1 files changed, 705 insertions, 0 deletions

diff --git a/crates/hir_def/src/item_tree/lower.rs b/crates/hir_def/src/item_tree/lower.rs
new file mode 100644
index 000000000..391ab5d39
--- /dev/null
+++ b/crates/hir_def/src/item_tree/lower.rs
@@ -0,0 +1,705 @@
+//! AST -> `ItemTree` lowering code.
+
+use std::{collections::hash_map::Entry, mem, sync::Arc};
+
+use arena::map::ArenaMap;
+use hir_expand::{ast_id_map::AstIdMap, hygiene::Hygiene, HirFileId};
+use smallvec::SmallVec;
+use syntax::{
+    ast::{self, ModuleItemOwner},
+    SyntaxNode,
+};
+
+use crate::{
+    attr::Attrs,
+    generics::{GenericParams, TypeParamData, TypeParamProvenance},
+};
+
+use super::*;
+
+fn id<N: ItemTreeNode>(index: Idx<N>) -> FileItemTreeId<N> {
+    FileItemTreeId { index, _p: PhantomData }
+}
+
+struct ModItems(SmallVec<[ModItem; 1]>);
+
+impl<T> From<T> for ModItems
+where
+    T: Into<ModItem>,
+{
+    fn from(t: T) -> Self {
+        ModItems(SmallVec::from_buf([t.into(); 1]))
+    }
+}
+
+pub(super) struct Ctx {
+    tree: ItemTree,
+    hygiene: Hygiene,
+    file: HirFileId,
+    source_ast_id_map: Arc<AstIdMap>,
+    body_ctx: crate::body::LowerCtx,
+    inner_items: Vec<ModItem>,
+    forced_visibility: Option<RawVisibilityId>,
+}
+
+impl Ctx {
+    pub(super) fn new(db: &dyn DefDatabase, hygiene: Hygiene, file: HirFileId) -> Self {
+        Self {
+            tree: ItemTree::empty(),
+            hygiene,
+            file,
+            source_ast_id_map: db.ast_id_map(file),
+            body_ctx: crate::body::LowerCtx::new(db, file),
+            inner_items: Vec::new(),
+            forced_visibility: None,
+        }
+    }
+
+    pub(super) fn lower_module_items(mut self, item_owner: &dyn ModuleItemOwner) -> ItemTree {
+        self.tree.top_level = item_owner
+            .items()
+            .flat_map(|item| self.lower_mod_item(&item, false))
+            .flat_map(|items| items.0)
+            .collect();
+        self.tree
+    }
+
+    pub(super) fn lower_inner_items(mut self, within: &SyntaxNode) -> ItemTree {
+        self.collect_inner_items(within);
+        self.tree
+    }
+
+    fn data(&mut self) -> &mut ItemTreeData {
+        self.tree.data_mut()
+    }
+
+    fn lower_mod_item(&mut self, item: &ast::Item, inner: bool) -> Option<ModItems> {
+        assert!(inner || self.inner_items.is_empty());
+
+        // Collect inner items for 1-to-1-lowered items.
+        match item {
+            ast::Item::Struct(_)
+            | ast::Item::Union(_)
+            | ast::Item::Enum(_)
+            | ast::Item::Fn(_)
+            | ast::Item::TypeAlias(_)
+            | ast::Item::Const(_)
+            | ast::Item::Static(_)
+            | ast::Item::MacroCall(_) => {
+                // Skip this if we're already collecting inner items. We'll descend into all nodes
+                // already.
+                if !inner {
+                    self.collect_inner_items(item.syntax());
+                }
+            }
+
+            // These are handled in their respective `lower_X` method (since we can't just blindly
+            // walk them).
+            ast::Item::Trait(_) | ast::Item::Impl(_) | ast::Item::ExternBlock(_) => {}
+
+            // These don't have inner items.
