23 files changed, 614 insertions, 271 deletions

diff --git a/crates/hir_def/Cargo.toml b/crates/hir_def/Cargo.toml
index 475d337f3..43324d8d9 100644
--- a/crates/hir_def/Cargo.toml
+++ b/crates/hir_def/Cargo.toml
@@ -11,6 +11,7 @@ doctest = false
 
 [dependencies]
 cov-mark = { version = "1.1", features = ["thread-local"] }
+dashmap = { version = "4.0.2", features = ["raw-api"] }
 log = "0.4.8"
 once_cell = "1.3.1"
 rustc-hash = "1.1.0"
diff --git a/crates/hir_def/src/adt.rs b/crates/hir_def/src/adt.rs
index 58e35353b..402fb1d8d 100644
--- a/crates/hir_def/src/adt.rs
+++ b/crates/hir_def/src/adt.rs
@@ -15,6 +15,7 @@ use tt::{Delimiter, DelimiterKind, Leaf, Subtree, TokenTree};
 use crate::{
     body::{CfgExpander, LowerCtx},
     db::DefDatabase,
+    intern::Interned,
     item_tree::{AttrOwner, Field, Fields, ItemTree, ModItem, RawVisibilityId},
     src::HasChildSource,
     src::HasSource,
@@ -58,7 +59,7 @@ pub enum VariantData {
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct FieldData {
     pub name: Name,
-    pub type_ref: TypeRef,
+    pub type_ref: Interned<TypeRef>,
     pub visibility: RawVisibility,
 }
 
@@ -292,7 +293,7 @@ fn lower_struct(
                     || Either::Left(fd.clone()),
                     || FieldData {
                         name: Name::new_tuple_field(i),
-                        type_ref: TypeRef::from_ast_opt(&ctx, fd.ty()),
+                        type_ref: Interned::new(TypeRef::from_ast_opt(&ctx, fd.ty())),
                         visibility: RawVisibility::from_ast(db, ast.with_value(fd.visibility())),
                     },
                 );
@@ -309,7 +310,7 @@ fn lower_struct(
                     || Either::Right(fd.clone()),
                     || FieldData {
                         name: fd.name().map(|n| n.as_name()).unwrap_or_else(Name::missing),
-                        type_ref: TypeRef::from_ast_opt(&ctx, fd.ty()),
+                        type_ref: Interned::new(TypeRef::from_ast_opt(&ctx, fd.ty())),
                         visibility: RawVisibility::from_ast(db, ast.with_value(fd.visibility())),
                     },
                 );
@@ -358,7 +359,7 @@ fn lower_field(
 ) -> FieldData {
     FieldData {
         name: field.name.clone(),
-        type_ref: item_tree[field.type_ref].clone(),
+        type_ref: field.type_ref.clone(),
         visibility: item_tree[override_visibility.unwrap_or(field.visibility)].clone(),
     }
 }
diff --git a/crates/hir_def/src/attr.rs b/crates/hir_def/src/attr.rs
index 74bb6de35..ab77d924a 100644
--- a/crates/hir_def/src/attr.rs
+++ b/crates/hir_def/src/attr.rs
@@ -22,6 +22,7 @@ use tt::Subtree;
 
 use crate::{
     db::DefDatabase,
+    intern::Interned,
     item_tree::{ItemTreeId, ItemTreeNode},
     nameres::ModuleSource,
     path::{ModPath, PathKind},
@@ -102,7 +103,7 @@ impl RawAttrs {
                 Either::Right(comment) => comment.doc_comment().map(|doc| Attr {
                     index: i as u32,
                     input: Some(AttrInput::Literal(SmolStr::new(doc))),
-                    path: ModPath::from(hir_expand::name!(doc)),
+                    path: Interned::new(ModPath::from(hir_expand::name!(doc))),
                 }),
             })
             .collect::<Arc<_>>();
@@ -601,7 +602,7 @@ impl DocsRangeMap {
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct Attr {
     index: u32,
-    pub(crate) path: ModPath,
+    pub(crate) path: Interned<ModPath>,
     pub(crate) input: Option<AttrInput>,
 }
 
@@ -615,7 +616,7 @@ pub enum AttrInput {
 
 impl Attr {
     fn from_src(ast: ast::Attr, hygiene: &Hygiene, index: u32) -> Option<Attr> {
-        let path = ModPath::from_src(ast.path()?, hygiene)?;
+        let path = Interned::new(ModPath::from_src(ast.path()?, hygiene)?);
         let input = if let Some(ast::Expr::Literal(lit)) = ast.expr() {
             let value = match lit.kind() {
                 ast::LiteralKind::String(string) => string.value()?.into(),
@@ -623,7 +624,7 @@ impl Attr {
             };
             Some(AttrInput::Literal(value))
         } else if let Some(tt) = ast.token_tree() {
-            Some(AttrInput::TokenTree(ast_to_token_tree(&tt)?.0))
+            Some(AttrInput::TokenTree(ast_to_token_tree(&tt).0))
         } else {
             None
         };
diff --git a/crates/hir_def/src/body.rs b/crates/hir_def/src/body.rs
index 1080d9c2c..96b959967 100644
--- a/crates/hir_def/src/body.rs
+++ b/crates/hir_def/src/body.rs
@@ -226,7 +226,7 @@ pub struct Body {
     /// The `ExprId` of the actual body expression.
     pub body_expr: ExprId,
     /// Block expressions in this body that may contain inner items.
-    pub block_scopes: Vec<BlockId>,
+    block_scopes: Vec<BlockId>,
     _c: Count<Self>,
 }
 
@@ -302,7 +302,8 @@ impl Body {
             }
         };
         let expander = Expander::new(db, file_id, module);
-        let (body, source_map) = Body::new(db, expander, params, body);
+        let (mut body, source_map) = Body::new(db, expander, params, body);
+        body.shrink_to_fit();
         (Arc::new(body), Arc::new(source_map))
     }
 
@@ -310,6 +311,16 @@ impl Body {
         db.body_with_source_map(def).0
     }
 
+    /// Returns an iterator over all block expressions in this body that define inner items.
+    pub fn blocks<'a>(
+        &'a self,
+        db: &'a dyn DefDatabase,
+    ) -> impl Iterator<Item = (BlockId, Arc<DefMap>)> + '_ {
+        self.block_scopes
+            .iter()
+            .map(move |block| (*block, db.block_def_map(*block).expect("block ID without DefMap")))
+    }
+
     fn new(
         db: &dyn DefDatabase,
         expander: Expander,
@@ -318,6 +329,15 @@ impl Body {
     ) -> (Body, BodySourceMap) {
         lower::lower(db, expander, params, body)
     }
+
+    fn shrink_to_fit(&mut self) {
+        let Self { _c: _, body_expr: _, block_scopes, exprs, labels, params, pats } = self;
+        block_scopes.shrink_to_fit();
+        exprs.shrink_to_fit();
+        labels.shrink_to_fit();
+        params.shrink_to_fit();
+        pats.shrink_to_fit();
+    }
 }
 
 impl Index<ExprId> for Body {
diff --git a/crates/hir_def/src/body/lower.rs b/crates/hir_def/src/body/lower.rs
index 229e81dd4..63e89a1f4 100644
--- a/crates/hir_def/src/body/lower.rs
+++ b/crates/hir_def/src/body/lower.rs
@@ -673,12 +673,14 @@ impl ExprCollector<'_> {
         let block_loc =
             BlockLoc { ast_id, module: self.expander.def_map.module_id(self.expander.module) };
         let block_id = self.db.intern_block(block_loc);
-        self.body.block_scopes.push(block_id);
 
