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//! Name resolution façade.
use std::sync::Arc;

use base_db::CrateId;
use hir_expand::{
    name::{name, Name},
    MacroDefId,
};
use indexmap::IndexMap;
use rustc_hash::FxHashSet;
use smallvec::SmallVec;

use crate::{
    body::scope::{ExprScopes, ScopeId},
    builtin_type::BuiltinType,
    db::DefDatabase,
    expr::{ExprId, LabelId, PatId},
    generics::GenericParams,
    intern::Interned,
    item_scope::{BuiltinShadowMode, BUILTIN_SCOPE},
    nameres::DefMap,
    path::{ModPath, PathKind},
    per_ns::PerNs,
    visibility::{RawVisibility, Visibility},
    AdtId, AssocContainerId, AssocItemId, ConstId, ConstParamId, DefWithBodyId, EnumId,
    EnumVariantId, FunctionId, GenericDefId, GenericParamId, HasModule, ImplId, LifetimeParamId,
    LocalModuleId, Lookup, ModuleDefId, ModuleId, StaticId, StructId, TraitId, TypeAliasId,
    TypeParamId, VariantId,
};

#[derive(Debug, Clone, Default)]
pub struct Resolver {
    /// The stack of scopes, where the inner-most scope is the last item.
    ///
    /// When using, you generally want to process the scopes in reverse order,
    /// there's `scopes` *method* for that.
    scopes: Vec<Scope>,
}

// FIXME how to store these best
#[derive(Debug, Clone)]
struct ModuleItemMap {
    def_map: Arc<DefMap>,
    module_id: LocalModuleId,
}

#[derive(Debug, Clone)]
struct ExprScope {
    owner: DefWithBodyId,
    expr_scopes: Arc<ExprScopes>,
    scope_id: ScopeId,
}

#[derive(Debug, Clone)]
enum Scope {
    /// All the items and imported names of a module
    ModuleScope(ModuleItemMap),
    /// Brings the generic parameters of an item into scope
    GenericParams { def: GenericDefId, params: Interned<GenericParams> },
    /// Brings `Self` in `impl` block into scope
    ImplDefScope(ImplId),
    /// Brings `Self` in enum, struct and union definitions into scope
    AdtScope(AdtId),
    /// Local bindings
    ExprScope(ExprScope),
}

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub enum TypeNs {
    SelfType(ImplId),
    GenericParam(TypeParamId),
    AdtId(AdtId),
    AdtSelfType(AdtId),
    // Yup, enum variants are added to the types ns, but any usage of variant as
    // type is an error.
    EnumVariantId(EnumVariantId),
    TypeAliasId(TypeAliasId),
    BuiltinType(BuiltinType),
    TraitId(TraitId),
    // Module belong to type ns, but the resolver is used when all module paths
    // are fully resolved.
    // ModuleId(ModuleId)
}

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub enum ResolveValueResult {
    ValueNs(ValueNs),
    Partial(TypeNs, usize),
}

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub enum ValueNs {
    ImplSelf(ImplId),
    LocalBinding(PatId),
    FunctionId(FunctionId),
    ConstId(ConstId),
    StaticId(StaticId),
    StructId(StructId),
    EnumVariantId(EnumVariantId),
    GenericParam(ConstParamId),
}

impl Resolver {
    /// Resolve known trait from std, like `std::futures::Future`
    pub fn resolve_known_trait(&self, db: &dyn DefDatabase, path: &ModPath) -> Option<TraitId> {
        let res = self.resolve_module_path(db, path, BuiltinShadowMode::Other).take_types()?;
        match res {
            ModuleDefId::TraitId(it) => Some(it),
            _ => None,
        }
    }

    /// Resolve known struct from std, like `std::boxed::Box`
    pub fn resolve_known_struct(&self, db: &dyn DefDatabase, path: &ModPath) -> Option<StructId> {
        let res = self.resolve_module_path(db, path, BuiltinShadowMode::Other).take_types()?;
        match res {
            ModuleDefId::AdtId(AdtId::StructId(it)) => Some(it),
            _ => None,
        }
    }

