we keep track of the current position in the module
    // graph in the `parent_scope.module` pointer. When we go to resolve a name in
    // the value or type namespaces, we first look through all the ribs and
    // then query the module graph. When we resolve a name in the module
    // namespace, we can skip all the ribs (since nested modules are not
    // allowed within blocks in Rust) and jump straight to the current module
    // graph node.
    //
    // Named implementations are handled separately. When we find a method
    // call, we consult the module node to find all of the implementations in
    // scope. This information is lazily cached in the module node. We then
    // generate a fake "implementation scope" containing all the
    // implementations thus found, for compatibility with old resolve pass.

    fn with_scope<F, T>(&mut self, id: NodeId, f: F) -> T
        where F: FnOnce(&mut LateResolutionVisitor<'_, '_>) -> T
    {
        let id = self.definitions.local_def_id(id);
        let module = self.module_map.get(&id).cloned(); // clones a reference
        if let Some(module) = module {
            // Move down in the graph.
            let orig_module = replace(&mut self.parent_scope.module, module);
            self.ribs[ValueNS].push(Rib::new(ModuleRibKind(module)));
            self.ribs[TypeNS].push(Rib::new(ModuleRibKind(module)));

            self.finalize_current_module_macro_resolutions(module);
            let ret = f(self);

            self.parent_scope.module = orig_module;
            self.ribs[ValueNS].pop();
            self.ribs[TypeNS].pop();
            ret
        } else {
            f(self)
        }
    }

    /// Searches the current set of local scopes for labels. Returns the first non-`None` label that
    /// is returned by the given predicate function
    ///
    /// Stops after meeting a closure.
    fn search_label<P, R>(&self, mut ident: Ident, pred: P) -> Option<R>
        where P: Fn(&Rib<'_, NodeId>, Ident) -> Option<R>
    {
        for rib in self.label_ribs.iter().rev() {
            match rib.kind {
                NormalRibKind => {}
                // If an invocation of this macro created `ident`, give up on `ident`
                // and switch to `ident`'s source from the macro definition.
                MacroDefinition(def) => {
                    if def == self.macro_def(ident.span.ctxt()) {
                        ident.span.remove_mark();
                    }
                }
                _ => {
                    // Do not resolve labels across function boundary
                    return None;
                }
            }
            let r = pred(rib, ident);
            if r.is_some() {
                return r;
            }
        }
        None
    }

    fn resolve_adt(&mut self, item: &Item, generics: &Generics) {
        debug!("resolve_adt");
        self.with_current_self_item(item, |this| {
            this.with_generic_param_rib(HasGenericParams(generics, ItemRibKind), |this| {
                let item_def_id = this.definitions.local_def_id(item.id);
                this.with_self_rib(Res::SelfTy(None, Some(item_def_id)), |this| {
                    visit::walk_item(this, item);
                });
            });
        });
    }

    fn future_proof_import(&mut self, use_tree: &UseTree) {
        let segments = &use_tree.prefix.segments;
        if !segments.is_empty() {
            let ident = segments[0].ident;
            if ident.is_path_segment_keyword() || ident.span.rust_2015() {
                return;
            }

            let nss = match use_tree.kind {
                UseTreeKind::Simple(..) if segments.len() == 1 => &[TypeNS, ValueNS][..],
                _ => &[TypeNS],
            };
            let report_error = |this: &Self, ns| {
                let what = if ns == TypeNS { "type parameters" } else { "local variables" };
                this.session.span_err(ident.span, &format!("imports cannot refer to {}", what));
            };

            for &ns in nss {
                match self.resolve_ident_in_lexical_scope(ident, ns, None, use_tree.prefix.span) {
                    Some(LexicalScopeBinding::Res(..)) => {
                        report_error(self, ns);
                    }
                    Some(LexicalScopeBinding::Item(binding)) => {
                        let orig_blacklisted_binding =
                            replace(&mut self.blacklisted_binding, Some(binding));
                        if let Some(LexicalScopeBinding::Res(..)) =
                                self.resolve_ident_in_lexical_scope(ident, ns, None,
                                                                    use_tree.prefix.span) {
                            report_error(self, ns);
                        }
                        self.blacklisted_binding = orig_blacklisted_binding;
                    }
                    None => {}
                }
            }
        } else if let UseTreeKind::Nested(use_trees) = &use_tree.kind {
            for (use_tree, _) in use_trees {
                self.future_proof_import(use_tree);
            }
        }
    }

    fn resolve_item(&mut self, item: &Item) {
        let name = item.ident.name;
        debug!("(resolving item) resolving {} ({:?})", name, item.node);

        match item.node {
            ItemKind::TyAlias(_, ref generics) |
            ItemKind::OpaqueTy(_, ref generics) |
            ItemKind::Fn(_, _, ref generics, _) => {
                self.with_generic_param_rib(
                    HasGenericParams(generics, ItemRibKind),
                    |this| visit::walk_item(this, item)
                );
            }

            ItemKind::Enum(_, ref generics) |
            ItemKind::Struct(_, ref generics) |
            ItemKind::Union(_, ref generics) => {
                self.resolve_adt(item, generics);
            }

            ItemKind::Impl(.., ref generics, ref opt_trait_ref, ref self_type, ref impl_items) =>
                self.resolve_implementation(generics,
                                            opt_trait_ref,
                                            &self_type,
                                            item.id,
                                            impl_items),

            ItemKind::Trait(.., ref generics, ref bounds, ref trait_items) => {
                // Create a new rib for the trait-wide type parameters.
                self.with_generic_param_rib(HasGenericParams(generics, ItemRibKind), |this| {
                    let local_def_id = this.definitions.local_def_id(item.id);
                    this.with_self_rib(Res::SelfTy(Some(local_def_id), None), |this| {
                        this.visit_generics(generics);
                        walk_list!(this, visit_param_bound, bounds);

                        for trait_item in trait_items {
                            this.with_trait_items(trait_items, |this| {
                                let generic_params = HasGenericParams(
                                    &trait_item.generics,
                                    AssocItemRibKind,
                                );
                                this.with_generic_param_rib(generic_params, |this| {
                                    match trait_item.node {
                                        TraitItemKind::Const(ref ty, ref default) => {
                                            this.visit_ty(ty);

                                            // Only impose the restrictions of
                                            // ConstRibKind for an actual constant
                                            // expression in a provided default.
                                            if let Some(ref expr) = *default{
                                                this.with_constant_rib(|this| {
                                                    this.visit_expr(expr);
                                                });
                                            }
                                        }
                                        TraitItemKind::Method(_, _) => {
                                            visit::walk_trait_item(this, trait_item)
                                        }
                                        TraitItemKind::Type(..) => {
                                            visit::walk_trait_item(this, trait_item)
                                        }
                                        TraitItemKind::Macro(_) => {
                                            panic!("unexpanded macro in resolve!")
                                        }
                                    };
                                });
                            });
                        }
                    });
                });
            }