-        let opt_def_map = self.db.block_def_map(block_id);
-        let has_def_map = opt_def_map.is_some();
-        let def_map = opt_def_map.unwrap_or_else(|| self.expander.def_map.clone());
-        let module = if has_def_map { def_map.root() } else { self.expander.module };
+        let (module, def_map) = match self.db.block_def_map(block_id) {
+            Some(def_map) => {
+                self.body.block_scopes.push(block_id);
+                (def_map.root(), def_map)
+            }
+            None => (self.expander.module, self.expander.def_map.clone()),
+        };
         let prev_def_map = mem::replace(&mut self.expander.def_map, def_map);
         let prev_local_module = mem::replace(&mut self.expander.module, module);
         let prev_statements = std::mem::take(&mut self.statements_in_scope);
diff --git a/crates/hir_def/src/child_by_source.rs b/crates/hir_def/src/child_by_source.rs
index 2a331dcaf..f40a7f80d 100644
--- a/crates/hir_def/src/child_by_source.rs
+++ b/crates/hir_def/src/child_by_source.rs
@@ -160,7 +160,7 @@ impl ChildBySource for EnumId {
 impl ChildBySource for DefWithBodyId {
     fn child_by_source_to(&self, db: &dyn DefDatabase, res: &mut DynMap) {
         let body = db.body(*self);
-        for def_map in body.block_scopes.iter().filter_map(|block| db.block_def_map(*block)) {
+        for (_, def_map) in body.blocks(db) {
             // All block expressions are merged into the same map, because they logically all add
             // inner items to the containing `DefWithBodyId`.
             def_map[def_map.root()].scope.child_by_source_to(db, res);
diff --git a/crates/hir_def/src/data.rs b/crates/hir_def/src/data.rs
index 0be868ba2..135a6698e 100644
--- a/crates/hir_def/src/data.rs
+++ b/crates/hir_def/src/data.rs
@@ -9,8 +9,9 @@ use crate::{
     attr::Attrs,
     body::Expander,
     db::DefDatabase,
-    item_tree::{AssocItem, FunctionQualifier, ItemTreeId, ModItem, Param},
-    type_ref::{TypeBound, TypeRef},
+    intern::Interned,
+    item_tree::{AssocItem, FnFlags, ItemTreeId, ModItem, Param},
+    type_ref::{TraitRef, TypeBound, TypeRef},
     visibility::RawVisibility,
     AssocContainerId, AssocItemId, ConstId, ConstLoc, FunctionId, FunctionLoc, HasModule, ImplId,
     Intern, Lookup, ModuleId, StaticId, TraitId, TypeAliasId, TypeAliasLoc,
@@ -19,17 +20,12 @@ use crate::{
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct FunctionData {
     pub name: Name,
-    pub params: Vec<TypeRef>,
-    pub ret_type: TypeRef,
+    pub params: Vec<Interned<TypeRef>>,
+    pub ret_type: Interned<TypeRef>,
     pub attrs: Attrs,
-    /// True if the first param is `self`. This is relevant to decide whether this
-    /// can be called as a method.
-    pub has_self_param: bool,
-    pub has_body: bool,
-    pub qualifier: FunctionQualifier,
-    pub is_in_extern_block: bool,
-    pub is_varargs: bool,
     pub visibility: RawVisibility,
+    pub abi: Option<Interned<str>>,
+    flags: FnFlags,
 }
 
 impl FunctionData {
@@ -52,31 +48,67 @@ impl FunctionData {
             .next_back()
             .map_or(false, |param| matches!(item_tree[param], Param::Varargs));
 
+        let mut flags = func.flags;
+        if is_varargs {
+            flags.bits |= FnFlags::IS_VARARGS;
+        }
+
         Arc::new(FunctionData {
             name: func.name.clone(),
             params: enabled_params
                 .clone()
                 .filter_map(|id| match &item_tree[id] {
-                    Param::Normal(ty) => Some(item_tree[*ty].clone()),
+                    Param::Normal(ty) => Some(ty.clone()),
                     Param::Varargs => None,
                 })
                 .collect(),
-            ret_type: item_tree[func.ret_type].clone(),
+            ret_type: func.ret_type.clone(),
             attrs: item_tree.attrs(db, krate, ModItem::from(loc.id.value).into()),
-            has_self_param: func.has_self_param,
-            has_body: func.has_body,
-            qualifier: func.qualifier.clone(),
-            is_in_extern_block: func.is_in_extern_block,
-            is_varargs,
             visibility: item_tree[func.visibility].clone(),
+            abi: func.abi.clone(),
+            flags,
         })
     }
+
+    pub fn has_body(&self) -> bool {
+        self.flags.bits & FnFlags::HAS_BODY != 0
+    }
+
+    /// True if the first param is `self`. This is relevant to decide whether this
+    /// can be called as a method.
+    pub fn has_self_param(&self) -> bool {
+        self.flags.bits & FnFlags::HAS_SELF_PARAM != 0
+    }
+
+    pub fn is_default(&self) -> bool {
+        self.flags.bits & FnFlags::IS_DEFAULT != 0
+    }
+
+    pub fn is_const(&self) -> bool {
+        self.flags.bits & FnFlags::IS_CONST != 0
+    }
+
+    pub fn is_async(&self) -> bool {
+        self.flags.bits & FnFlags::IS_ASYNC != 0
+    }
+
+    pub fn is_unsafe(&self) -> bool {
+        self.flags.bits & FnFlags::IS_UNSAFE != 0
+    }
+
+    pub fn is_in_extern_block(&self) -> bool {
+        self.flags.bits & FnFlags::IS_IN_EXTERN_BLOCK != 0
+    }
+
+    pub fn is_varargs(&self) -> bool {
+        self.flags.bits & FnFlags::IS_VARARGS != 0
+    }
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct TypeAliasData {
     pub name: Name,
-    pub type_ref: Option<TypeRef>,
+    pub type_ref: Option<Interned<TypeRef>>,
     pub visibility: RawVisibility,
     pub is_extern: bool,
     /// Bounds restricting the type alias itself (eg. `type Ty: Bound;` in a trait or impl).
@@ -94,7 +126,7 @@ impl TypeAliasData {
 
         Arc::new(TypeAliasData {
             name: typ.name.clone(),
-            type_ref: typ.type_ref.map(|id| item_tree[id].clone()),
+            type_ref: typ.type_ref.clone(),
             visibility: item_tree[typ.visibility].clone(),
             is_extern: typ.is_extern,
             bounds: typ.bounds.to_vec(),
@@ -156,8 +188,8 @@ impl TraitData {
 
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct ImplData {
-    pub target_trait: Option<TypeRef>,
-    pub target_type: TypeRef,
+    pub target_trait: Option<Interned<TraitRef>>,
+    pub self_ty: Interned<TypeRef>,
     pub items: Vec<AssocItemId>,
     pub is_negative: bool,
 }
@@ -169,8 +201,8 @@ impl ImplData {
 
         let item_tree = impl_loc.id.item_tree(db);
         let impl_def = &item_tree[impl_loc.id.value];
-        let target_trait = impl_def.target_trait.map(|id| item_tree[id].clone());
-        let target_type = item_tree[impl_def.target_type].clone();
+        let target_trait = impl_def.target_trait.clone();
+        let self_ty = impl_def.self_ty.clone();
         let is_negative = impl_def.is_negative;
         let module_id = impl_loc.container;
         let container = AssocContainerId::ImplId(id);
@@ -187,7 +219,7 @@ impl ImplData {
         );
         let items = items.into_iter().map(|(_, item)| item).collect();
 
-        Arc::new(ImplData { target_trait, target_type, items, is_negative })
+        Arc::new(ImplData { target_trait, self_ty, items, is_negative })
     }
 }
 
@@ -195,7 +227,7 @@ impl ImplData {
 pub struct ConstData {
     /// const _: () = ();
     pub name: Option<Name>,
-    pub type_ref: TypeRef,
+    pub type_ref: Interned<TypeRef>,
     pub visibility: RawVisibility,
 }
 
@@ -207,7 +239,7 @@ impl ConstData {
 
         Arc::new(ConstData {
             name: konst.name.clone(),
-            type_ref: item_tree[konst.type_ref].clone(),
+            type_ref: konst.type_ref.clone(),
             visibility: item_tree[konst.visibility].clone(),
         })
     }
@@ -216,7 +248,7 @@ impl ConstData {
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct StaticData {
     pub name: Option<Name>,
-    pub type_ref: TypeRef,
+    pub type_ref: Interned<TypeRef>,
     pub visibility: RawVisibility,
     pub mutable: bool,
     pub is_extern: bool,
@@ -230,7 +262,7 @@ impl StaticData {
 