    /// Resolve known enum from std, like `std::result::Result`
    pub fn resolve_known_enum(&self, db: &dyn DefDatabase, path: &ModPath) -> Option<EnumId> {
        let res = self.resolve_module_path(db, path, BuiltinShadowMode::Other).take_types()?;
        match res {
            ModuleDefId::AdtId(AdtId::EnumId(it)) => Some(it),
            _ => None,
        }
    }

    fn scopes(&self) -> impl Iterator<Item = &Scope> {
        self.scopes.iter().rev()
    }

    fn resolve_module_path(
        &self,
        db: &dyn DefDatabase,
        path: &ModPath,
        shadow: BuiltinShadowMode,
    ) -> PerNs {
        let (item_map, module) = match self.module_scope() {
            Some(it) => it,
            None => return PerNs::none(),
        };
        let (module_res, segment_index) = item_map.resolve_path(db, module, path, shadow);
        if segment_index.is_some() {
            return PerNs::none();
        }
        module_res
    }

    pub fn resolve_module_path_in_items(&self, db: &dyn DefDatabase, path: &ModPath) -> PerNs {
        self.resolve_module_path(db, path, BuiltinShadowMode::Module)
    }

    pub fn resolve_module_path_in_trait_assoc_items(
        &self,
        db: &dyn DefDatabase,
        path: &ModPath,
    ) -> Option<PerNs> {
        let (item_map, module) = self.module_scope()?;
        let (module_res, idx) = item_map.resolve_path(db, module, path, BuiltinShadowMode::Module);
        match module_res.take_types()? {
            ModuleDefId::TraitId(it) => {
                let idx = idx?;
                let unresolved = &path.segments()[idx..];
                let assoc = match unresolved {
                    [it] => it,
                    _ => return None,
                };
                let &(_, assoc) = db.trait_data(it).items.iter().find(|(n, _)| n == assoc)?;
                Some(match assoc {
                    AssocItemId::FunctionId(it) => PerNs::values(it.into(), Visibility::Public),
                    AssocItemId::ConstId(it) => PerNs::values(it.into(), Visibility::Public),
                    AssocItemId::TypeAliasId(it) => PerNs::types(it.into(), Visibility::Public),
                })
            }
            _ => None,
        }
    }

    pub fn resolve_path_in_type_ns(
        &self,
        db: &dyn DefDatabase,
        path: &ModPath,
    ) -> Option<(TypeNs, Option<usize>)> {
        let first_name = path.segments().first()?;
        let skip_to_mod = path.kind != PathKind::Plain;
        for scope in self.scopes() {
            match scope {
                Scope::ExprScope(_) => continue,
                Scope::GenericParams { .. } | Scope::ImplDefScope(_) if skip_to_mod => continue,

                Scope::GenericParams { params, def } => {
                    if let Some(local_id) = params.find_type_by_name(first_name) {
                        let idx = if path.segments().len() == 1 { None } else { Some(1) };
                        return Some((
                            TypeNs::GenericParam(TypeParamId { local_id, parent: *def }),
                            idx,
                        ));
                    }
                }
                Scope::ImplDefScope(impl_) => {
                    if first_name == &name![Self] {
                        let idx = if path.segments().len() == 1 { None } else { Some(1) };
                        return Some((TypeNs::SelfType(*impl_), idx));
                    }
                }
                Scope::AdtScope(adt) => {
                    if first_name == &name![Self] {
                        let idx = if path.segments().len() == 1 { None } else { Some(1) };
                        return Some((TypeNs::AdtSelfType(*adt), idx));
                    }
                }
                Scope::ModuleScope(m) => {
                    if let Some(res) = m.resolve_path_in_type_ns(db, path) {
                        return Some(res);
                    }
                }
            }
        }
        None
    }

    pub fn resolve_path_in_type_ns_fully(
        &self,
        db: &dyn DefDatabase,
        path: &ModPath,
    ) -> Option<TypeNs> {
        let (res, unresolved) = self.resolve_path_in_type_ns(db, path)?;
        if unresolved.is_some() {
            return None;
        }
        Some(res)
    }

    pub fn resolve_visibility(
        &self,
        db: &dyn DefDatabase,
        visibility: &RawVisibility,
    ) -> Option<Visibility> {
        match visibility {
            RawVisibility::Module(_) => {
                let (item_map, module) = match self.module_scope() {
                    Some(it) => it,
                    None => return None,
                };
                item_map.resolve_visibility(db, module, visibility)
            }
            RawVisibility::Public => Some(Visibility::Public),
        }
    }