            ItemKind::TraitAlias(ref generics, ref bounds) => {
                // Create a new rib for the trait-wide type parameters.
                self.with_generic_param_rib(HasGenericParams(generics, ItemRibKind), |this| {
                    let local_def_id = this.definitions.local_def_id(item.id);
                    this.with_self_rib(Res::SelfTy(Some(local_def_id), None), |this| {
                        this.visit_generics(generics);
                        walk_list!(this, visit_param_bound, bounds);
                    });
                });
            }

            ItemKind::Mod(_) | ItemKind::ForeignMod(_) => {
                self.with_scope(item.id, |this| {
                    visit::walk_item(this, item);
                });
            }

            ItemKind::Static(ref ty, _, ref expr) |
            ItemKind::Const(ref ty, ref expr) => {
                debug!("resolve_item ItemKind::Const");
                self.with_item_rib(|this| {
                    this.visit_ty(ty);
                    this.with_constant_rib(|this| {
                        this.visit_expr(expr);
                    });
                });
            }

            ItemKind::Use(ref use_tree) => {
                self.future_proof_import(use_tree);
            }

            ItemKind::ExternCrate(..) |
            ItemKind::MacroDef(..) | ItemKind::GlobalAsm(..) => {
                // do nothing, these are just around to be encoded
            }

            ItemKind::Mac(_) => panic!("unexpanded macro in resolve!"),
        }
    }

    fn with_generic_param_rib<'c, F>(&'c mut self, generic_params: GenericParameters<'a, 'c>, f: F)
        where F: FnOnce(&mut LateResolutionVisitor<'_, '_>)
    {
        debug!("with_generic_param_rib");
        match generic_params {
            HasGenericParams(generics, rib_kind) => {
                let mut function_type_rib = Rib::new(rib_kind);
                let mut function_value_rib = Rib::new(rib_kind);
                let mut seen_bindings = FxHashMap::default();
                for param in &generics.params {
                    match param.kind {
                        GenericParamKind::Lifetime { .. } => {}
                        GenericParamKind::Type { .. } => {
                            let ident = param.ident.modern();
                            debug!("with_generic_param_rib: {}", param.id);

                            if seen_bindings.contains_key(&ident) {
                                let span = seen_bindings.get(&ident).unwrap();
                                let err = ResolutionError::NameAlreadyUsedInParameterList(
                                    ident.name,
                                    *span,
                                );
                                resolve_error(self, param.ident.span, err);
                            }
                            seen_bindings.entry(ident).or_insert(param.ident.span);

                            // Plain insert (no renaming).
                            let res = Res::Def(
                                DefKind::TyParam,
                                self.definitions.local_def_id(param.id),
                            );
                            function_type_rib.bindings.insert(ident, res);
                            self.record_partial_res(param.id, PartialRes::new(res));
                        }
                        GenericParamKind::Const { .. } => {
                            let ident = param.ident.modern();
                            debug!("with_generic_param_rib: {}", param.id);

                            if seen_bindings.contains_key(&ident) {
                                let span = seen_bindings.get(&ident).unwrap();
                                let err = ResolutionError::NameAlreadyUsedInParameterList(
                                    ident.name,
                                    *span,
                                );
                                resolve_error(self, param.ident.span, err);
                            }
                            seen_bindings.entry(ident).or_insert(param.ident.span);

                            let res = Res::Def(
                                DefKind::ConstParam,
                                self.definitions.local_def_id(param.id),
                            );
                            function_value_rib.bindings.insert(ident, res);
                            self.record_partial_res(param.id, PartialRes::new(res));
                        }
                    }
                }
                self.ribs[ValueNS].push(function_value_rib);
                self.ribs[TypeNS].push(function_type_rib);
            }

            NoGenericParams => {
                // Nothing to do.
            }
        }

        f(self);

        if let HasGenericParams(..) = generic_params {
            self.ribs[TypeNS].pop();
            self.ribs[ValueNS].pop();
        }
    }

    fn with_label_rib<F>(&mut self, f: F)
        where F: FnOnce(&mut LateResolutionVisitor<'_, '_>)
    {
        self.label_ribs.push(Rib::new(NormalRibKind));
        f(self);
        self.label_ribs.pop();
    }

    fn with_item_rib<F>(&mut self, f: F)
        where F: FnOnce(&mut LateResolutionVisitor<'_, '_>)
    {
        self.ribs[ValueNS].push(Rib::new(ItemRibKind));
        self.ribs[TypeNS].push(Rib::new(ItemRibKind));
        f(self);
        self.ribs[TypeNS].pop();
        self.ribs[ValueNS].pop();
    }

    fn with_constant_rib<F>(&mut self, f: F)
        where F: FnOnce(&mut LateResolutionVisitor<'_, '_>)
    {
        debug!("with_constant_rib");
        self.ribs[ValueNS].push(Rib::new(ConstantItemRibKind));
        self.label_ribs.push(Rib::new(ConstantItemRibKind));
        f(self);
        self.label_ribs.pop();
        self.ribs[ValueNS].pop();
    }

    fn with_current_self_type<T, F>(&mut self, self_type: &Ty, f: F) -> T
        where F: FnOnce(&mut LateResolutionVisitor<'_, '_>) -> T
    {
        // Handle nested impls (inside fn bodies)
        let previous_value = replace(&mut self.current_self_type, Some(self_type.clone()));
        let result = f(self);
        self.current_self_type = previous_value;
        result
    }

    fn with_current_self_item<T, F>(&mut self, self_item: &Item, f: F) -> T
        where F: FnOnce(&mut LateResolutionVisitor<'_, '_>) -> T
    {
        let previous_value = replace(&mut self.current_self_item, Some(self_item.id));
        let result = f(self);
        self.current_self_item = previous_value;
        result
    }

    /// When evaluating a `trait` use its associated types' idents for suggestionsa in E0412.
    fn with_trait_items<T, F>(&mut self, trait_items: &Vec<TraitItem>, f: F) -> T
        where F: FnOnce(&mut LateResolutionVisitor<'_, '_>) -> T
    {
        let trait_assoc_types = replace(
            &mut self.current_trait_assoc_types,
            trait_items.iter().filter_map(|item| match &item.node {
                TraitItemKind::Type(bounds, _) if bounds.len() == 0 => Some(item.ident),
                _ => None,
            }).collect(),
        );
        let result = f(self);
        self.current_trait_assoc_types = trait_assoc_types;
        result
    }