         Arc::new(StaticData {
             name: Some(statik.name.clone()),
-            type_ref: item_tree[statik.type_ref].clone(),
+            type_ref: statik.type_ref.clone(),
             visibility: item_tree[statik.visibility].clone(),
             mutable: statik.mutable,
             is_extern: statik.is_extern,
diff --git a/crates/hir_def/src/db.rs b/crates/hir_def/src/db.rs
index 068b2ee38..9b7a213a1 100644
--- a/crates/hir_def/src/db.rs
+++ b/crates/hir_def/src/db.rs
@@ -13,6 +13,7 @@ use crate::{
     data::{ConstData, FunctionData, ImplData, StaticData, TraitData, TypeAliasData},
     generics::GenericParams,
     import_map::ImportMap,
+    intern::Interned,
     item_tree::ItemTree,
     lang_item::{LangItemTarget, LangItems},
     nameres::DefMap,
@@ -113,7 +114,7 @@ pub trait DefDatabase: InternDatabase + AstDatabase + Upcast<dyn AstDatabase> {
     fn expr_scopes(&self, def: DefWithBodyId) -> Arc<ExprScopes>;
 
     #[salsa::invoke(GenericParams::generic_params_query)]
-    fn generic_params(&self, def: GenericDefId) -> Arc<GenericParams>;
+    fn generic_params(&self, def: GenericDefId) -> Interned<GenericParams>;
 
     #[salsa::invoke(Attrs::variants_attrs_query)]
     fn variants_attrs(&self, def: EnumId) -> Arc<ArenaMap<LocalEnumVariantId, Attrs>>;
diff --git a/crates/hir_def/src/generics.rs b/crates/hir_def/src/generics.rs
index 7c6cbff11..de5acced8 100644
--- a/crates/hir_def/src/generics.rs
+++ b/crates/hir_def/src/generics.rs
@@ -2,7 +2,6 @@
 //! structs, impls, traits, etc. This module provides a common HIR for these
 //! generic parameters. See also the `Generics` type and the `generics_of` query
 //! in rustc.
-use std::sync::Arc;
 
 use base_db::FileId;
 use either::Either;
@@ -18,6 +17,7 @@ use crate::{
     child_by_source::ChildBySource,
     db::DefDatabase,
     dyn_map::DynMap,
+    intern::Interned,
     keys,
     src::{HasChildSource, HasSource},
     type_ref::{LifetimeRef, TypeBound, TypeRef},
@@ -26,27 +26,27 @@ use crate::{
 };
 
 /// Data about a generic type parameter (to a function, struct, impl, ...).
-#[derive(Clone, PartialEq, Eq, Debug)]
+#[derive(Clone, PartialEq, Eq, Debug, Hash)]
 pub struct TypeParamData {
     pub name: Option<Name>,
-    pub default: Option<TypeRef>,
+    pub default: Option<Interned<TypeRef>>,
     pub provenance: TypeParamProvenance,
 }
 
 /// Data about a generic lifetime parameter (to a function, struct, impl, ...).
-#[derive(Clone, PartialEq, Eq, Debug)]
+#[derive(Clone, PartialEq, Eq, Debug, Hash)]
 pub struct LifetimeParamData {
     pub name: Name,
 }
 
 /// Data about a generic const parameter (to a function, struct, impl, ...).
-#[derive(Clone, PartialEq, Eq, Debug)]
+#[derive(Clone, PartialEq, Eq, Debug, Hash)]
 pub struct ConstParamData {
     pub name: Name,
-    pub ty: TypeRef,
+    pub ty: Interned<TypeRef>,
 }
 
-#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+#[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]
 pub enum TypeParamProvenance {
     TypeParamList,
     TraitSelf,
@@ -54,7 +54,7 @@ pub enum TypeParamProvenance {
 }
 
 /// Data about the generic parameters of a function, struct, impl, etc.
-#[derive(Clone, PartialEq, Eq, Debug, Default)]
+#[derive(Clone, PartialEq, Eq, Debug, Default, Hash)]
 pub struct GenericParams {
     pub types: Arena<TypeParamData>,
     pub lifetimes: Arena<LifetimeParamData>,
@@ -66,16 +66,16 @@ pub struct GenericParams {
 /// where clauses like `where T: Foo + Bar` are turned into multiple of these.
 /// It might still result in multiple actual predicates though, because of
 /// associated type bindings like `Iterator<Item = u32>`.
-#[derive(Clone, PartialEq, Eq, Debug)]
+#[derive(Clone, PartialEq, Eq, Debug, Hash)]
 pub enum WherePredicate {
     TypeBound { target: WherePredicateTypeTarget, bound: TypeBound },
     Lifetime { target: LifetimeRef, bound: LifetimeRef },
     ForLifetime { lifetimes: Box<[Name]>, target: WherePredicateTypeTarget, bound: TypeBound },
 }
 
-#[derive(Clone, PartialEq, Eq, Debug)]
+#[derive(Clone, PartialEq, Eq, Debug, Hash)]
 pub enum WherePredicateTypeTarget {
-    TypeRef(TypeRef),
+    TypeRef(Interned<TypeRef>),
     /// For desugared where predicates that can directly refer to a type param.
     TypeParam(LocalTypeParamId),
 }
@@ -91,7 +91,7 @@ impl GenericParams {
     pub(crate) fn generic_params_query(
         db: &dyn DefDatabase,
         def: GenericDefId,
-    ) -> Arc<GenericParams> {
+    ) -> Interned<GenericParams> {
         let _p = profile::span("generic_params_query");
 
         let generics = match def {
@@ -99,47 +99,49 @@ impl GenericParams {
                 let id = id.lookup(db).id;
                 let tree = id.item_tree(db);
                 let item = &tree[id.value];
-                tree[item.generic_params].clone()
+                item.generic_params.clone()
             }
             GenericDefId::AdtId(AdtId::StructId(id)) => {
                 let id = id.lookup(db).id;
                 let tree = id.item_tree(db);
                 let item = &tree[id.value];
-                tree[item.generic_params].clone()
+                item.generic_params.clone()
             }
             GenericDefId::AdtId(AdtId::EnumId(id)) => {
                 let id = id.lookup(db).id;
                 let tree = id.item_tree(db);
                 let item = &tree[id.value];
-                tree[item.generic_params].clone()
+                item.generic_params.clone()
             }
             GenericDefId::AdtId(AdtId::UnionId(id)) => {
                 let id = id.lookup(db).id;
                 let tree = id.item_tree(db);
                 let item = &tree[id.value];
-                tree[item.generic_params].clone()
+                item.generic_params.clone()
             }
             GenericDefId::TraitId(id) => {
                 let id = id.lookup(db).id;
                 let tree = id.item_tree(db);
                 let item = &tree[id.value];
-                tree[item.generic_params].clone()
+                item.generic_params.clone()
             }
             GenericDefId::TypeAliasId(id) => {
                 let id = id.lookup(db).id;
                 let tree = id.item_tree(db);
                 let item = &tree[id.value];
-                tree[item.generic_params].clone()
+                item.generic_params.clone()
             }
             GenericDefId::ImplId(id) => {
                 let id = id.lookup(db).id;
                 let tree = id.item_tree(db);
                 let item = &tree[id.value];
-                tree[item.generic_params].clone()
+                item.generic_params.clone()
+            }
+            GenericDefId::EnumVariantId(_) | GenericDefId::ConstId(_) => {
+                Interned::new(GenericParams::default())
             }
-            GenericDefId::EnumVariantId(_) | GenericDefId::ConstId(_) => GenericParams::default(),
         };
-        Arc::new(generics)
+        generics
     }
 
     fn new(db: &dyn DefDatabase, def: GenericDefId) -> (GenericParams, InFile<SourceMap>) {
@@ -217,6 +219,7 @@ impl GenericParams {
             GenericDefId::EnumVariantId(_) | GenericDefId::ConstId(_) => FileId(!0).into(),
         };
 