+            ast::Item::Module(_) | ast::Item::ExternCrate(_) | ast::Item::Use(_) => {}
+        };
+
+        let attrs = Attrs::new(item, &self.hygiene);
+        let items = match item {
+            ast::Item::Struct(ast) => self.lower_struct(ast).map(Into::into),
+            ast::Item::Union(ast) => self.lower_union(ast).map(Into::into),
+            ast::Item::Enum(ast) => self.lower_enum(ast).map(Into::into),
+            ast::Item::Fn(ast) => self.lower_function(ast).map(Into::into),
+            ast::Item::TypeAlias(ast) => self.lower_type_alias(ast).map(Into::into),
+            ast::Item::Static(ast) => self.lower_static(ast).map(Into::into),
+            ast::Item::Const(ast) => Some(self.lower_const(ast).into()),
+            ast::Item::Module(ast) => self.lower_module(ast).map(Into::into),
+            ast::Item::Trait(ast) => self.lower_trait(ast).map(Into::into),
+            ast::Item::Impl(ast) => self.lower_impl(ast).map(Into::into),
+            ast::Item::Use(ast) => Some(ModItems(
+                self.lower_use(ast).into_iter().map(Into::into).collect::<SmallVec<_>>(),
+            )),
+            ast::Item::ExternCrate(ast) => self.lower_extern_crate(ast).map(Into::into),
+            ast::Item::MacroCall(ast) => self.lower_macro_call(ast).map(Into::into),
+            ast::Item::ExternBlock(ast) => {
+                Some(ModItems(self.lower_extern_block(ast).into_iter().collect::<SmallVec<_>>()))
+            }
+        };
+
+        if !attrs.is_empty() {
+            for item in items.iter().flat_map(|items| &items.0) {
+                self.add_attrs((*item).into(), attrs.clone());
+            }
+        }
+
+        items
+    }
+
+    fn add_attrs(&mut self, item: AttrOwner, attrs: Attrs) {
+        match self.tree.attrs.entry(item) {
+            Entry::Occupied(mut entry) => {
+                *entry.get_mut() = entry.get().merge(attrs);
+            }
+            Entry::Vacant(entry) => {
+                entry.insert(attrs);
+            }
+        }
+    }
+
+    fn collect_inner_items(&mut self, container: &SyntaxNode) {
+        let forced_vis = self.forced_visibility.take();
+        let mut inner_items = mem::take(&mut self.tree.inner_items);
+        inner_items.extend(container.descendants().skip(1).filter_map(ast::Item::cast).filter_map(
+            |item| {
+                let ast_id = self.source_ast_id_map.ast_id(&item);
+                Some((ast_id, self.lower_mod_item(&item, true)?.0))
+            },
+        ));
+        self.tree.inner_items = inner_items;
+        self.forced_visibility = forced_vis;
+    }
+
+    fn lower_assoc_item(&mut self, item: &ast::AssocItem) -> Option<AssocItem> {
+        match item {
+            ast::AssocItem::Fn(ast) => self.lower_function(ast).map(Into::into),
+            ast::AssocItem::TypeAlias(ast) => self.lower_type_alias(ast).map(Into::into),
+            ast::AssocItem::Const(ast) => Some(self.lower_const(ast).into()),
+            ast::AssocItem::MacroCall(ast) => self.lower_macro_call(ast).map(Into::into),
+        }
+    }
+
+    fn lower_struct(&mut self, strukt: &ast::Struct) -> Option<FileItemTreeId<Struct>> {
+        let visibility = self.lower_visibility(strukt);
+        let name = strukt.name()?.as_name();
+        let generic_params = self.lower_generic_params(GenericsOwner::Struct, strukt);
+        let fields = self.lower_fields(&strukt.kind());
+        let ast_id = self.source_ast_id_map.ast_id(strukt);
+        let kind = match strukt.kind() {
+            ast::StructKind::Record(_) => StructDefKind::Record,
+            ast::StructKind::Tuple(_) => StructDefKind::Tuple,
+            ast::StructKind::Unit => StructDefKind::Unit,
+        };
+        let res = Struct { name, visibility, generic_params, fields, ast_id, kind };
+        Some(id(self.data().structs.alloc(res)))
+    }
+
+    fn lower_fields(&mut self, strukt_kind: &ast::StructKind) -> Fields {
+        match strukt_kind {
+            ast::StructKind::Record(it) => {
+                let range = self.lower_record_fields(it);
+                Fields::Record(range)
+            }
+            ast::StructKind::Tuple(it) => {