    /// This is called to resolve a trait reference from an `impl` (i.e., `impl Trait for Foo`).
    fn with_optional_trait_ref<T, F>(&mut self, opt_trait_ref: Option<&TraitRef>, f: F) -> T
        where F: FnOnce(&mut LateResolutionVisitor<'_, '_>, Option<DefId>) -> T
    {
        let mut new_val = None;
        let mut new_id = None;
        if let Some(trait_ref) = opt_trait_ref {
            let path: Vec<_> = Segment::from_path(&trait_ref.path);
            let res = self.smart_resolve_path_fragment(
                trait_ref.ref_id,
                None,
                &path,
                trait_ref.path.span,
                PathSource::Trait(AliasPossibility::No),
                CrateLint::SimplePath(trait_ref.ref_id),
            ).base_res();
            if res != Res::Err {
                new_id = Some(res.def_id());
                let span = trait_ref.path.span;
                if let PathResult::Module(ModuleOrUniformRoot::Module(module)) =
                    self.resolve_path(
                        &path,
                        Some(TypeNS),
                        false,
                        span,
                        CrateLint::SimplePath(trait_ref.ref_id),
                    )
                {
                    new_val = Some((module, trait_ref.clone()));
                }
            }
        }
        let original_trait_ref = replace(&mut self.current_trait_ref, new_val);
        let result = f(self, new_id);
        self.current_trait_ref = original_trait_ref;
        result
    }

    fn with_self_rib<F>(&mut self, self_res: Res, f: F)
        where F: FnOnce(&mut LateResolutionVisitor<'_, '_>)
    {
        let mut self_type_rib = Rib::new(NormalRibKind);

        // Plain insert (no renaming, since types are not currently hygienic)
        self_type_rib.bindings.insert(Ident::with_empty_ctxt(kw::SelfUpper), self_res);
        self.ribs[TypeNS].push(self_type_rib);
        f(self);
        self.ribs[TypeNS].pop();
    }

    fn with_self_struct_ctor_rib<F>(&mut self, impl_id: DefId, f: F)
        where F: FnOnce(&mut LateResolutionVisitor<'_, '_>)
    {
        let self_res = Res::SelfCtor(impl_id);
        let mut self_type_rib = Rib::new(NormalRibKind);
        self_type_rib.bindings.insert(Ident::with_empty_ctxt(kw::SelfUpper), self_res);
        self.ribs[ValueNS].push(self_type_rib);
        f(self);
        self.ribs[ValueNS].pop();
    }

    fn resolve_implementation(&mut self,
                              generics: &Generics,
                              opt_trait_reference: &Option<TraitRef>,
                              self_type: &Ty,
                              item_id: NodeId,
                              impl_items: &[ImplItem]) {
        debug!("resolve_implementation");
        // If applicable, create a rib for the type parameters.
        self.with_generic_param_rib(HasGenericParams(generics, ItemRibKind), |this| {
            // Dummy self type for better errors if `Self` is used in the trait path.
            this.with_self_rib(Res::SelfTy(None, None), |this| {
                // Resolve the trait reference, if necessary.
                this.with_optional_trait_ref(opt_trait_reference.as_ref(), |this, trait_id| {
                    let item_def_id = this.definitions.local_def_id(item_id);
                    this.with_self_rib(Res::SelfTy(trait_id, Some(item_def_id)), |this| {
                        if let Some(trait_ref) = opt_trait_reference.as_ref() {
                            // Resolve type arguments in the trait path.
                            visit::walk_trait_ref(this, trait_ref);
                        }
                        // Resolve the self type.
                        this.visit_ty(self_type);
                        // Resolve the generic parameters.
                        this.visit_generics(generics);
                        // Resolve the items within the impl.
                        this.with_current_self_type(self_type, |this| {
                            this.with_self_struct_ctor_rib(item_def_id, |this| {
                                debug!("resolve_implementation with_self_struct_ctor_rib");
                                for impl_item in impl_items {
                                    this.resolver.resolve_visibility(
                                        &impl_item.vis, &this.parent_scope
                                    );
                                    // We also need a new scope for the impl item type parameters.
                                    let generic_params = HasGenericParams(&impl_item.generics,
                                                                          AssocItemRibKind);
                                    this.with_generic_param_rib(generic_params, |this| {
                                        use crate::ResolutionError::*;
                                        match impl_item.node {
                                            ImplItemKind::Const(..) => {
                                                debug!(
                                                    "resolve_implementation ImplItemKind::Const",
                                                );
                                                // If this is a trait impl, ensure the const
                                                // exists in trait
                                                this.check_trait_item(
                                                    impl_item.ident,
                                                    ValueNS,
                                                    impl_item.span,
                                                    |n, s| ConstNotMemberOfTrait(n, s),
                                                );

                                                this.with_constant_rib(|this| {
                                                    visit::walk_impl_item(this, impl_item)
                                                });
                                            }
                                            ImplItemKind::Method(..) => {
                                                // If this is a trait impl, ensure the method
                                                // exists in trait
                                                this.check_trait_item(impl_item.ident,
                                                                      ValueNS,
                                                                      impl_item.span,
                                                    |n, s| MethodNotMemberOfTrait(n, s));

                                                visit::walk_impl_item(this, impl_item);
                                            }
                                            ImplItemKind::TyAlias(ref ty) => {
                                                // If this is a trait impl, ensure the type
                                                // exists in trait
                                                this.check_trait_item(impl_item.ident,
                                                                      TypeNS,
                                                                      impl_item.span,
                                                    |n, s| TypeNotMemberOfTrait(n, s));

                                                this.visit_ty(ty);
                                            }
                                            ImplItemKind::OpaqueTy(ref bounds) => {
                                                // If this is a trait impl, ensure the type
                                                // exists in trait
                                                this.check_trait_item(impl_item.ident,
                                                                      TypeNS,
                                                                      impl_item.span,
                                                    |n, s| TypeNotMemberOfTrait(n, s));

                                                for bound in bounds {
                                                    this.visit_param_bound(bound);
                                                }
                                            }
                                            ImplItemKind::Macro(_) =>
                                                panic!("unexpanded macro in resolve!"),
                                        }
                                    });
                                }
                            });
                        });
                    });
                });
            });
        });
    }

    fn check_trait_item<F>(&mut self, ident: Ident, ns: Namespace, span: Span, err: F)
        where F: FnOnce(Name, &str) -> ResolutionError<'_>
    {
        // If there is a TraitRef in scope for an impl, then the method must be in the
        // trait.
        if let Some((module, _)) = self.current_trait_ref {
            if self.resolver.resolve_ident_in_module(
                ModuleOrUniformRoot::Module(module),
                ident,
                ns,
                &self.parent_scope,
                false,
                span,
            ).is_err() {
                let path = &self.current_trait_ref.as_ref().unwrap().1.path;
                resolve_error(self, span, err(ident.name, &path_names_to_string(path)));
            }
        }
    }

    fn resolve_local(&mut self, local: &Local) {
        // Resolve the type.
        walk_list!(self, visit_ty, &local.ty);