+        generics.shrink_to_fit();
         (generics, InFile::new(file_id, sm))
     }
 
@@ -256,7 +259,8 @@ impl GenericParams {
         for type_param in params.type_params() {
             let name = type_param.name().map_or_else(Name::missing, |it| it.as_name());
             // FIXME: Use `Path::from_src`
-            let default = type_param.default_type().map(|it| TypeRef::from_ast(lower_ctx, it));
+            let default =
+                type_param.default_type().map(|it| Interned::new(TypeRef::from_ast(lower_ctx, it)));
             let param = TypeParamData {
                 name: Some(name.clone()),
                 default,
@@ -280,7 +284,7 @@ impl GenericParams {
         for const_param in params.const_params() {
             let name = const_param.name().map_or_else(Name::missing, |it| it.as_name());
             let ty = const_param.ty().map_or(TypeRef::Error, |it| TypeRef::from_ast(lower_ctx, it));
-            let param = ConstParamData { name, ty };
+            let param = ConstParamData { name, ty: Interned::new(ty) };
             let param_id = self.consts.alloc(param);
             sm.const_params.insert(param_id, const_param.clone());
         }
@@ -334,11 +338,11 @@ impl GenericParams {
             (Either::Left(type_ref), bound) => match hrtb_lifetimes {
                 Some(hrtb_lifetimes) => WherePredicate::ForLifetime {
                     lifetimes: hrtb_lifetimes.clone(),
-                    target: WherePredicateTypeTarget::TypeRef(type_ref),
+                    target: WherePredicateTypeTarget::TypeRef(Interned::new(type_ref)),
                     bound,
                 },
                 None => WherePredicate::TypeBound {
-                    target: WherePredicateTypeTarget::TypeRef(type_ref),
+                    target: WherePredicateTypeTarget::TypeRef(Interned::new(type_ref)),
                     bound,
                 },
             },
@@ -369,6 +373,14 @@ impl GenericParams {
         });
     }
 
+    pub(crate) fn shrink_to_fit(&mut self) {
+        let Self { consts, lifetimes, types, where_predicates } = self;
+        consts.shrink_to_fit();
+        lifetimes.shrink_to_fit();
+        types.shrink_to_fit();
+        where_predicates.shrink_to_fit();
+    }
+
     pub fn find_type_by_name(&self, name: &Name) -> Option<LocalTypeParamId> {
         self.types
             .iter()
diff --git a/crates/hir_def/src/intern.rs b/crates/hir_def/src/intern.rs
new file mode 100644
index 000000000..2467e9299
--- /dev/null
+++ b/crates/hir_def/src/intern.rs
@@ -0,0 +1,205 @@
+//! Global `Arc`-based object interning infrastructure.
+//!
+//! Eventually this should probably be replaced with salsa-based interning.
+
+use std::{
+    collections::HashMap,
+    fmt::{self, Debug},
+    hash::{BuildHasherDefault, Hash},
+    ops::Deref,
+    sync::Arc,
+};
+
+use dashmap::{lock::RwLockWriteGuard, DashMap, SharedValue};
+use once_cell::sync::OnceCell;
+use rustc_hash::FxHasher;
+
+use crate::generics::GenericParams;
+
+type InternMap<T> = DashMap<Arc<T>, (), BuildHasherDefault<FxHasher>>;
+type Guard<T> =
+    RwLockWriteGuard<'static, HashMap<Arc<T>, SharedValue<()>, BuildHasherDefault<FxHasher>>>;
+
+#[derive(Hash)]
+pub struct Interned<T: Internable + ?Sized> {
+    arc: Arc<T>,
+}
+
+impl<T: Internable> Interned<T> {
+    pub fn new(obj: T) -> Self {
+        match Interned::lookup(&obj) {
+            Ok(this) => this,
+            Err(shard) => {
+                let arc = Arc::new(obj);
+                Self::alloc(arc, shard)
+            }
+        }
+    }
+}
+
+impl<T: Internable + ?Sized> Interned<T> {
+    fn lookup(obj: &T) -> Result<Self, Guard<T>> {
+        let storage = T::storage().get();
+        let shard_idx = storage.determine_map(obj);
+        let shard = &storage.shards()[shard_idx];
+        let shard = shard.write();
+
+        // Atomically,
+        // - check if `obj` is already in the map
+        //   - if so, clone its `Arc` and return it
+        //   - if not, box it up, insert it, and return a clone
+        // This needs to be atomic (locking the shard) to avoid races with other thread, which could
+        // insert the same object between us looking it up and inserting it.
+
+        // FIXME: avoid double lookup/hashing by using raw entry API (once stable, or when
+        // hashbrown can be plugged into dashmap)
+        match shard.get_key_value(obj) {
+            Some((arc, _)) => Ok(Self { arc: arc.clone() }),
+            None => Err(shard),
+        }
+    }
+
+    fn alloc(arc: Arc<T>, mut shard: Guard<T>) -> Self {
+        let arc2 = arc.clone();
+
+        shard.insert(arc2, SharedValue::new(()));
+
+        Self { arc }
+    }
+}
+
+impl Interned<str> {
+    pub fn new_str(s: &str) -> Self {
+        match Interned::lookup(s) {
+            Ok(this) => this,
+            Err(shard) => {
+                let arc = Arc::<str>::from(s);
+                Self::alloc(arc, shard)
+            }
+        }
+    }
+}
+
+impl<T: Internable + ?Sized> Drop for Interned<T> {
+    #[inline]
+    fn drop(&mut self) {
+        // When the last `Ref` is dropped, remove the object from the global map.
+        if Arc::strong_count(&self.arc) == 2 {
+            // Only `self` and the global map point to the object.
+
+            self.drop_slow();
+        }
+    }
+}
+
+impl<T: Internable + ?Sized> Interned<T> {
+    #[cold]
+    fn drop_slow(&mut self) {
+        let storage = T::storage().get();
+        let shard_idx = storage.determine_map(&self.arc);
+        let shard = &storage.shards()[shard_idx];
+        let mut shard = shard.write();
+
+        // FIXME: avoid double lookup
+        let (arc, _) = shard.get_key_value(&self.arc).expect("interned value removed prematurely");
+
+        if Arc::strong_count(arc) != 2 {
+            // Another thread has interned another copy
+            return;
+        }
+
+        shard.remove(&self.arc);
+
+        // Shrink the backing storage if the shard is less than 50% occupied.
+        if shard.len() * 2 < shard.capacity() {
+            shard.shrink_to_fit();
+        }
+    }
+}
+
+/// Compares interned `Ref`s using pointer equality.
+impl<T: Internable> PartialEq for Interned<T> {
+    // NOTE: No `?Sized` because `ptr_eq` doesn't work right with trait objects.
+
+    #[inline]
+    fn eq(&self, other: &Self) -> bool {
+        Arc::ptr_eq(&self.arc, &other.arc)
+    }
+}
+
+impl<T: Internable> Eq for Interned<T> {}
+
+impl PartialEq for Interned<str> {
+    fn eq(&self, other: &Self) -> bool {
+        Arc::ptr_eq(&self.arc, &other.arc)
+    }
+}
+
+impl Eq for Interned<str> {}
+
+impl<T: Internable + ?Sized> AsRef<T> for Interned<T> {
+    #[inline]
+    fn as_ref(&self) -> &T {
+        &self.arc
+    }
+}
+
+impl<T: Internable + ?Sized> Deref for Interned<T> {
+    type Target = T;
+
+    #[inline]
+    fn deref(&self) -> &Self::Target {
+        &self.arc
+    }
+}
+
+impl<T: Internable + ?Sized> Clone for Interned<T> {
+    fn clone(&self) -> Self {
+        Self { arc: self.arc.clone() }
+    }
+}
+
+impl<T: Debug + Internable + ?Sized> Debug for Interned<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        (*self.arc).fmt(f)
+    }
+}
+
+pub struct InternStorage<T: ?Sized> {
+    map: OnceCell<InternMap<T>>,
+}
+
+impl<T: ?Sized> InternStorage<T> {
+    pub const fn new() -> Self {
+        Self { map: OnceCell::new() }
+    }
+}
+
+impl<T: Internable + ?Sized> InternStorage<T> {
+    fn get(&self) -> &InternMap<T> {
+        self.map.get_or_init(DashMap::default)
+    }
+}
+
+pub trait Internable: Hash + Eq + 'static {
+    fn storage() -> &'static InternStorage<Self>;
+}
+
+macro_rules! impl_internable {
+    ( $($t:path),+ $(,)? ) => { $(
+        impl Internable for $t {
+            fn storage() -> &'static InternStorage<Self> {
+                static STORAGE: InternStorage<$t> = InternStorage::new();
+                &STORAGE
+            }
+        }
+    )+ };
+}
+
+impl_internable!(
+    crate::type_ref::TypeRef,
+    crate::type_ref::TraitRef,
+    crate::path::ModPath,
+    GenericParams,
+    str
+);
diff --git a/crates/hir_def/src/item_scope.rs b/crates/hir_def/src/item_scope.rs
index f3ebe7c72..a8ee5eeac 100644
--- a/crates/hir_def/src/item_scope.rs
+++ b/crates/hir_def/src/item_scope.rs
@@ -285,6 +285,28 @@ impl ItemScope {
             buf.push('\n');
         }
     }
+
+    pub(crate) fn shrink_to_fit(&mut self) {
+        // Exhaustive match to require handling new fields.
+        let Self {
+            types,
+            values,
+            macros,
+            unresolved,
+            defs,
+            impls,
+            unnamed_trait_imports,
+            legacy_macros,
+        } = self;
+        types.shrink_to_fit();
+        values.shrink_to_fit();
+        macros.shrink_to_fit();
+        unresolved.shrink_to_fit();
+        defs.shrink_to_fit();
+        impls.shrink_to_fit();
+        unnamed_trait_imports.shrink_to_fit();
+        legacy_macros.shrink_to_fit();
+    }
 }
 