+                let range = self.lower_tuple_fields(it);
+                Fields::Tuple(range)
+            }
+            ast::StructKind::Unit => Fields::Unit,
+        }
+    }
+
+    fn lower_record_fields(&mut self, fields: &ast::RecordFieldList) -> IdRange<Field> {
+        let start = self.next_field_idx();
+        for field in fields.fields() {
+            if let Some(data) = self.lower_record_field(&field) {
+                let idx = self.data().fields.alloc(data);
+                self.add_attrs(idx.into(), Attrs::new(&field, &self.hygiene));
+            }
+        }
+        let end = self.next_field_idx();
+        IdRange::new(start..end)
+    }
+
+    fn lower_record_field(&mut self, field: &ast::RecordField) -> Option<Field> {
+        let name = field.name()?.as_name();
+        let visibility = self.lower_visibility(field);
+        let type_ref = self.lower_type_ref_opt(field.ty());
+        let res = Field { name, type_ref, visibility };
+        Some(res)
+    }
+
+    fn lower_tuple_fields(&mut self, fields: &ast::TupleFieldList) -> IdRange<Field> {
+        let start = self.next_field_idx();
+        for (i, field) in fields.fields().enumerate() {
+            let data = self.lower_tuple_field(i, &field);
+            let idx = self.data().fields.alloc(data);
+            self.add_attrs(idx.into(), Attrs::new(&field, &self.hygiene));
+        }
+        let end = self.next_field_idx();
+        IdRange::new(start..end)
+    }
+
+    fn lower_tuple_field(&mut self, idx: usize, field: &ast::TupleField) -> Field {
+        let name = Name::new_tuple_field(idx);
+        let visibility = self.lower_visibility(field);
+        let type_ref = self.lower_type_ref_opt(field.ty());
+        let res = Field { name, type_ref, visibility };
+        res
+    }
+
+    fn lower_union(&mut self, union: &ast::Union) -> Option<FileItemTreeId<Union>> {
+        let visibility = self.lower_visibility(union);
+        let name = union.name()?.as_name();
+        let generic_params = self.lower_generic_params(GenericsOwner::Union, union);
+        let fields = match union.record_field_list() {
+            Some(record_field_list) => self.lower_fields(&StructKind::Record(record_field_list)),
+            None => Fields::Record(IdRange::new(self.next_field_idx()..self.next_field_idx())),
+        };
+        let ast_id = self.source_ast_id_map.ast_id(union);
+        let res = Union { name, visibility, generic_params, fields, ast_id };
+        Some(id(self.data().unions.alloc(res)))
+    }
+
+    fn lower_enum(&mut self, enum_: &ast::Enum) -> Option<FileItemTreeId<Enum>> {
+        let visibility = self.lower_visibility(enum_);
+        let name = enum_.name()?.as_name();
+        let generic_params = self.lower_generic_params(GenericsOwner::Enum, enum_);
+        let variants = match &enum_.variant_list() {
+            Some(variant_list) => self.lower_variants(variant_list),
+            None => IdRange::new(self.next_variant_idx()..self.next_variant_idx()),
+        };
+        let ast_id = self.source_ast_id_map.ast_id(enum_);
+        let res = Enum { name, visibility, generic_params, variants, ast_id };
+        Some(id(self.data().enums.alloc(res)))
+    }
+
+    fn lower_variants(&mut self, variants: &ast::VariantList) -> IdRange<Variant> {
+        let start = self.next_variant_idx();
+        for variant in variants.variants() {
+            if let Some(data) = self.lower_variant(&variant) {
+                let idx = self.data().variants.alloc(data);
+                self.add_attrs(idx.into(), Attrs::new(&variant, &self.hygiene));
+            }
+        }
+        let end = self.next_variant_idx();
+        IdRange::new(start..end)
+    }
+
+    fn lower_variant(&mut self, variant: &ast::Variant) -> Option<Variant> {
+        let name = variant.name()?.as_name();
+        let fields = self.lower_fields(&variant.kind());
+        let res = Variant { name, fields };
+        Some(res)
+    }
+
+    fn lower_function(&mut self, func: &ast::Fn) -> Option<FileItemTreeId<Function>> {
+        let visibility = self.lower_visibility(func);