        // Resolve the initializer.
        walk_list!(self, visit_expr, &local.init);

        // Resolve the pattern.
        self.resolve_pattern(&local.pat, PatternSource::Let, &mut FxHashMap::default());
    }

    // build a map from pattern identifiers to binding-info's.
    // this is done hygienically. This could arise for a macro
    // that expands into an or-pattern where one 'x' was from the
    // user and one 'x' came from the macro.
    fn binding_mode_map(&mut self, pat: &Pat) -> BindingMap {
        let mut binding_map = FxHashMap::default();

        pat.walk(&mut |pat| {
            if let PatKind::Ident(binding_mode, ident, ref sub_pat) = pat.node {
                if sub_pat.is_some() || match self.partial_res_map.get(&pat.id)
                                                                  .map(|res| res.base_res()) {
                    Some(Res::Local(..)) => true,
                    _ => false,
                } {
                    let binding_info = BindingInfo { span: ident.span, binding_mode: binding_mode };
                    binding_map.insert(ident, binding_info);
                }
            }
            true
        });

        binding_map
    }

    // Checks that all of the arms in an or-pattern have exactly the
    // same set of bindings, with the same binding modes for each.
    fn check_consistent_bindings(&mut self, pats: &[P<Pat>]) {
        if pats.is_empty() {
            return;
        }

        let mut missing_vars = FxHashMap::default();
        let mut inconsistent_vars = FxHashMap::default();
        for (i, p) in pats.iter().enumerate() {
            let map_i = self.binding_mode_map(&p);

            for (j, q) in pats.iter().enumerate() {
                if i == j {
                    continue;
                }

                let map_j = self.binding_mode_map(&q);
                for (&key, &binding_i) in &map_i {
                    if map_j.is_empty() {                   // Account for missing bindings when
                        let binding_error = missing_vars    // `map_j` has none.
                            .entry(key.name)
                            .or_insert(BindingError {
                                name: key.name,
                                origin: BTreeSet::new(),
                                target: BTreeSet::new(),
                            });
                        binding_error.origin.insert(binding_i.span);
                        binding_error.target.insert(q.span);
                    }
                    for (&key_j, &binding_j) in &map_j {
                        match map_i.get(&key_j) {
                            None => {  // missing binding
                                let binding_error = missing_vars
                                    .entry(key_j.name)
                                    .or_insert(BindingError {
                                        name: key_j.name,
                                        origin: BTreeSet::new(),
                                        target: BTreeSet::new(),
                                    });
                                binding_error.origin.insert(binding_j.span);
                                binding_error.target.insert(p.span);
                            }
                            Some(binding_i) => {  // check consistent binding
                                if binding_i.binding_mode != binding_j.binding_mode {
                                    inconsistent_vars
                                        .entry(key.name)
                                        .or_insert((binding_j.span, binding_i.span));
                                }
                            }
                        }
                    }
                }
            }
        }
        let mut missing_vars = missing_vars.iter().collect::<Vec<_>>();
        missing_vars.sort();
        for (_, v) in missing_vars {
            resolve_error(self,
                          *v.origin.iter().next().unwrap(),
                          ResolutionError::VariableNotBoundInPattern(v));
        }
        let mut inconsistent_vars = inconsistent_vars.iter().collect::<Vec<_>>();
        inconsistent_vars.sort();
        for (name, v) in inconsistent_vars {
            resolve_error(self, v.0, ResolutionError::VariableBoundWithDifferentMode(*name, v.1));
        }
    }

    fn resolve_arm(&mut self, arm: &Arm) {
        self.ribs[ValueNS].push(Rib::new(NormalRibKind));

        self.resolve_pats(&arm.pats, PatternSource::Match);

        if let Some(ref expr) = arm.guard {
            self.visit_expr(expr)
        }
        self.visit_expr(&arm.body);

        self.ribs[ValueNS].pop();
    }

    /// Arising from `source`, resolve a sequence of patterns (top level or-patterns).
    fn resolve_pats(&mut self, pats: &[P<Pat>], source: PatternSource) {
        let mut bindings_list = FxHashMap::default();
        for pat in pats {
            self.resolve_pattern(pat, source, &mut bindings_list);
        }
        // This has to happen *after* we determine which pat_idents are variants
        self.check_consistent_bindings(pats);
    }

    fn resolve_block(&mut self, block: &Block) {
        debug!("(resolving block) entering block");
        // Move down in the graph, if there's an anonymous module rooted here.
        let orig_module = self.parent_scope.module;
        let anonymous_module = self.block_map.get(&block.id).cloned(); // clones a reference

        let mut num_macro_definition_ribs = 0;
        if let Some(anonymous_module) = anonymous_module {
            debug!("(resolving block) found anonymous module, moving down");
            self.ribs[ValueNS].push(Rib::new(ModuleRibKind(anonymous_module)));
            self.ribs[TypeNS].push(Rib::new(ModuleRibKind(anonymous_module)));
            self.parent_scope.module = anonymous_module;
            self.finalize_current_module_macro_resolutions(anonymous_module);
        } else {
            self.ribs[ValueNS].push(Rib::new(NormalRibKind));
        }

        // Descend into the block.
        for stmt in &block.stmts {
            if let StmtKind::Item(ref item) = stmt.node {
                if let ItemKind::MacroDef(..) = item.node {
                    num_macro_definition_ribs += 1;
                    let res = self.definitions.local_def_id(item.id);
                    self.ribs[ValueNS].push(Rib::new(MacroDefinition(res)));
                    self.label_ribs.push(Rib::new(MacroDefinition(res)));
                }
            }

            self.visit_stmt(stmt);
        }

        // Move back up.
        self.parent_scope.module = orig_module;
        for _ in 0 .. num_macro_definition_ribs {
            self.ribs[ValueNS].pop();
            self.label_ribs.pop();
        }
        self.ribs[ValueNS].pop();
        if anonymous_module.is_some() {
            self.ribs[TypeNS].pop();
        }
        debug!("(resolving block) leaving block");
    }