 impl PerNs {
diff --git a/crates/hir_def/src/item_tree.rs b/crates/hir_def/src/item_tree.rs
index ca0048b16..240662486 100644
--- a/crates/hir_def/src/item_tree.rs
+++ b/crates/hir_def/src/item_tree.rs
@@ -24,14 +24,15 @@ use la_arena::{Arena, Idx, RawIdx};
 use profile::Count;
 use rustc_hash::FxHashMap;
 use smallvec::SmallVec;
-use syntax::{ast, match_ast, SmolStr, SyntaxKind};
+use syntax::{ast, match_ast, SyntaxKind};
 
 use crate::{
     attr::{Attrs, RawAttrs},
     db::DefDatabase,
     generics::GenericParams,
+    intern::Interned,
     path::{path, AssociatedTypeBinding, GenericArgs, ImportAlias, ModPath, Path, PathKind},
-    type_ref::{Mutability, TypeBound, TypeRef},
+    type_ref::{Mutability, TraitRef, TypeBound, TypeRef},
     visibility::RawVisibility,
 };
 
@@ -57,13 +58,6 @@ impl fmt::Debug for RawVisibilityId {
     }
 }
 
-#[derive(Debug, Copy, Clone, Eq, PartialEq)]
-pub struct GenericParamsId(u32);
-
-impl GenericParamsId {
-    pub const EMPTY: Self = GenericParamsId(u32::max_value());
-}
-
 /// The item tree of a source file.
 #[derive(Debug, Default, Eq, PartialEq)]
 pub struct ItemTree {
@@ -145,8 +139,6 @@ impl ItemTree {
                 macro_rules,
                 macro_defs,
                 vis,
-                generics,
-                type_refs,
                 inner_items,
             } = &mut **data;
 
@@ -170,9 +162,6 @@ impl ItemTree {
             macro_defs.shrink_to_fit();
 
             vis.arena.shrink_to_fit();
-            generics.arena.shrink_to_fit();
-            type_refs.arena.shrink_to_fit();
-            type_refs.map.shrink_to_fit();
 
             inner_items.shrink_to_fit();
         }
@@ -244,58 +233,6 @@ static VIS_PRIV: RawVisibility = RawVisibility::Module(ModPath::from_kind(PathKi
 static VIS_PUB_CRATE: RawVisibility = RawVisibility::Module(ModPath::from_kind(PathKind::Crate));
 
 #[derive(Default, Debug, Eq, PartialEq)]
-struct GenericParamsStorage {
-    arena: Arena<GenericParams>,
-}
-
-impl GenericParamsStorage {
-    fn alloc(&mut self, params: GenericParams) -> GenericParamsId {
-        if params.types.is_empty()
-            && params.lifetimes.is_empty()
-            && params.consts.is_empty()
-            && params.where_predicates.is_empty()
-        {
-            return GenericParamsId::EMPTY;
-        }
-
-        GenericParamsId(self.arena.alloc(params).into_raw().into())
-    }
-}
-
-static EMPTY_GENERICS: GenericParams = GenericParams {
-    types: Arena::new(),
-    lifetimes: Arena::new(),
-    consts: Arena::new(),
-    where_predicates: Vec::new(),
-};
-
-/// `TypeRef` interner.
-#[derive(Default, Debug, Eq, PartialEq)]
-struct TypeRefStorage {
-    arena: Arena<Arc<TypeRef>>,
-    map: FxHashMap<Arc<TypeRef>, Idx<Arc<TypeRef>>>,
-}
-
-impl TypeRefStorage {
-    // Note: We lie about the `Idx<TypeRef>` to hide the interner details.
-
-    fn intern(&mut self, ty: TypeRef) -> Idx<TypeRef> {
-        if let Some(id) = self.map.get(&ty) {
-            return Idx::from_raw(id.into_raw());
-        }
-
-        let ty = Arc::new(ty);
-        let idx = self.arena.alloc(ty.clone());
-        self.map.insert(ty, idx);
-        Idx::from_raw(idx.into_raw())
-    }
-
-    fn lookup(&self, id: Idx<TypeRef>) -> &TypeRef {
-        &self.arena[Idx::from_raw(id.into_raw())]
-    }
-}
-
-#[derive(Default, Debug, Eq, PartialEq)]
 struct ItemTreeData {
     imports: Arena<Import>,
     extern_crates: Arena<ExternCrate>,
@@ -317,8 +254,6 @@ struct ItemTreeData {
     macro_defs: Arena<MacroDef>,
 
     vis: ItemVisibilities,
-    generics: GenericParamsStorage,
-    type_refs: TypeRefStorage,
 
     inner_items: FxHashMap<FileAstId<ast::BlockExpr>, SmallVec<[ModItem; 1]>>,
 }
@@ -537,25 +472,6 @@ impl Index<RawVisibilityId> for ItemTree {
     }
 }
 
-impl Index<GenericParamsId> for ItemTree {
-    type Output = GenericParams;
-
-    fn index(&self, index: GenericParamsId) -> &Self::Output {
-        match index {
-            GenericParamsId::EMPTY => &EMPTY_GENERICS,
-            _ => &self.data().generics.arena[Idx::from_raw(index.0.into())],
-        }
-    }
-}
-
-impl Index<Idx<TypeRef>> for ItemTree {
-    type Output = TypeRef;
-
-    fn index(&self, id: Idx<TypeRef>) -> &Self::Output {
-        self.data().type_refs.lookup(id)
-    }
-}
-
 impl<N: ItemTreeNode> Index<FileItemTreeId<N>> for ItemTree {
     type Output = N;
     fn index(&self, id: FileItemTreeId<N>) -> &N {
@@ -566,7 +482,7 @@ impl<N: ItemTreeNode> Index<FileItemTreeId<N>> for ItemTree {
 /// A desugared `use` import.
 #[derive(Debug, Clone, Eq, PartialEq)]
 pub struct Import {
-    pub path: ModPath,
+    pub path: Interned<ModPath>,
     pub alias: Option<ImportAlias>,
     pub visibility: RawVisibilityId,
     pub is_glob: bool,
@@ -592,38 +508,42 @@ pub struct ExternCrate {
 pub struct Function {
     pub name: Name,
     pub visibility: RawVisibilityId,
-    pub generic_params: GenericParamsId,
-    pub has_self_param: bool,
-    pub has_body: bool,
-    pub qualifier: FunctionQualifier,
-    /// Whether the function is located in an `extern` block (*not* whether it is an
-    /// `extern "abi" fn`).
-    pub is_in_extern_block: bool,
+    pub generic_params: Interned<GenericParams>,
+    pub abi: Option<Interned<str>>,
     pub params: IdRange<Param>,
-    pub ret_type: Idx<TypeRef>,
+    pub ret_type: Interned<TypeRef>,
     pub ast_id: FileAstId<ast::Fn>,
+    pub(crate) flags: FnFlags,
 }
 