    pub fn resolve_path_in_value_ns(
        &self,
        db: &dyn DefDatabase,
        path: &ModPath,
    ) -> Option<ResolveValueResult> {
        let n_segments = path.segments().len();
        let tmp = name![self];
        let first_name = if path.is_self() { &tmp } else { path.segments().first()? };
        let skip_to_mod = path.kind != PathKind::Plain && !path.is_self();
        for scope in self.scopes() {
            match scope {
                Scope::AdtScope(_)
                | Scope::ExprScope(_)
                | Scope::GenericParams { .. }
                | Scope::ImplDefScope(_)
                    if skip_to_mod =>
                {
                    continue
                }

                Scope::ExprScope(scope) if n_segments <= 1 => {
                    let entry = scope
                        .expr_scopes
                        .entries(scope.scope_id)
                        .iter()
                        .find(|entry| entry.name() == first_name);

                    if let Some(e) = entry {
                        return Some(ResolveValueResult::ValueNs(ValueNs::LocalBinding(e.pat())));
                    }
                }
                Scope::ExprScope(_) => continue,

                Scope::GenericParams { params, def } if n_segments > 1 => {
                    if let Some(local_id) = params.find_type_by_name(first_name) {
                        let ty = TypeNs::GenericParam(TypeParamId { local_id, parent: *def });
                        return Some(ResolveValueResult::Partial(ty, 1));
                    }
                }
                Scope::GenericParams { params, def } if n_segments == 1 => {
                    if let Some(local_id) = params.find_const_by_name(first_name) {
                        let val = ValueNs::GenericParam(ConstParamId { local_id, parent: *def });
                        return Some(ResolveValueResult::ValueNs(val));
                    }
                }
                Scope::GenericParams { .. } => continue,

                Scope::ImplDefScope(impl_) => {
                    if first_name == &name![Self] {
                        if n_segments > 1 {
                            let ty = TypeNs::SelfType(*impl_);
                            return Some(ResolveValueResult::Partial(ty, 1));
                        } else {
                            return Some(ResolveValueResult::ValueNs(ValueNs::ImplSelf(*impl_)));
                        }
                    }
                }
                Scope::AdtScope(adt) => {
                    if n_segments == 1 {
                        // bare `Self` doesn't work in the value namespace in a struct/enum definition
                        continue;
                    }
                    if first_name == &name![Self] {
                        let ty = TypeNs::AdtSelfType(*adt);
                        return Some(ResolveValueResult::Partial(ty, 1));
                    }
                }

                Scope::ModuleScope(m) => {
                    if let Some(def) = m.resolve_path_in_value_ns(db, path) {
                        return Some(def);
                    }
                }
            }
        }

        None
    }

    pub fn resolve_path_in_value_ns_fully(
        &self,
        db: &dyn DefDatabase,
        path: &ModPath,
    ) -> Option<ValueNs> {
        match self.resolve_path_in_value_ns(db, path)? {
            ResolveValueResult::ValueNs(it) => Some(it),
            ResolveValueResult::Partial(..) => None,
        }
    }

    pub fn resolve_path_as_macro(
        &self,
        db: &dyn DefDatabase,
        path: &ModPath,
    ) -> Option<MacroDefId> {
        let (item_map, module) = self.module_scope()?;
        item_map.resolve_path(db, module, path, BuiltinShadowMode::Other).0.take_macros()
    }

    /// Returns a set of names available in the current scope.
    ///
    /// Note that this is a somewhat fuzzy concept -- internally, the compiler
    /// doesn't necessary follow a strict scoping discipline. Rathe, it just
    /// tells for each ident what it resolves to.
    ///
    /// A good example is something like `str::from_utf8`. From scopes point of
    /// view, this code is erroneous -- both `str` module and `str` type occupy
    /// the same type namespace.
    ///
    /// We don't try to model that super-correctly -- this functionality is
    /// primarily exposed for completions.
    ///
    /// Note that in Rust one name can be bound to several items:
    ///
    /// ```
    /// macro_rules! t { () => (()) }
    /// type t = t!();
    /// const t: t = t!()
    /// ```
    ///
    /// That's why we return a multimap.
    ///
    /// The shadowing is accounted for: in
    ///
    /// ```
    /// let x = 92;
    /// {
    ///     let x = 92;
    ///     $0
    /// }
    /// ```
    ///
    /// there will be only one entry for `x` in the result.
    ///
    /// The result is ordered *roughly* from the innermost scope to the
    /// outermost: when the name is introduced in two namespaces in two scopes,
    /// we use the position of the first scope.
    pub fn names_in_scope(&self, db: &dyn DefDatabase) -> IndexMap<Name, SmallVec<[ScopeDef; 1]>> {
        let mut res = ScopeNames::default();
        for scope in self.scopes() {
            scope.process_names(db, &mut res);
        }
        res.map
    }