    fn fresh_binding(&mut self,
                     ident: Ident,
                     pat_id: NodeId,
                     outer_pat_id: NodeId,
                     pat_src: PatternSource,
                     bindings: &mut FxHashMap<Ident, NodeId>)
                     -> Res {
        // Add the binding to the local ribs, if it
        // doesn't already exist in the bindings map. (We
        // must not add it if it's in the bindings map
        // because that breaks the assumptions later
        // passes make about or-patterns.)
        let ident = ident.modern_and_legacy();
        let mut res = Res::Local(pat_id);
        match bindings.get(&ident).cloned() {
            Some(id) if id == outer_pat_id => {
                // `Variant(a, a)`, error
                resolve_error(
                    self,
                    ident.span,
                    ResolutionError::IdentifierBoundMoreThanOnceInSamePattern(
                        &ident.as_str())
                );
            }
            Some(..) if pat_src == PatternSource::FnParam => {
                // `fn f(a: u8, a: u8)`, error
                resolve_error(
                    self,
                    ident.span,
                    ResolutionError::IdentifierBoundMoreThanOnceInParameterList(
                        &ident.as_str())
                );
            }
            Some(..) if pat_src == PatternSource::Match ||
                        pat_src == PatternSource::Let => {
                // `Variant1(a) | Variant2(a)`, ok
                // Reuse definition from the first `a`.
                res = self.ribs[ValueNS].last_mut().unwrap().bindings[&ident];
            }
            Some(..) => {
                span_bug!(ident.span, "two bindings with the same name from \
                                       unexpected pattern source {:?}", pat_src);
            }
            None => {
                // A completely fresh binding, add to the lists if it's valid.
                if ident.name != kw::Invalid {
                    bindings.insert(ident, outer_pat_id);
                    self.ribs[ValueNS].last_mut().unwrap().bindings.insert(ident, res);
                }
            }
        }

        res
    }

    fn resolve_pattern(&mut self,
                       pat: &Pat,
                       pat_src: PatternSource,
                       // Maps idents to the node ID for the
                       // outermost pattern that binds them.
                       bindings: &mut FxHashMap<Ident, NodeId>) {
        // Visit all direct subpatterns of this pattern.
        let outer_pat_id = pat.id;
        pat.walk(&mut |pat| {
            debug!("resolve_pattern pat={:?} node={:?}", pat, pat.node);
            match pat.node {
                PatKind::Ident(bmode, ident, ref opt_pat) => {
                    // First try to resolve the identifier as some existing
                    // entity, then fall back to a fresh binding.
                    let binding = self.resolve_ident_in_lexical_scope(ident, ValueNS,
                                                                      None, pat.span)
                                      .and_then(LexicalScopeBinding::item);
                    let res = binding.map(NameBinding::res).and_then(|res| {
                        let is_syntactic_ambiguity = opt_pat.is_none() &&
                            bmode == BindingMode::ByValue(Mutability::Immutable);
                        match res {
                            Res::Def(DefKind::Ctor(_, CtorKind::Const), _) |
                            Res::Def(DefKind::Const, _) if is_syntactic_ambiguity => {
                                // Disambiguate in favor of a unit struct/variant
                                // or constant pattern.
                                self.record_use(ident, ValueNS, binding.unwrap(), false);
                                Some(res)
                            }
                            Res::Def(DefKind::Ctor(..), _)
                            | Res::Def(DefKind::Const, _)
                            | Res::Def(DefKind::Static, _) => {
                                // This is unambiguously a fresh binding, either syntactically
                                // (e.g., `IDENT @ PAT` or `ref IDENT`) or because `IDENT` resolves
                                // to something unusable as a pattern (e.g., constructor function),
                                // but we still conservatively report an error, see
                                // issues/33118#issuecomment-233962221 for one reason why.
                                resolve_error(
                                    self,
                                    ident.span,
                                    ResolutionError::BindingShadowsSomethingUnacceptable(
                                        pat_src.descr(), ident.name, binding.unwrap())
                                );
                                None
                            }
                            Res::Def(DefKind::Fn, _) | Res::Err => {
                                // These entities are explicitly allowed
                                // to be shadowed by fresh bindings.
                                None
                            }
                            res => {
                                span_bug!(ident.span, "unexpected resolution for an \
                                                       identifier in pattern: {:?}", res);
                            }
                        }
                    }).unwrap_or_else(|| {
                        self.fresh_binding(ident, pat.id, outer_pat_id, pat_src, bindings)
                    });

                    self.record_partial_res(pat.id, PartialRes::new(res));
                }

                PatKind::TupleStruct(ref path, ..) => {
                    self.smart_resolve_path(pat.id, None, path, PathSource::TupleStruct);
                }

                PatKind::Path(ref qself, ref path) => {
                    self.smart_resolve_path(pat.id, qself.as_ref(), path, PathSource::Pat);
                }

                PatKind::Struct(ref path, ..) => {
                    self.smart_resolve_path(pat.id, None, path, PathSource::Struct);
                }

                _ => {}
            }
            true
        });

        visit::walk_pat(self, pat);
    }

    // High-level and context dependent path resolution routine.
    // Resolves the path and records the resolution into definition map.
    // If resolution fails tries several techniques to find likely
    // resolution candidates, suggest imports or other help, and report
    // errors in user friendly way.
    fn smart_resolve_path(&mut self,
                          id: NodeId,
                          qself: Option<&QSelf>,
                          path: &Path,
                          source: PathSource<'_>) {
        self.smart_resolve_path_fragment(
            id,
            qself,
            &Segment::from_path(path),
            path.span,
            source,
            CrateLint::SimplePath(id),
        );
    }

    fn smart_resolve_path_fragment(&mut self,
                                   id: NodeId,
                                   qself: Option<&QSelf>,
                                   path: &[Segment],
                                   span: Span,
                                   source: PathSource<'_>,
                                   crate_lint: CrateLint)
                                   -> PartialRes {
        let ns = source.namespace();
        let is_expected = &|res| source.is_expected(res);

        let report_errors = |this: &mut Self, res: Option<Res>| {
            let (err, candidates) = this.smart_resolve_report_errors(path, span, source, res);
            let def_id = this.parent_scope.module.normal_ancestor_id;
            let node_id = this.definitions.as_local_node_id(def_id).unwrap();
            let better = res.is_some();
            this.use_injections.push(UseError { err, candidates, node_id, better });
            PartialRes::new(Res::Err)
        };

        let partial_res = match self.resolve_qpath_anywhere(
            id,
            qself,
            path,
            ns,
            span,
            source.defer_to_typeck(),
            crate_lint,
        ) {
            Some(partial_res) if partial_res.unresolved_segments() == 0 => {
                if is_expected(partial_res.base_res()) || partial_res.base_res() == Res::Err {
                    partial_res
                } else {
                    // Add a temporary hack to smooth the transition to new struct ctor
                    // visibility rules. See #38932 for more details.
                    let mut res = None;
                    if let Res::Def(DefKind::Struct, def_id) = partial_res.base_res() {
                        if let Some((ctor_res, ctor_vis))
                                = self.struct_constructors.get(&def_id).cloned() {
                            if is_expected(ctor_res) &&
                               self.is_accessible_from(ctor_vis, self.parent_scope.module) {
                                let lint = lint::builtin::LEGACY_CONSTRUCTOR_VISIBILITY;
                                self.session.buffer_lint(lint, id, span,
                                    "private struct constructors are not usable through \
                                     re-exports in outer modules",
                                );
                                res = Some(PartialRes::new(ctor_res));
                            }
                        }
                    }