 #[derive(Debug, Clone, Eq, PartialEq)]
 pub enum Param {
-    Normal(Idx<TypeRef>),
+    Normal(Interned<TypeRef>),
     Varargs,
 }
 
-#[derive(Debug, Clone, PartialEq, Eq)]
-pub struct FunctionQualifier {
-    pub is_default: bool,
-    pub is_const: bool,
-    pub is_async: bool,
-    pub is_unsafe: bool,
-    pub abi: Option<SmolStr>,
+#[derive(Debug, Clone, Copy, Eq, PartialEq, Default)]
+pub(crate) struct FnFlags {
+    pub(crate) bits: u8,
+}
+impl FnFlags {
+    pub(crate) const HAS_SELF_PARAM: u8 = 1 << 0;
+    pub(crate) const HAS_BODY: u8 = 1 << 1;
+    pub(crate) const IS_DEFAULT: u8 = 1 << 2;
+    pub(crate) const IS_CONST: u8 = 1 << 3;
+    pub(crate) const IS_ASYNC: u8 = 1 << 4;
+    pub(crate) const IS_UNSAFE: u8 = 1 << 5;
+    /// Whether the function is located in an `extern` block (*not* whether it is an
+    /// `extern "abi" fn`).
+    pub(crate) const IS_IN_EXTERN_BLOCK: u8 = 1 << 6;
+    pub(crate) const IS_VARARGS: u8 = 1 << 7;
 }
 
 #[derive(Debug, Clone, Eq, PartialEq)]
 pub struct Struct {
     pub name: Name,
     pub visibility: RawVisibilityId,
-    pub generic_params: GenericParamsId,
+    pub generic_params: Interned<GenericParams>,
     pub fields: Fields,
     pub ast_id: FileAstId<ast::Struct>,
     pub kind: StructDefKind,
@@ -643,7 +563,7 @@ pub enum StructDefKind {
 pub struct Union {
     pub name: Name,
     pub visibility: RawVisibilityId,
-    pub generic_params: GenericParamsId,
+    pub generic_params: Interned<GenericParams>,
     pub fields: Fields,
     pub ast_id: FileAstId<ast::Union>,
 }
@@ -652,7 +572,7 @@ pub struct Union {
 pub struct Enum {
     pub name: Name,
     pub visibility: RawVisibilityId,
-    pub generic_params: GenericParamsId,
+    pub generic_params: Interned<GenericParams>,
     pub variants: IdRange<Variant>,
     pub ast_id: FileAstId<ast::Enum>,
 }
@@ -662,7 +582,7 @@ pub struct Const {
     /// const _: () = ();
     pub name: Option<Name>,
     pub visibility: RawVisibilityId,
-    pub type_ref: Idx<TypeRef>,
+    pub type_ref: Interned<TypeRef>,
     pub ast_id: FileAstId<ast::Const>,
 }
 
@@ -673,7 +593,7 @@ pub struct Static {
     pub mutable: bool,
     /// Whether the static is in an `extern` block.
     pub is_extern: bool,
-    pub type_ref: Idx<TypeRef>,
+    pub type_ref: Interned<TypeRef>,
     pub ast_id: FileAstId<ast::Static>,
 }
 
@@ -681,7 +601,7 @@ pub struct Static {
 pub struct Trait {
     pub name: Name,
     pub visibility: RawVisibilityId,
-    pub generic_params: GenericParamsId,
+    pub generic_params: Interned<GenericParams>,
     pub is_auto: bool,
     pub is_unsafe: bool,
     pub bounds: Box<[TypeBound]>,
@@ -691,9 +611,9 @@ pub struct Trait {
 
 #[derive(Debug, Clone, Eq, PartialEq)]
 pub struct Impl {
-    pub generic_params: GenericParamsId,
-    pub target_trait: Option<Idx<TypeRef>>,
-    pub target_type: Idx<TypeRef>,
+    pub generic_params: Interned<GenericParams>,
+    pub target_trait: Option<Interned<TraitRef>>,
+    pub self_ty: Interned<TypeRef>,
     pub is_negative: bool,
     pub items: Box<[AssocItem]>,
     pub ast_id: FileAstId<ast::Impl>,
@@ -705,8 +625,8 @@ pub struct TypeAlias {
     pub visibility: RawVisibilityId,
     /// Bounds on the type alias itself. Only valid in trait declarations, eg. `type Assoc: Copy;`.
     pub bounds: Box<[TypeBound]>,
-    pub generic_params: GenericParamsId,
-    pub type_ref: Option<Idx<TypeRef>>,
+    pub generic_params: Interned<GenericParams>,
+    pub type_ref: Option<Interned<TypeRef>>,
     pub is_extern: bool,
     pub ast_id: FileAstId<ast::TypeAlias>,
 }
@@ -731,7 +651,7 @@ pub enum ModKind {
 #[derive(Debug, Clone, Eq, PartialEq)]
 pub struct MacroCall {
     /// Path to the called macro.
-    pub path: ModPath,
+    pub path: Interned<ModPath>,
     pub ast_id: FileAstId<ast::MacroCall>,
 }
 
@@ -896,6 +816,6 @@ pub enum Fields {
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct Field {
     pub name: Name,
-    pub type_ref: Idx<TypeRef>,
+    pub type_ref: Interned<TypeRef>,
     pub visibility: RawVisibilityId,
 }
diff --git a/crates/hir_def/src/item_tree/lower.rs b/crates/hir_def/src/item_tree/lower.rs
index 3f558edd8..c5629af24 100644
--- a/crates/hir_def/src/item_tree/lower.rs
+++ b/crates/hir_def/src/item_tree/lower.rs
@@ -11,7 +11,7 @@ use syntax::{
 
 use crate::{
     generics::{GenericParams, TypeParamData, TypeParamProvenance},
-    type_ref::LifetimeRef,
+    type_ref::{LifetimeRef, TraitRef},
 };
 
 use super::*;
@@ -174,6 +174,12 @@ impl Ctx {
         let forced_vis = self.forced_visibility.take();
 
         let mut block_stack = Vec::new();
+
+        // if container itself is block, add it to the stack
+        if let Some(block) = ast::BlockExpr::cast(container.clone()) {
+            block_stack.push(self.source_ast_id_map.ast_id(&block));
+        }
+
         for event in container.preorder().skip(1) {
             match event {
                 WalkEvent::Enter(node) => {
@@ -356,7 +362,7 @@ impl Ctx {
                         }
                     }
                 };
-                let ty = self.data().type_refs.intern(self_type);
+                let ty = Interned::new(self_type);
                 let idx = self.data().params.alloc(Param::Normal(ty));
                 self.add_attrs(idx.into(), RawAttrs::new(&self_param, &self.hygiene));
                 has_self_param = true;
@@ -366,7 +372,7 @@ impl Ctx {
                     Some(_) => self.data().params.alloc(Param::Varargs),
                     None => {
                         let type_ref = TypeRef::from_ast_opt(&self.body_ctx, param.ty());
-                        let ty = self.data().type_refs.intern(type_ref);
+                        let ty = Interned::new(type_ref);
                         self.data().params.alloc(Param::Normal(ty))
                     }
                 };
@@ -389,41 +395,51 @@ impl Ctx {
             ret_type
         };
 