    pub fn traits_in_scope(&self, db: &dyn DefDatabase) -> FxHashSet<TraitId> {
        let mut traits = FxHashSet::default();
        for scope in self.scopes() {
            match scope {
                Scope::ModuleScope(m) => {
                    if let Some(prelude) = m.def_map.prelude() {
                        let prelude_def_map = prelude.def_map(db);
                        traits.extend(prelude_def_map[prelude.local_id].scope.traits());
                    }
                    traits.extend(m.def_map[m.module_id].scope.traits());

                    // Add all traits that are in scope because of the containing DefMaps
                    m.def_map.with_ancestor_maps(db, m.module_id, &mut |def_map, module| {
                        if let Some(prelude) = def_map.prelude() {
                            let prelude_def_map = prelude.def_map(db);
                            traits.extend(prelude_def_map[prelude.local_id].scope.traits());
                        }
                        traits.extend(def_map[module].scope.traits());
                        None::<()>
                    });
                }
                &Scope::ImplDefScope(impl_) => {
                    if let Some(target_trait) = &db.impl_data(impl_).target_trait {
                        if let Some(TypeNs::TraitId(trait_)) =
                            self.resolve_path_in_type_ns_fully(db, target_trait.path.mod_path())
                        {
                            traits.insert(trait_);
                        }
                    }
                }
                _ => (),
            }
        }
        traits
    }

    fn module_scope(&self) -> Option<(&DefMap, LocalModuleId)> {
        self.scopes().find_map(|scope| match scope {
            Scope::ModuleScope(m) => Some((&*m.def_map, m.module_id)),

            _ => None,
        })
    }

    pub fn module(&self) -> Option<ModuleId> {
        let (def_map, local_id) = self.module_scope()?;
        Some(def_map.module_id(local_id))
    }

    pub fn krate(&self) -> Option<CrateId> {
        // FIXME: can this ever be `None`?
        self.module_scope().map(|t| t.0.krate())
    }

    pub fn where_predicates_in_scope(
        &self,
    ) -> impl Iterator<Item = &crate::generics::WherePredicate> {
        self.scopes()
            .filter_map(|scope| match scope {
                Scope::GenericParams { params, .. } => Some(params),
                _ => None,
            })
            .flat_map(|params| params.where_predicates.iter())
    }

    pub fn generic_def(&self) -> Option<GenericDefId> {
        self.scopes().find_map(|scope| match scope {
            Scope::GenericParams { def, .. } => Some(*def),
            _ => None,
        })
    }

    pub fn body_owner(&self) -> Option<DefWithBodyId> {
        self.scopes().find_map(|scope| match scope {
            Scope::ExprScope(it) => Some(it.owner),
            _ => None,
        })
    }
}

#[derive(Debug, PartialEq, Eq)]
pub enum ScopeDef {
    ModuleDef(ModuleDefId),
    MacroDef(MacroDefId),
    Unknown,
    ImplSelfType(ImplId),
    AdtSelfType(AdtId),
    GenericParam(GenericParamId),
    Local(PatId),
    Label(LabelId),
}