                    res.unwrap_or_else(|| report_errors(self, Some(partial_res.base_res())))
                }
            }
            Some(partial_res) if source.defer_to_typeck() => {
                // Not fully resolved associated item `T::A::B` or `<T as Tr>::A::B`
                // or `<T>::A::B`. If `B` should be resolved in value namespace then
                // it needs to be added to the trait map.
                if ns == ValueNS {
                    let item_name = path.last().unwrap().ident;
                    let traits = self.get_traits_containing_item(item_name, ns);
                    self.trait_map.insert(id, traits);
                }

                let mut std_path = vec![Segment::from_ident(Ident::with_empty_ctxt(sym::std))];
                std_path.extend(path);
                if self.primitive_type_table.primitive_types.contains_key(&path[0].ident.name) {
                    let cl = CrateLint::No;
                    let ns = Some(ns);
                    if let PathResult::Module(_) | PathResult::NonModule(_) =
                            self.resolve_path(&std_path, ns, false, span, cl) {
                        // check if we wrote `str::from_utf8` instead of `std::str::from_utf8`
                        let item_span = path.iter().last().map(|segment| segment.ident.span)
                            .unwrap_or(span);
                        debug!("accessed item from `std` submodule as a bare type {:?}", std_path);
                        let mut hm = self.session.confused_type_with_std_module.borrow_mut();
                        hm.insert(item_span, span);
                        // In some places (E0223) we only have access to the full path
                        hm.insert(span, span);
                    }
                }
                partial_res
            }
            _ => report_errors(self, None)
        };

        if let PathSource::TraitItem(..) = source {} else {
            // Avoid recording definition of `A::B` in `<T as A>::B::C`.
            self.record_partial_res(id, partial_res);
        }
        partial_res
    }

    fn self_type_is_available(&mut self, span: Span) -> bool {
        let binding = self.resolve_ident_in_lexical_scope(
            Ident::with_empty_ctxt(kw::SelfUpper),
            TypeNS,
            None,
            span,
        );
        if let Some(LexicalScopeBinding::Res(res)) = binding { res != Res::Err } else { false }
    }

    fn self_value_is_available(&mut self, self_span: Span, path_span: Span) -> bool {
        let ident = Ident::new(kw::SelfLower, self_span);
        let binding = self.resolve_ident_in_lexical_scope(ident, ValueNS, None, path_span);
        if let Some(LexicalScopeBinding::Res(res)) = binding { res != Res::Err } else { false }
    }

    // Resolve in alternative namespaces if resolution in the primary namespace fails.
    fn resolve_qpath_anywhere(
        &mut self,
        id: NodeId,
        qself: Option<&QSelf>,
        path: &[Segment],
        primary_ns: Namespace,
        span: Span,
        defer_to_typeck: bool,
        crate_lint: CrateLint,
    ) -> Option<PartialRes> {
        let mut fin_res = None;
        for (i, ns) in [primary_ns, TypeNS, ValueNS].iter().cloned().enumerate() {
            if i == 0 || ns != primary_ns {
                match self.resolve_qpath(id, qself, path, ns, span, crate_lint) {
                    // If defer_to_typeck, then resolution > no resolution,
                    // otherwise full resolution > partial resolution > no resolution.
                    Some(partial_res) if partial_res.unresolved_segments() == 0 ||
                                         defer_to_typeck =>
                        return Some(partial_res),
                    partial_res => if fin_res.is_none() { fin_res = partial_res },
                }
            }
        }

        // `MacroNS`
        assert!(primary_ns != MacroNS);
        if qself.is_none() {
            let path_seg = |seg: &Segment| PathSegment::from_ident(seg.ident);
            let path = Path { segments: path.iter().map(path_seg).collect(), span };
            if let Ok((_, res)) = self.resolver.resolve_macro_path(
                &path, None, &self.parent_scope, false, false
            ) {
                return Some(PartialRes::new(res));
            }
        }

        fin_res
    }

    /// Handles paths that may refer to associated items.
    fn resolve_qpath(
        &mut self,
        id: NodeId,
        qself: Option<&QSelf>,
        path: &[Segment],
        ns: Namespace,
        span: Span,
        crate_lint: CrateLint,
    ) -> Option<PartialRes> {
        debug!(
            "resolve_qpath(id={:?}, qself={:?}, path={:?}, ns={:?}, span={:?})",
            id,
            qself,
            path,
            ns,
            span,
        );

        if let Some(qself) = qself {
            if qself.position == 0 {
                // This is a case like `<T>::B`, where there is no
                // trait to resolve.  In that case, we leave the `B`
                // segment to be resolved by type-check.
                return Some(PartialRes::with_unresolved_segments(
                    Res::Def(DefKind::Mod, DefId::local(CRATE_DEF_INDEX)), path.len()
                ));
            }

            // Make sure `A::B` in `<T as A::B>::C` is a trait item.
            //
            // Currently, `path` names the full item (`A::B::C`, in
            // our example).  so we extract the prefix of that that is
            // the trait (the slice upto and including
            // `qself.position`). And then we recursively resolve that,
            // but with `qself` set to `None`.
            //
            // However, setting `qself` to none (but not changing the
            // span) loses the information about where this path
            // *actually* appears, so for the purposes of the crate
            // lint we pass along information that this is the trait
            // name from a fully qualified path, and this also
            // contains the full span (the `CrateLint::QPathTrait`).
            let ns = if qself.position + 1 == path.len() { ns } else { TypeNS };
            let partial_res = self.smart_resolve_path_fragment(
                id,
                None,
                &path[..=qself.position],
                span,
                PathSource::TraitItem(ns),
                CrateLint::QPathTrait {
                    qpath_id: id,
                    qpath_span: qself.path_span,
                },
            );