-        let ret_type = self.data().type_refs.intern(ret_type);
-
-        let has_body = func.body().is_some();
+        let abi = func.abi().map(|abi| {
+            // FIXME: Abi::abi() -> Option<SyntaxToken>?
+            match abi.syntax().last_token() {
+                Some(tok) if tok.kind() == SyntaxKind::STRING => {
+                    // FIXME: Better way to unescape?
+                    Interned::new_str(tok.text().trim_matches('"'))
+                }
+                _ => {
+                    // `extern` default to be `extern "C"`.
+                    Interned::new_str("C")
+                }
+            }
+        });
 
         let ast_id = self.source_ast_id_map.ast_id(func);
-        let qualifier = FunctionQualifier {
-            is_default: func.default_token().is_some(),
-            is_const: func.const_token().is_some(),
-            is_async: func.async_token().is_some(),
-            is_unsafe: func.unsafe_token().is_some(),
-            abi: func.abi().map(|abi| {
-                // FIXME: Abi::abi() -> Option<SyntaxToken>?
-                match abi.syntax().last_token() {
-                    Some(tok) if tok.kind() == SyntaxKind::STRING => {
-                        // FIXME: Better way to unescape?
-                        tok.text().trim_matches('"').into()
-                    }
-                    _ => {
-                        // `extern` default to be `extern "C"`.
-                        "C".into()
-                    }
-                }
-            }),
-        };
+
+        let mut flags = FnFlags::default();
+        if func.body().is_some() {
+            flags.bits |= FnFlags::HAS_BODY;
+        }
+        if has_self_param {
+            flags.bits |= FnFlags::HAS_SELF_PARAM;
+        }
+        if func.default_token().is_some() {
+            flags.bits |= FnFlags::IS_DEFAULT;
+        }
+        if func.const_token().is_some() {
+            flags.bits |= FnFlags::IS_CONST;
+        }
+        if func.async_token().is_some() {
+            flags.bits |= FnFlags::IS_ASYNC;
+        }
+        if func.unsafe_token().is_some() {
+            flags.bits |= FnFlags::IS_UNSAFE;
+        }
+
         let mut res = Function {
             name,
             visibility,
-            generic_params: GenericParamsId::EMPTY,
-            has_self_param,
-            has_body,
-            qualifier,
-            is_in_extern_block: false,
+            generic_params: Interned::new(GenericParams::default()),
+            abi,
             params,
-            ret_type,
+            ret_type: Interned::new(ret_type),
             ast_id,
+            flags,
         };
         res.generic_params = self.lower_generic_params(GenericsOwner::Function(&res), func);
 
@@ -536,8 +552,11 @@ impl Ctx {
     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()?);
+        // FIXME: If trait lowering fails, due to a non PathType for example, we treat this impl
+        // as if it was an non-trait impl. Ideally we want to create a unique missing ref that only
+        // equals itself.
+        let target_trait = impl_def.trait_().and_then(|tr| self.lower_trait_ref(&tr));
+        let self_ty = 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.
@@ -554,7 +573,7 @@ impl Ctx {
             })
             .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 };
+        let res = Impl { generic_params, target_trait, self_ty, is_negative, items, ast_id };
         Some(id(self.data().impls.alloc(res)))
     }
 
@@ -570,7 +589,7 @@ impl Ctx {
             &self.hygiene,
             |path, _use_tree, is_glob, alias| {
                 imports.push(id(tree.imports.alloc(Import {
-                    path,
+                    path: Interned::new(path),
                     alias,
                     visibility,
                     is_glob,
@@ -599,7 +618,7 @@ impl Ctx {
     }
 
     fn lower_macro_call(&mut self, m: &ast::MacroCall) -> Option<FileItemTreeId<MacroCall>> {
-        let path = ModPath::from_src(m.path()?, &self.hygiene)?;
+        let path = Interned::new(ModPath::from_src(m.path()?, &self.hygiene)?);
         let ast_id = self.source_ast_id_map.ast_id(m);
         let res = MacroCall { path, ast_id };
         Some(id(self.data().macro_calls.alloc(res)))
@@ -633,8 +652,10 @@ impl Ctx {
                         ast::ExternItem::Fn(ast) => {
                             let func_id = self.lower_function(&ast)?;
                             let func = &mut self.data().functions[func_id.index];
-                            func.qualifier.is_unsafe = is_intrinsic_fn_unsafe(&func.name);
-                            func.is_in_extern_block = true;
+                            if is_intrinsic_fn_unsafe(&func.name) {
+                                func.flags.bits |= FnFlags::IS_UNSAFE;
+                            }
+                            func.flags.bits |= FnFlags::IS_IN_EXTERN_BLOCK;
                             func_id.into()
                         }
                         ast::ExternItem::Static(ast) => {
@@ -661,7 +682,7 @@ impl Ctx {
         &mut self,
         owner: GenericsOwner<'_>,
         node: &impl ast::GenericParamsOwner,
-    ) -> GenericParamsId {
+    ) -> Interned<GenericParams> {
         // 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());
@@ -677,7 +698,7 @@ impl Ctx {
         &mut self,
         owner: GenericsOwner<'_>,
         node: &impl ast::GenericParamsOwner,
-    ) -> GenericParamsId {
+    ) -> Interned<GenericParams> {
         let mut sm = &mut Default::default();
         let mut generics = GenericParams::default();
         match owner {
@@ -685,8 +706,7 @@ impl Ctx {
                 generics.fill(&self.body_ctx, sm, node);
                 // lower `impl Trait` in arguments
                 for id in func.params.clone() {
-                    if let Param::Normal(ty) = self.data().params[id] {
-                        let ty = self.data().type_refs.lookup(ty);
+                    if let Param::Normal(ty) = &self.data().params[id] {
                         generics.fill_implicit_impl_trait_args(ty);
                     }
                 }
@@ -719,7 +739,8 @@ impl Ctx {
             }
         }
 
-        self.data().generics.alloc(generics)
+        generics.shrink_to_fit();
+        Interned::new(generics)
     }
 
     fn lower_type_bounds(&mut self, node: &impl ast::TypeBoundsOwner) -> Vec<TypeBound> {
@@ -740,14 +761,20 @@ impl Ctx {
         self.data().vis.alloc(vis)
     }
 
-    fn lower_type_ref(&mut self, type_ref: &ast::Type) -> Idx<TypeRef> {
+    fn lower_trait_ref(&mut self, trait_ref: &ast::Type) -> Option<Interned<TraitRef>> {
+        let trait_ref = TraitRef::from_ast(&self.body_ctx, trait_ref.clone())?;
+        Some(Interned::new(trait_ref))
+    }
+
+    fn lower_type_ref(&mut self, type_ref: &ast::Type) -> Interned<TypeRef> {
         let tyref = TypeRef::from_ast(&self.body_ctx, type_ref.clone());
-        self.data().type_refs.intern(tyref)
+        Interned::new(tyref)
     }
-    fn lower_type_ref_opt(&mut self, type_ref: Option<ast::Type>) -> Idx<TypeRef> {
+
+    fn lower_type_ref_opt(&mut self, type_ref: Option<ast::Type>) -> Interned<TypeRef> {
         match type_ref.map(|ty| self.lower_type_ref(&ty)) {
             Some(it) => it,
-            None => self.data().type_refs.intern(TypeRef::Error),
+            None => Interned::new(TypeRef::Error),
         }
     }
 
diff --git a/crates/hir_def/src/lib.rs b/crates/hir_def/src/lib.rs
index c9e07de86..be9a5e1a0 100644
--- a/crates/hir_def/src/lib.rs
+++ b/crates/hir_def/src/lib.rs
@@ -27,6 +27,7 @@ pub mod dyn_map;
 pub mod keys;
 
 pub mod item_tree;
+pub mod intern;
 