impl Scope {
    fn process_names(&self, db: &dyn DefDatabase, acc: &mut ScopeNames) {
        match self {
            Scope::ModuleScope(m) => {
                // FIXME: should we provide `self` here?
                // f(
                //     Name::self_param(),
                //     PerNs::types(Resolution::Def {
                //         def: m.module.into(),
                //     }),
                // );
                m.def_map[m.module_id].scope.entries().for_each(|(name, def)| {
                    acc.add_per_ns(name, def);
                });
                m.def_map[m.module_id].scope.legacy_macros().for_each(|(name, mac)| {
                    acc.add(name, ScopeDef::MacroDef(mac));
                });
                m.def_map.extern_prelude().for_each(|(name, &def)| {
                    acc.add(name, ScopeDef::ModuleDef(def));
                });
                BUILTIN_SCOPE.iter().for_each(|(name, &def)| {
                    acc.add_per_ns(name, def);
                });
                if let Some(prelude) = m.def_map.prelude() {
                    let prelude_def_map = prelude.def_map(db);
                    for (name, def) in prelude_def_map[prelude.local_id].scope.entries() {
                        acc.add_per_ns(name, def)
                    }
                }
            }
            Scope::GenericParams { params, def: parent } => {
                let parent = *parent;
                for (local_id, param) in params.types.iter() {
                    if let Some(name) = &param.name {
                        let id = TypeParamId { parent, local_id };
                        acc.add(name, ScopeDef::GenericParam(id.into()))
                    }
                }
                for (local_id, param) in params.consts.iter() {
                    let id = ConstParamId { parent, local_id };
                    acc.add(&param.name, ScopeDef::GenericParam(id.into()))
                }
                for (local_id, param) in params.lifetimes.iter() {
                    let id = LifetimeParamId { parent, local_id };
                    acc.add(&param.name, ScopeDef::GenericParam(id.into()))
                }
            }
            Scope::ImplDefScope(i) => {
                acc.add(&name![Self], ScopeDef::ImplSelfType(*i));
            }
            Scope::AdtScope(i) => {
                acc.add(&name![Self], ScopeDef::AdtSelfType(*i));
            }
            Scope::ExprScope(scope) => {
                if let Some((label, name)) = scope.expr_scopes.label(scope.scope_id) {
                    acc.add(&name, ScopeDef::Label(label))
                }
                scope.expr_scopes.entries(scope.scope_id).iter().for_each(|e| {
                    acc.add_local(e.name(), e.pat());
                });
            }
        }
    }
}

// needs arbitrary_self_types to be a method... or maybe move to the def?
pub fn resolver_for_expr(db: &dyn DefDatabase, owner: DefWithBodyId, expr_id: ExprId) -> Resolver {
    let scopes = db.expr_scopes(owner);
    resolver_for_scope(db, owner, scopes.scope_for(expr_id))
}

pub fn resolver_for_scope(
    db: &dyn DefDatabase,
    owner: DefWithBodyId,
    scope_id: Option<ScopeId>,
) -> Resolver {
    let mut r = owner.resolver(db);
    let scopes = db.expr_scopes(owner);
    let scope_chain = scopes.scope_chain(scope_id).collect::<Vec<_>>();
    for scope in scope_chain.into_iter().rev() {
        if let Some(block) = scopes.block(scope) {
            if let Some(def_map) = db.block_def_map(block) {
                let root = def_map.root();
                r = r.push_module_scope(def_map, root);
                // FIXME: This adds as many module scopes as there are blocks, but resolving in each
                // already traverses all parents, so this is O(n²). I think we could only store the
                // innermost module scope instead?
            }
        }

        r = r.push_expr_scope(owner, Arc::clone(&scopes), scope);
    }
    r
}

impl Resolver {
    fn push_scope(mut self, scope: Scope) -> Resolver {
        self.scopes.push(scope);
        self
    }

    fn push_generic_params_scope(self, db: &dyn DefDatabase, def: GenericDefId) -> Resolver {
        let params = db.generic_params(def);
        self.push_scope(Scope::GenericParams { def, params })
    }

    fn push_impl_def_scope(self, impl_def: ImplId) -> Resolver {
        self.push_scope(Scope::ImplDefScope(impl_def))
    }

    fn push_module_scope(self, def_map: Arc<DefMap>, module_id: LocalModuleId) -> Resolver {
        self.push_scope(Scope::ModuleScope(ModuleItemMap { def_map, module_id }))
    }

    fn push_expr_scope(
        self,
        owner: DefWithBodyId,
        expr_scopes: Arc<ExprScopes>,
        scope_id: ScopeId,
    ) -> Resolver {
        self.push_scope(Scope::ExprScope(ExprScope { owner, expr_scopes, scope_id }))
    }
}