+        let name = func.name()?.as_name();
+
+        let mut params = Vec::new();
+        let mut has_self_param = false;
+        if let Some(param_list) = func.param_list() {
+            if let Some(self_param) = param_list.self_param() {
+                let self_type = match self_param.ty() {
+                    Some(type_ref) => TypeRef::from_ast(&self.body_ctx, type_ref),
+                    None => {
+                        let self_type = TypeRef::Path(name![Self].into());
+                        match self_param.kind() {
+                            ast::SelfParamKind::Owned => self_type,
+                            ast::SelfParamKind::Ref => {
+                                TypeRef::Reference(Box::new(self_type), Mutability::Shared)
+                            }
+                            ast::SelfParamKind::MutRef => {
+                                TypeRef::Reference(Box::new(self_type), Mutability::Mut)
+                            }
+                        }
+                    }
+                };
+                params.push(self_type);
+                has_self_param = true;
+            }
+            for param in param_list.params() {
+                let type_ref = TypeRef::from_ast_opt(&self.body_ctx, param.ty());
+                params.push(type_ref);
+            }
+        }
+
+        let mut is_varargs = false;
+        if let Some(params) = func.param_list() {
+            if let Some(last) = params.params().last() {
+                is_varargs = last.dotdotdot_token().is_some();
+            }
+        }
+
+        let ret_type = match func.ret_type().and_then(|rt| rt.ty()) {
+            Some(type_ref) => TypeRef::from_ast(&self.body_ctx, type_ref),
+            _ => TypeRef::unit(),
+        };
+
+        let ret_type = if func.async_token().is_some() {
+            let future_impl = desugar_future_path(ret_type);
+            let ty_bound = TypeBound::Path(future_impl);
+            TypeRef::ImplTrait(vec![ty_bound])
+        } else {
+            ret_type
+        };
+
+        let ast_id = self.source_ast_id_map.ast_id(func);
+        let mut res = Function {
+            name,
+            visibility,
+            generic_params: GenericParamsId::EMPTY,
+            has_self_param,
+            is_unsafe: func.unsafe_token().is_some(),
+            params: params.into_boxed_slice(),
+            is_varargs,
+            ret_type,
+            ast_id,
+        };
+        res.generic_params = self.lower_generic_params(GenericsOwner::Function(&res), func);
+
+        Some(id(self.data().functions.alloc(res)))
+    }
+
+    fn lower_type_alias(
+        &mut self,
+        type_alias: &ast::TypeAlias,
+    ) -> Option<FileItemTreeId<TypeAlias>> {
+        let name = type_alias.name()?.as_name();
+        let type_ref = type_alias.ty().map(|it| self.lower_type_ref(&it));
+        let visibility = self.lower_visibility(type_alias);
+        let bounds = self.lower_type_bounds(type_alias);
+        let generic_params = self.lower_generic_params(GenericsOwner::TypeAlias, type_alias);
+        let ast_id = self.source_ast_id_map.ast_id(type_alias);
+        let res = TypeAlias {
+            name,
+            visibility,
+            bounds: bounds.into_boxed_slice(),
+            generic_params,
+            type_ref,
+            ast_id,
+        };
+        Some(id(self.data().type_aliases.alloc(res)))
+    }
+
+    fn lower_static(&mut self, static_: &ast::Static) -> Option<FileItemTreeId<Static>> {
+        let name = static_.name()?.as_name();
+        let type_ref = self.lower_type_ref_opt(static_.ty());
+        let visibility = self.lower_visibility(static_);
+        let mutable = static_.mut_token().is_some();
+        let ast_id = self.source_ast_id_map.ast_id(static_);
+        let res = Static { name, visibility, mutable, type_ref, ast_id };
+        Some(id(self.data().statics.alloc(res)))
+    }
+
+    fn lower_const(&mut self, konst: &ast::Const) -> FileItemTreeId<Const> {
+        let name = konst.name().map(|it| it.as_name());
+        let type_ref = self.lower_type_ref_opt(konst.ty());
+        let visibility = self.lower_visibility(konst);