            // The remaining segments (the `C` in our example) will
            // have to be resolved by type-check, since that requires doing
            // trait resolution.
            return Some(PartialRes::with_unresolved_segments(
                partial_res.base_res(),
                partial_res.unresolved_segments() + path.len() - qself.position - 1,
            ));
        }

        let result = match self.resolve_path(&path, Some(ns), true, span, crate_lint) {
            PathResult::NonModule(path_res) => path_res,
            PathResult::Module(ModuleOrUniformRoot::Module(module)) if !module.is_normal() => {
                PartialRes::new(module.res().unwrap())
            }
            // In `a(::assoc_item)*` `a` cannot be a module. If `a` does resolve to a module we
            // don't report an error right away, but try to fallback to a primitive type.
            // So, we are still able to successfully resolve something like
            //
            // use std::u8; // bring module u8 in scope
            // fn f() -> u8 { // OK, resolves to primitive u8, not to std::u8
            //     u8::max_value() // OK, resolves to associated function <u8>::max_value,
            //                     // not to non-existent std::u8::max_value
            // }
            //
            // Such behavior is required for backward compatibility.
            // The same fallback is used when `a` resolves to nothing.
            PathResult::Module(ModuleOrUniformRoot::Module(_)) |
            PathResult::Failed { .. }
                    if (ns == TypeNS || path.len() > 1) &&
                       self.primitive_type_table.primitive_types
                           .contains_key(&path[0].ident.name) => {
                let prim = self.primitive_type_table.primitive_types[&path[0].ident.name];
                PartialRes::with_unresolved_segments(Res::PrimTy(prim), path.len() - 1)
            }
            PathResult::Module(ModuleOrUniformRoot::Module(module)) =>
                PartialRes::new(module.res().unwrap()),
            PathResult::Failed { is_error_from_last_segment: false, span, label, suggestion } => {
                resolve_error(self, span, ResolutionError::FailedToResolve { label, suggestion });
                PartialRes::new(Res::Err)
            }
            PathResult::Module(..) | PathResult::Failed { .. } => return None,
            PathResult::Indeterminate => bug!("indetermined path result in resolve_qpath"),
        };

        if path.len() > 1 && result.base_res() != Res::Err &&
           path[0].ident.name != kw::PathRoot &&
           path[0].ident.name != kw::DollarCrate {
            let unqualified_result = {
                match self.resolve_path(
                    &[*path.last().unwrap()],
                    Some(ns),
                    false,
                    span,
                    CrateLint::No,
                ) {
                    PathResult::NonModule(path_res) => path_res.base_res(),
                    PathResult::Module(ModuleOrUniformRoot::Module(module)) =>
                        module.res().unwrap(),
                    _ => return Some(result),
                }
            };
            if result.base_res() == unqualified_result {
                let lint = lint::builtin::UNUSED_QUALIFICATIONS;
                self.session.buffer_lint(lint, id, span, "unnecessary qualification")
            }
        }

        Some(result)
    }

    fn with_resolved_label<F>(&mut self, label: Option<Label>, id: NodeId, f: F)
        where F: FnOnce(&mut LateResolutionVisitor<'_, '_>)
    {
        if let Some(label) = label {
            self.unused_labels.insert(id, label.ident.span);
            self.with_label_rib(|this| {
                let ident = label.ident.modern_and_legacy();
                this.label_ribs.last_mut().unwrap().bindings.insert(ident, id);
                f(this);
            });
        } else {
            f(self);
        }
    }

    fn resolve_labeled_block(&mut self, label: Option<Label>, id: NodeId, block: &Block) {
        self.with_resolved_label(label, id, |this| this.visit_block(block));
    }

    fn resolve_expr(&mut self, expr: &Expr, parent: Option<&Expr>) {
        // First, record candidate traits for this expression if it could
        // result in the invocation of a method call.

        self.record_candidate_traits_for_expr_if_necessary(expr);

        // Next, resolve the node.
        match expr.node {
            ExprKind::Path(ref qself, ref path) => {
                self.smart_resolve_path(expr.id, qself.as_ref(), path, PathSource::Expr(parent));
                visit::walk_expr(self, expr);
            }

            ExprKind::Struct(ref path, ..) => {
                self.smart_resolve_path(expr.id, None, path, PathSource::Struct);
                visit::walk_expr(self, expr);
            }

            ExprKind::Break(Some(label), _) | ExprKind::Continue(Some(label)) => {
                let node_id = self.search_label(label.ident, |rib, ident| {
                    rib.bindings.get(&ident.modern_and_legacy()).cloned()
                });
                match node_id {
                    None => {
                        // Search again for close matches...
                        // Picks the first label that is "close enough", which is not necessarily
                        // the closest match
                        let close_match = self.search_label(label.ident, |rib, ident| {
                            let names = rib.bindings.iter().filter_map(|(id, _)| {
                                if id.span.ctxt() == label.ident.span.ctxt() {
                                    Some(&id.name)
                                } else {
                                    None
                                }
                            });
                            find_best_match_for_name(names, &*ident.as_str(), None)
                        });
                        self.record_partial_res(expr.id, PartialRes::new(Res::Err));
                        resolve_error(self,
                                      label.ident.span,
                                      ResolutionError::UndeclaredLabel(&label.ident.as_str(),
                                                                       close_match));
                    }
                    Some(node_id) => {
                        // Since this res is a label, it is never read.
                        self.label_res_map.insert(expr.id, node_id);
                        self.unused_labels.remove(&node_id);
                    }
                }

                // visit `break` argument if any
                visit::walk_expr(self, expr);
            }

            ExprKind::Let(ref pats, ref scrutinee) => {
                self.visit_expr(scrutinee);
                self.resolve_pats(pats, PatternSource::Let);
            }

            ExprKind::If(ref cond, ref then, ref opt_else) => {
                self.ribs[ValueNS].push(Rib::new(NormalRibKind));
                self.visit_expr(cond);
                self.visit_block(then);
                self.ribs[ValueNS].pop();

                opt_else.as_ref().map(|expr| self.visit_expr(expr));
            }

            ExprKind::Loop(ref block, label) => self.resolve_labeled_block(label, expr.id, &block),

            ExprKind::While(ref subexpression, ref block, label) => {
                self.with_resolved_label(label, expr.id, |this| {
                    this.ribs[ValueNS].push(Rib::new(NormalRibKind));
                    this.visit_expr(subexpression);
                    this.visit_block(block);
                    this.ribs[ValueNS].pop();
                });
            }

            ExprKind::ForLoop(ref pattern, ref subexpression, ref block, label) => {
                self.visit_expr(subexpression);
                self.ribs[ValueNS].push(Rib::new(NormalRibKind));
                self.resolve_pattern(pattern, PatternSource::For, &mut FxHashMap::default());

                self.resolve_labeled_block(label, expr.id, block);

                self.ribs[ValueNS].pop();
            }

            ExprKind::Block(ref block, label) => self.resolve_labeled_block(label, block.id, block),

            // Equivalent to `visit::walk_expr` + passing some context to children.
            ExprKind::Field(ref subexpression, _) => {
                self.resolve_expr(subexpression, Some(expr));
            }
            ExprKind::MethodCall(ref segment, ref arguments) => {
                let mut arguments = arguments.iter();
                self.resolve_expr(arguments.next().unwrap(), Some(expr));
                for argument in arguments {
                    self.resolve_expr(argument, None);
                }
                self.visit_path_segment(expr.span, segment);
            }