impl ModuleItemMap {
    fn resolve_path_in_value_ns(
        &self,
        db: &dyn DefDatabase,
        path: &ModPath,
    ) -> Option<ResolveValueResult> {
        let (module_def, idx) =
            self.def_map.resolve_path_locally(db, self.module_id, path, BuiltinShadowMode::Other);
        match idx {
            None => {
                let value = to_value_ns(module_def)?;
                Some(ResolveValueResult::ValueNs(value))
            }
            Some(idx) => {
                let ty = match module_def.take_types()? {
                    ModuleDefId::AdtId(it) => TypeNs::AdtId(it),
                    ModuleDefId::TraitId(it) => TypeNs::TraitId(it),
                    ModuleDefId::TypeAliasId(it) => TypeNs::TypeAliasId(it),
                    ModuleDefId::BuiltinType(it) => TypeNs::BuiltinType(it),

                    ModuleDefId::ModuleId(_)
                    | ModuleDefId::FunctionId(_)
                    | ModuleDefId::EnumVariantId(_)
                    | ModuleDefId::ConstId(_)
                    | ModuleDefId::StaticId(_) => return None,
                };
                Some(ResolveValueResult::Partial(ty, idx))
            }
        }
    }

    fn resolve_path_in_type_ns(
        &self,
        db: &dyn DefDatabase,
        path: &ModPath,
    ) -> Option<(TypeNs, Option<usize>)> {
        let (module_def, idx) =
            self.def_map.resolve_path_locally(db, self.module_id, path, BuiltinShadowMode::Other);
        let res = to_type_ns(module_def)?;
        Some((res, idx))
    }
}

fn to_value_ns(per_ns: PerNs) -> Option<ValueNs> {
    let res = match per_ns.take_values()? {
        ModuleDefId::FunctionId(it) => ValueNs::FunctionId(it),
        ModuleDefId::AdtId(AdtId::StructId(it)) => ValueNs::StructId(it),
        ModuleDefId::EnumVariantId(it) => ValueNs::EnumVariantId(it),
        ModuleDefId::ConstId(it) => ValueNs::ConstId(it),
        ModuleDefId::StaticId(it) => ValueNs::StaticId(it),

        ModuleDefId::AdtId(AdtId::EnumId(_) | AdtId::UnionId(_))
        | ModuleDefId::TraitId(_)
        | ModuleDefId::TypeAliasId(_)
        | ModuleDefId::BuiltinType(_)
        | ModuleDefId::ModuleId(_) => return None,
    };
    Some(res)
}

fn to_type_ns(per_ns: PerNs) -> Option<TypeNs> {
    let res = match per_ns.take_types()? {
        ModuleDefId::AdtId(it) => TypeNs::AdtId(it),
        ModuleDefId::EnumVariantId(it) => TypeNs::EnumVariantId(it),

        ModuleDefId::TypeAliasId(it) => TypeNs::TypeAliasId(it),
        ModuleDefId::BuiltinType(it) => TypeNs::BuiltinType(it),

        ModuleDefId::TraitId(it) => TypeNs::TraitId(it),

        ModuleDefId::FunctionId(_)
        | ModuleDefId::ConstId(_)
        | ModuleDefId::StaticId(_)
        | ModuleDefId::ModuleId(_) => return None,
    };
    Some(res)
}

#[derive(Default)]
struct ScopeNames {
    map: IndexMap<Name, SmallVec<[ScopeDef; 1]>>,
}

impl ScopeNames {
    fn add(&mut self, name: &Name, def: ScopeDef) {
        let set = self.map.entry(name.clone()).or_default();
        if !set.contains(&def) {
            set.push(def)
        }
    }
    fn add_per_ns(&mut self, name: &Name, def: PerNs) {
        if let Some(ty) = &def.types {
            self.add(name, ScopeDef::ModuleDef(ty.0))
        }
        if let Some(val) = &def.values {
            self.add(name, ScopeDef::ModuleDef(val.0))
        }
        if let Some(mac) = &def.macros {
            self.add(name, ScopeDef::MacroDef(mac.0))
        }
        if def.is_none() {
            self.add(name, ScopeDef::Unknown)
        }
    }
    fn add_local(&mut self, name: &Name, pat: PatId) {
        let set = self.map.entry(name.clone()).or_default();
        // XXX: hack, account for local (and only local) shadowing.
        //
        // This should be somewhat more principled and take namespaces into
        // accounts, but, alas, scoping rules are a hoax. `str` type and `str`
        // module can be both available in the same scope.
        if set.iter().any(|it| matches!(it, &ScopeDef::Local(_))) {
            cov_mark::hit!(shadowing_shows_single_completion);
            return;
        }
        set.push(ScopeDef::Local(pat))
    }
}