+        let ast_id = self.source_ast_id_map.ast_id(konst);
+        let res = Const { name, visibility, type_ref, ast_id };
+        id(self.data().consts.alloc(res))
+    }
+
+    fn lower_module(&mut self, module: &ast::Module) -> Option<FileItemTreeId<Mod>> {
+        let name = module.name()?.as_name();
+        let visibility = self.lower_visibility(module);
+        let kind = if module.semicolon_token().is_some() {
+            ModKind::Outline {}
+        } else {
+            ModKind::Inline {
+                items: module
+                    .item_list()
+                    .map(|list| {
+                        list.items()
+                            .flat_map(|item| self.lower_mod_item(&item, false))
+                            .flat_map(|items| items.0)
+                            .collect()
+                    })
+                    .unwrap_or_else(|| {
+                        mark::hit!(name_res_works_for_broken_modules);
+                        Box::new([]) as Box<[_]>
+                    }),
+            }
+        };
+        let ast_id = self.source_ast_id_map.ast_id(module);
+        let res = Mod { name, visibility, kind, ast_id };
+        Some(id(self.data().mods.alloc(res)))
+    }
+
+    fn lower_trait(&mut self, trait_def: &ast::Trait) -> Option<FileItemTreeId<Trait>> {
+        let name = trait_def.name()?.as_name();
+        let visibility = self.lower_visibility(trait_def);
+        let generic_params =
+            self.lower_generic_params_and_inner_items(GenericsOwner::Trait(trait_def), trait_def);
+        let auto = trait_def.auto_token().is_some();
+        let items = trait_def.assoc_item_list().map(|list| {
+            self.with_inherited_visibility(visibility, |this| {
+                list.assoc_items()
+                    .filter_map(|item| {
+                        let attrs = Attrs::new(&item, &this.hygiene);
+                        this.collect_inner_items(item.syntax());
+                        this.lower_assoc_item(&item).map(|item| {
+                            this.add_attrs(ModItem::from(item).into(), attrs);
+                            item
+                        })
+                    })
+                    .collect()
+            })
+        });
+        let ast_id = self.source_ast_id_map.ast_id(trait_def);
+        let res = Trait {
+            name,
+            visibility,
+            generic_params,
+            auto,
+            items: items.unwrap_or_default(),
+            ast_id,
+        };
+        Some(id(self.data().traits.alloc(res)))
+    }
+
+    fn lower_impl(&mut self, impl_def: &ast::Impl) -> Option<FileItemTreeId<Impl>> {
+        let generic_params =
+            self.lower_generic_params_and_inner_items(GenericsOwner::Impl, impl_def);
+        let target_trait = impl_def.trait_().map(|tr| self.lower_type_ref(&tr));
+        let target_type = self.lower_type_ref(&impl_def.self_ty()?);
+        let is_negative = impl_def.excl_token().is_some();
+
+        // We cannot use `assoc_items()` here as that does not include macro calls.
+        let items = impl_def
+            .assoc_item_list()
+            .into_iter()
+            .flat_map(|it| it.assoc_items())
+            .filter_map(|item| {
+                self.collect_inner_items(item.syntax());
+                let assoc = self.lower_assoc_item(&item)?;
+                let attrs = Attrs::new(&item, &self.hygiene);
+                self.add_attrs(ModItem::from(assoc).into(), attrs);
+                Some(assoc)
+            })
+            .collect();
+        let ast_id = self.source_ast_id_map.ast_id(impl_def);
+        let res = Impl { generic_params, target_trait, target_type, is_negative, items, ast_id };
+        Some(id(self.data().impls.alloc(res)))
+    }
+
+    fn lower_use(&mut self, use_item: &ast::Use) -> Vec<FileItemTreeId<Import>> {
+        // FIXME: cfg_attr
+        let is_prelude = use_item.has_atom_attr("prelude_import");
+        let visibility = self.lower_visibility(use_item);
+        let ast_id = self.source_ast_id_map.ast_id(use_item);
+
+        // Every use item can expand to many `Import`s.