            ExprKind::Call(ref callee, ref arguments) => {
                self.resolve_expr(callee, Some(expr));
                for argument in arguments {
                    self.resolve_expr(argument, None);
                }
            }
            ExprKind::Type(ref type_expr, _) => {
                self.current_type_ascription.push(type_expr.span);
                visit::walk_expr(self, expr);
                self.current_type_ascription.pop();
            }
            // `async |x| ...` gets desugared to `|x| future_from_generator(|| ...)`, so we need to
            // resolve the arguments within the proper scopes so that usages of them inside the
            // closure are detected as upvars rather than normal closure arg usages.
            ExprKind::Closure(
                _, IsAsync::Async { .. }, _,
                ref fn_decl, ref body, _span,
            ) => {
                let rib_kind = NormalRibKind;
                self.ribs[ValueNS].push(Rib::new(rib_kind));
                // Resolve arguments:
                let mut bindings_list = FxHashMap::default();
                for argument in &fn_decl.inputs {
                    self.resolve_pattern(&argument.pat, PatternSource::FnParam, &mut bindings_list);
                    self.visit_ty(&argument.ty);
                }
                // No need to resolve return type-- the outer closure return type is
                // FunctionRetTy::Default

                // Now resolve the inner closure
                {
                    // No need to resolve arguments: the inner closure has none.
                    // Resolve the return type:
                    visit::walk_fn_ret_ty(self, &fn_decl.output);
                    // Resolve the body
                    self.visit_expr(body);
                }
                self.ribs[ValueNS].pop();
            }
            _ => {
                visit::walk_expr(self, expr);
            }
        }
    }

    fn record_candidate_traits_for_expr_if_necessary(&mut self, expr: &Expr) {
        match expr.node {
            ExprKind::Field(_, ident) => {
                // FIXME(#6890): Even though you can't treat a method like a
                // field, we need to add any trait methods we find that match
                // the field name so that we can do some nice error reporting
                // later on in typeck.
                let traits = self.get_traits_containing_item(ident, ValueNS);
                self.trait_map.insert(expr.id, traits);
            }
            ExprKind::MethodCall(ref segment, ..) => {
                debug!("(recording candidate traits for expr) recording traits for {}",
                       expr.id);
                let traits = self.get_traits_containing_item(segment.ident, ValueNS);
                self.trait_map.insert(expr.id, traits);
            }
            _ => {
                // Nothing to do.
            }
        }
    }

    fn get_traits_containing_item(&mut self, mut ident: Ident, ns: Namespace)
                                  -> Vec<TraitCandidate> {
        debug!("(getting traits containing item) looking for '{}'", ident.name);

        let mut found_traits = Vec::new();
        // Look for the current trait.
        if let Some((module, _)) = self.current_trait_ref {
            if self.resolver.resolve_ident_in_module(
                ModuleOrUniformRoot::Module(module),
                ident,
                ns,
                &self.parent_scope,
                false,
                module.span,
            ).is_ok() {
                let def_id = module.def_id().unwrap();
                found_traits.push(TraitCandidate { def_id: def_id, import_ids: smallvec![] });
            }
        }

        ident.span = ident.span.modern();
        let mut search_module = self.parent_scope.module;
        loop {
            self.get_traits_in_module_containing_item(ident, ns, search_module, &mut found_traits);
            search_module = unwrap_or!(
                self.hygienic_lexical_parent(search_module, &mut ident.span), break
            );
        }

        if let Some(prelude) = self.prelude {
            if !search_module.no_implicit_prelude {
                self.get_traits_in_module_containing_item(ident, ns, prelude, &mut found_traits);
            }
        }

        found_traits
    }

    fn get_traits_in_module_containing_item(&mut self,
                                            ident: Ident,
                                            ns: Namespace,
                                            module: Module<'a>,
                                            found_traits: &mut Vec<TraitCandidate>) {
        assert!(ns == TypeNS || ns == ValueNS);
        let mut traits = module.traits.borrow_mut();
        if traits.is_none() {
            let mut collected_traits = Vec::new();
            module.for_each_child(|name, ns, binding| {
                if ns != TypeNS { return }
                match binding.res() {
                    Res::Def(DefKind::Trait, _) |
                    Res::Def(DefKind::TraitAlias, _) => collected_traits.push((name, binding)),
                    _ => (),
                }
            });
            *traits = Some(collected_traits.into_boxed_slice());
        }

        for &(trait_name, binding) in traits.as_ref().unwrap().iter() {
            // Traits have pseudo-modules that can be used to search for the given ident.
            if let Some(module) = binding.module() {
                let mut ident = ident;
                if ident.span.glob_adjust(
                    module.expansion,
                    binding.span,
                ).is_none() {
                    continue
                }
                if self.resolver.resolve_ident_in_module_unadjusted(
                    ModuleOrUniformRoot::Module(module),
                    ident,
                    ns,
                    &self.parent_scope,
                    false,
                    module.span,
                ).is_ok() {
                    let import_ids = self.find_transitive_imports(&binding.kind, trait_name);
                    let trait_def_id = module.def_id().unwrap();
                    found_traits.push(TraitCandidate { def_id: trait_def_id, import_ids });
                }
            } else if let Res::Def(DefKind::TraitAlias, _) = binding.res() {
                // For now, just treat all trait aliases as possible candidates, since we don't
                // know if the ident is somewhere in the transitive bounds.
                let import_ids = self.find_transitive_imports(&binding.kind, trait_name);
                let trait_def_id = binding.res().def_id();
                found_traits.push(TraitCandidate { def_id: trait_def_id, import_ids });
            } else {
                bug!("candidate is not trait or trait alias?")
            }
        }
    }

    fn find_transitive_imports(&mut self, mut kind: &NameBindingKind<'_>,
                               trait_name: Ident) -> SmallVec<[NodeId; 1]> {
        let mut import_ids = smallvec![];
        while let NameBindingKind::Import { directive, binding, .. } = kind {
            self.maybe_unused_trait_imports.insert(directive.id);
            self.add_to_glob_map(&directive, trait_name);
            import_ids.push(directive.id);
            kind = &binding.kind;
        };
        import_ids
    }
}

impl<'a> Resolver<'a> {
    pub(crate) fn late_resolve_crate(&mut self, krate: &Crate) {
        let mut late_resolution_visitor = LateResolutionVisitor::new(self);
        let module = late_resolution_visitor.parent_scope.module;
        late_resolution_visitor.finalize_current_module_macro_resolutions(module);
        visit::walk_crate(&mut late_resolution_visitor, krate);
        for (id, span) in late_resolution_visitor.unused_labels.iter() {
            self.session.buffer_lint(lint::builtin::UNUSED_LABELS, *id, *span, "unused label");
        }
    }
}