pub trait HasResolver: Copy {
    /// Builds a resolver for type references inside this def.
    fn resolver(self, db: &dyn DefDatabase) -> Resolver;
}

impl HasResolver for ModuleId {
    fn resolver(self, db: &dyn DefDatabase) -> Resolver {
        let mut def_map = self.def_map(db);
        let mut modules = vec![(def_map.clone(), self.local_id)];
        while let Some(parent) = def_map.parent() {
            def_map = parent.def_map(db);
            modules.push((def_map.clone(), parent.local_id));
        }
        let mut resolver = Resolver::default();
        for (def_map, module) in modules.into_iter().rev() {
            resolver = resolver.push_module_scope(def_map, module);
        }
        resolver
    }
}

impl HasResolver for TraitId {
    fn resolver(self, db: &dyn DefDatabase) -> Resolver {
        self.lookup(db).container.resolver(db).push_generic_params_scope(db, self.into())
    }
}

impl<T: Into<AdtId> + Copy> HasResolver for T {
    fn resolver(self, db: &dyn DefDatabase) -> Resolver {
        let def = self.into();
        def.module(db)
            .resolver(db)
            .push_generic_params_scope(db, def.into())
            .push_scope(Scope::AdtScope(def))
    }
}

impl HasResolver for FunctionId {
    fn resolver(self, db: &dyn DefDatabase) -> Resolver {
        self.lookup(db).container.resolver(db).push_generic_params_scope(db, self.into())
    }
}

impl HasResolver for ConstId {
    fn resolver(self, db: &dyn DefDatabase) -> Resolver {
        self.lookup(db).container.resolver(db)
    }
}

impl HasResolver for StaticId {
    fn resolver(self, db: &dyn DefDatabase) -> Resolver {
        self.lookup(db).container.resolver(db)
    }
}

impl HasResolver for TypeAliasId {
    fn resolver(self, db: &dyn DefDatabase) -> Resolver {
        self.lookup(db).container.resolver(db).push_generic_params_scope(db, self.into())
    }
}

impl HasResolver for ImplId {
    fn resolver(self, db: &dyn DefDatabase) -> Resolver {
        self.lookup(db)
            .container
            .resolver(db)
            .push_generic_params_scope(db, self.into())
            .push_impl_def_scope(self)
    }
}

impl HasResolver for DefWithBodyId {
    fn resolver(self, db: &dyn DefDatabase) -> Resolver {
        match self {
            DefWithBodyId::ConstId(c) => c.resolver(db),
            DefWithBodyId::FunctionId(f) => f.resolver(db),
            DefWithBodyId::StaticId(s) => s.resolver(db),
        }
    }
}

impl HasResolver for AssocContainerId {
    fn resolver(self, db: &dyn DefDatabase) -> Resolver {
        match self {
            AssocContainerId::ModuleId(it) => it.resolver(db),
            AssocContainerId::TraitId(it) => it.resolver(db),
            AssocContainerId::ImplId(it) => it.resolver(db),
        }
    }
}

impl HasResolver for GenericDefId {
    fn resolver(self, db: &dyn DefDatabase) -> Resolver {
        match self {
            GenericDefId::FunctionId(inner) => inner.resolver(db),
            GenericDefId::AdtId(adt) => adt.resolver(db),
            GenericDefId::TraitId(inner) => inner.resolver(db),
            GenericDefId::TypeAliasId(inner) => inner.resolver(db),
            GenericDefId::ImplId(inner) => inner.resolver(db),
            GenericDefId::EnumVariantId(inner) => inner.parent.resolver(db),
            GenericDefId::ConstId(inner) => inner.resolver(db),
        }
    }
}

impl HasResolver for VariantId {
    fn resolver(self, db: &dyn DefDatabase) -> Resolver {
        match self {
            VariantId::EnumVariantId(it) => it.parent.resolver(db),
            VariantId::StructId(it) => it.resolver(db),
            VariantId::UnionId(it) => it.resolver(db),
        }
    }
}