+        let mut imports = Vec::new();
+        let tree = self.tree.data_mut();
+        ModPath::expand_use_item(
+            InFile::new(self.file, use_item.clone()),
+            &self.hygiene,
+            |path, _tree, is_glob, alias| {
+                imports.push(id(tree.imports.alloc(Import {
+                    path,
+                    alias,
+                    visibility,
+                    is_glob,
+                    is_prelude,
+                    ast_id,
+                })));
+            },
+        );
+
+        imports
+    }
+
+    fn lower_extern_crate(
+        &mut self,
+        extern_crate: &ast::ExternCrate,
+    ) -> Option<FileItemTreeId<ExternCrate>> {
+        let path = ModPath::from_name_ref(&extern_crate.name_ref()?);
+        let alias = extern_crate.rename().map(|a| {
+            a.name().map(|it| it.as_name()).map_or(ImportAlias::Underscore, ImportAlias::Alias)
+        });
+        let visibility = self.lower_visibility(extern_crate);
+        let ast_id = self.source_ast_id_map.ast_id(extern_crate);
+        // FIXME: cfg_attr
+        let is_macro_use = extern_crate.has_atom_attr("macro_use");
+
+        let res = ExternCrate { path, alias, visibility, is_macro_use, ast_id };
+        Some(id(self.data().extern_crates.alloc(res)))
+    }
+
+    fn lower_macro_call(&mut self, m: &ast::MacroCall) -> Option<FileItemTreeId<MacroCall>> {
+        let name = m.name().map(|it| it.as_name());
+        let attrs = Attrs::new(m, &self.hygiene);
+        let path = ModPath::from_src(m.path()?, &self.hygiene)?;
+
+        let ast_id = self.source_ast_id_map.ast_id(m);
+
+        // FIXME: cfg_attr
+        let export_attr = attrs.by_key("macro_export");
+
+        let is_export = export_attr.exists();
+        let is_local_inner = if is_export {
+            export_attr.tt_values().map(|it| &it.token_trees).flatten().any(|it| match it {
+                tt::TokenTree::Leaf(tt::Leaf::Ident(ident)) => {
+                    ident.text.contains("local_inner_macros")
+                }
+                _ => false,
+            })
+        } else {
+            false
+        };
+
+        let is_builtin = attrs.by_key("rustc_builtin_macro").exists();
+        let res = MacroCall { name, path, is_export, is_builtin, is_local_inner, ast_id };
+        Some(id(self.data().macro_calls.alloc(res)))
+    }
+
+    fn lower_extern_block(&mut self, block: &ast::ExternBlock) -> Vec<ModItem> {
+        block.extern_item_list().map_or(Vec::new(), |list| {
+            list.extern_items()
+                .filter_map(|item| {
+                    self.collect_inner_items(item.syntax());
+                    let attrs = Attrs::new(&item, &self.hygiene);
+                    let id: ModItem = match item {
+                        ast::ExternItem::Fn(ast) => {
+                            let func = self.lower_function(&ast)?;
+                            self.data().functions[func.index].is_unsafe = true;
+                            func.into()
+                        }
+                        ast::ExternItem::Static(ast) => {
+                            let statik = self.lower_static(&ast)?;
+                            statik.into()
+                        }
+                        ast::ExternItem::MacroCall(_) => return None,
+                    };
+                    self.add_attrs(id.into(), attrs);
+                    Some(id)
+                })
+                .collect()
+        })
+    }
+
+    /// Lowers generics defined on `node` and collects inner items defined within.
+    fn lower_generic_params_and_inner_items(
+        &mut self,
+        owner: GenericsOwner<'_>,
+        node: &impl ast::GenericParamsOwner,
+    ) -> GenericParamsId {
+        // Generics are part of item headers and may contain inner items we need to collect.
+        if let Some(params) = node.generic_param_list() {
+            self.collect_inner_items(params.syntax());
+        }
+        if let Some(clause) = node.where_clause() {
+            self.collect_inner_items(clause.syntax());
+        }
+
+        self.lower_generic_params(owner, node)
+    }
+
+    fn lower_generic_params(
+        &mut self,
+        owner: GenericsOwner<'_>,
+        node: &impl ast::GenericParamsOwner,
+    ) -> GenericParamsId {
+        let mut sm = &mut ArenaMap::default();
+        let mut generics = GenericParams::default();
+        match owner {
+            GenericsOwner::Function(func) => {
+                generics.fill(&self.body_ctx, sm, node);
+                // lower `impl Trait` in arguments
+                for param in &*func.params {
+                    generics.fill_implicit_impl_trait_args(param);
+                }
+            }
+            GenericsOwner::Struct
+            | GenericsOwner::Enum
+            | GenericsOwner::Union
+            | GenericsOwner::TypeAlias => {
+                generics.fill(&self.body_ctx, sm, node);
+            }
+            GenericsOwner::Trait(trait_def) => {
+                // traits get the Self type as an implicit first type parameter
+                let self_param_id = generics.types.alloc(TypeParamData {
+                    name: Some(name![Self]),
+                    default: None,
+                    provenance: TypeParamProvenance::TraitSelf,
+                });
+                sm.insert(self_param_id, Either::Left(trait_def.clone()));
+                // add super traits as bounds on Self
+                // i.e., trait Foo: Bar is equivalent to trait Foo where Self: Bar
+                let self_param = TypeRef::Path(name![Self].into());
+                generics.fill_bounds(&self.body_ctx, trait_def, self_param);
+
+                generics.fill(&self.body_ctx, &mut sm, node);
+            }
+            GenericsOwner::Impl => {
+                // Note that we don't add `Self` here: in `impl`s, `Self` is not a
+                // type-parameter, but rather is a type-alias for impl's target
+                // type, so this is handled by the resolver.
+                generics.fill(&self.body_ctx, &mut sm, node);
+            }
+        }
+
+        self.data().generics.alloc(generics)
+    }
+
+    fn lower_type_bounds(&mut self, node: &impl ast::TypeBoundsOwner) -> Vec<TypeBound> {
+        match node.type_bound_list() {
+            Some(bound_list) => {
+                bound_list.bounds().map(|it| TypeBound::from_ast(&self.body_ctx, it)).collect()
+            }
+            None => Vec::new(),
+        }
+    }
+
+    fn lower_visibility(&mut self, item: &impl ast::VisibilityOwner) -> RawVisibilityId {
+        let vis = match self.forced_visibility {
+            Some(vis) => return vis,
+            None => RawVisibility::from_ast_with_hygiene(item.visibility(), &self.hygiene),
+        };
+
+        self.data().vis.alloc(vis)
+    }
+
+    fn lower_type_ref(&self, type_ref: &ast::Type) -> TypeRef {
+        TypeRef::from_ast(&self.body_ctx, type_ref.clone())
+    }
+    fn lower_type_ref_opt(&self, type_ref: Option<ast::Type>) -> TypeRef {
+        type_ref.map(|ty| self.lower_type_ref(&ty)).unwrap_or(TypeRef::Error)
+    }
+
+    /// Forces the visibility `vis` to be used for all items lowered during execution of `f`.
+    fn with_inherited_visibility<R>(
+        &mut self,
+        vis: RawVisibilityId,
+        f: impl FnOnce(&mut Self) -> R,
+    ) -> R {
+        let old = mem::replace(&mut self.forced_visibility, Some(vis));
+        let res = f(self);
+        self.forced_visibility = old;
+        res
+    }
+
+    fn next_field_idx(&self) -> Idx<Field> {
+        Idx::from_raw(RawId::from(
+            self.tree.data.as_ref().map_or(0, |data| data.fields.len() as u32),
+        ))
+    }
+    fn next_variant_idx(&self) -> Idx<Variant> {
+        Idx::from_raw(RawId::from(
+            self.tree.data.as_ref().map_or(0, |data| data.variants.len() as u32),
+        ))
+    }
+}
+
+fn desugar_future_path(orig: TypeRef) -> Path {
+    let path = path![core::future::Future];
+    let mut generic_args: Vec<_> = std::iter::repeat(None).take(path.segments.len() - 1).collect();
+    let mut last = GenericArgs::empty();
+    let binding =
+        AssociatedTypeBinding { name: name![Output], type_ref: Some(orig), bounds: Vec::new() };
+    last.bindings.push(binding);
+    generic_args.push(Some(Arc::new(last)));
+
+    Path::from_known_path(path, generic_args)
+}
+
+enum GenericsOwner<'a> {
+    /// We need access to the partially-lowered `Function` for lowering `impl Trait` in argument
+    /// position.
+    Function(&'a Function),
+    Struct,
+    Enum,
+    Union,
+    /// The `TraitDef` is needed to fill the source map for the implicit `Self` parameter.
+    Trait(&'a ast::Trait),
+    TypeAlias,
+    Impl,
+}