 pub mod adt;
 pub mod data;
diff --git a/crates/hir_def/src/nameres.rs b/crates/hir_def/src/nameres.rs
index 9e8e4e9ec..7dd68219f 100644
--- a/crates/hir_def/src/nameres.rs
+++ b/crates/hir_def/src/nameres.rs
@@ -409,6 +409,31 @@ impl DefMap {
             }
         }
     }
+
+    fn shrink_to_fit(&mut self) {
+        // Exhaustive match to require handling new fields.
+        let Self {
+            _c: _,
+            exported_proc_macros,
+            extern_prelude,
+            diagnostics,
+            modules,
+            block: _,
+            edition: _,
+            krate: _,
+            prelude: _,
+            root: _,
+        } = self;
+
+        extern_prelude.shrink_to_fit();
+        exported_proc_macros.shrink_to_fit();
+        diagnostics.shrink_to_fit();
+        modules.shrink_to_fit();
+        for (_, module) in modules.iter_mut() {
+            module.children.shrink_to_fit();
+            module.scope.shrink_to_fit();
+        }
+    }
 }
 
 impl ModuleData {
diff --git a/crates/hir_def/src/nameres/collector.rs b/crates/hir_def/src/nameres/collector.rs
index d58135ec9..4ddc791ce 100644
--- a/crates/hir_def/src/nameres/collector.rs
+++ b/crates/hir_def/src/nameres/collector.rs
@@ -23,6 +23,7 @@ use crate::{
     attr::Attrs,
     db::DefDatabase,
     derive_macro_as_call_id,
+    intern::Interned,
     item_scope::{ImportType, PerNsGlobImports},
     item_tree::{
         self, FileItemTreeId, ItemTree, ItemTreeId, MacroCall, MacroDef, MacroRules, Mod, ModItem,
@@ -54,20 +55,22 @@ pub(super) fn collect_defs(
 ) -> DefMap {
     let crate_graph = db.crate_graph();
 
-    // populate external prelude
-    for dep in &crate_graph[def_map.krate].dependencies {
-        log::debug!("crate dep {:?} -> {:?}", dep.name, dep.crate_id);
-        let dep_def_map = db.crate_def_map(dep.crate_id);
-        def_map
-            .extern_prelude
-            .insert(dep.as_name(), dep_def_map.module_id(dep_def_map.root).into());
-
-        // look for the prelude
-        // If the dependency defines a prelude, we overwrite an already defined
-        // prelude. This is necessary to import the "std" prelude if a crate
-        // depends on both "core" and "std".
-        if dep_def_map.prelude.is_some() {
-            def_map.prelude = dep_def_map.prelude;
+    if block.is_none() {
+        // populate external prelude
+        for dep in &crate_graph[def_map.krate].dependencies {
+            log::debug!("crate dep {:?} -> {:?}", dep.name, dep.crate_id);
+            let dep_def_map = db.crate_def_map(dep.crate_id);
+            def_map
+                .extern_prelude
+                .insert(dep.as_name(), dep_def_map.module_id(dep_def_map.root).into());
+
+            // look for the prelude
+            // If the dependency defines a prelude, we overwrite an already defined
+            // prelude. This is necessary to import the "std" prelude if a crate
+            // depends on both "core" and "std".
+            if dep_def_map.prelude.is_some() {
+                def_map.prelude = dep_def_map.prelude;
+            }
         }
     }
 
@@ -106,7 +109,9 @@ pub(super) fn collect_defs(
         }
     }
     collector.collect();
-    collector.finish()
+    let mut def_map = collector.finish();
+    def_map.shrink_to_fit();
+    def_map
 }
 
 #[derive(Copy, Clone, Debug, Eq, PartialEq)]
@@ -137,7 +142,7 @@ enum ImportSource {
 
 #[derive(Clone, Debug, Eq, PartialEq)]
 struct Import {
-    path: ModPath,
+    path: Interned<ModPath>,
     alias: Option<ImportAlias>,
     visibility: RawVisibility,
     is_glob: bool,
@@ -179,7 +184,10 @@ impl Import {
         let attrs = &tree.attrs(db, krate, ModItem::from(id.value).into());
         let visibility = &tree[it.visibility];
         Self {
-            path: ModPath::from_segments(PathKind::Plain, iter::once(it.name.clone())),
+            path: Interned::new(ModPath::from_segments(
+                PathKind::Plain,
+                iter::once(it.name.clone()),
+            )),
             alias: it.alias.clone(),
             visibility: visibility.clone(),
             is_glob: false,
@@ -1400,8 +1408,18 @@ impl ModCollector<'_, '_> {
 
         // Case 1: builtin macros
         if attrs.by_key("rustc_builtin_macro").exists() {
+            // `#[rustc_builtin_macro = "builtin_name"]` overrides the `macro_rules!` name.
+            let name;
+            let name = match attrs.by_key("rustc_builtin_macro").string_value() {
+                Some(it) => {
+                    // FIXME: a hacky way to create a Name from string.
+                    name = tt::Ident { text: it.clone(), id: tt::TokenId::unspecified() }.as_name();
+                    &name
+                }
+                None => &mac.name,
+            };
             let krate = self.def_collector.def_map.krate;
-            if let Some(macro_id) = find_builtin_macro(&mac.name, krate, ast_id) {
+            if let Some(macro_id) = find_builtin_macro(name, krate, ast_id) {
                 self.def_collector.define_macro_rules(
                     self.module_id,
                     mac.name.clone(),
@@ -1464,7 +1482,7 @@ impl ModCollector<'_, '_> {
     }
 
     fn collect_macro_call(&mut self, mac: &MacroCall) {
-        let mut ast_id = AstIdWithPath::new(self.file_id, mac.ast_id, mac.path.clone());
+        let mut ast_id = AstIdWithPath::new(self.file_id, mac.ast_id, (*mac.path).clone());
 
         // Case 1: try to resolve in legacy scope and expand macro_rules
         let mut error = None;
diff --git a/crates/hir_def/src/nameres/tests/diagnostics.rs b/crates/hir_def/src/nameres/tests/diagnostics.rs
index a89061c2e..fefdadb22 100644
--- a/crates/hir_def/src/nameres/tests/diagnostics.rs
+++ b/crates/hir_def/src/nameres/tests/diagnostics.rs
@@ -7,6 +7,11 @@ fn check_diagnostics(ra_fixture: &str) {
     db.check_diagnostics();
 }
 
+fn check_no_diagnostics(ra_fixture: &str) {
+    let db: TestDB = TestDB::with_files(ra_fixture);
+    db.check_no_diagnostics();
+}
+
 #[test]
 fn unresolved_import() {
     check_diagnostics(
@@ -202,6 +207,21 @@ fn builtin_macro_fails_expansion() {
 }
 
 #[test]
+fn include_macro_should_allow_empty_content() {
+    check_no_diagnostics(
+        r#"
+        //- /lib.rs
+          #[rustc_builtin_macro]
+          macro_rules! include { () => {} }
+
+          include!("bar.rs");
+        //- /bar.rs
+          // empty
+        "#,
+    );
+}
+
+#[test]
 fn good_out_dir_diagnostic() {
     check_diagnostics(
         r#"
diff --git a/crates/hir_def/src/path.rs b/crates/hir_def/src/path.rs
index 8c923bb7b..f9c8328f0 100644
--- a/crates/hir_def/src/path.rs
+++ b/crates/hir_def/src/path.rs
@@ -7,7 +7,7 @@ use std::{
     sync::Arc,
 };
 
-use crate::{body::LowerCtx, type_ref::LifetimeRef};
+use crate::{body::LowerCtx, intern::Interned, type_ref::LifetimeRef};
 use base_db::CrateId;
 use hir_expand::{
     hygiene::Hygiene,
@@ -48,7 +48,7 @@ pub enum ImportAlias {
 
 impl ModPath {
     pub fn from_src(path: ast::Path, hygiene: &Hygiene) -> Option<ModPath> {
-        lower::lower_path(path, hygiene).map(|it| it.mod_path)
+        lower::lower_path(path, hygiene).map(|it| (*it.mod_path).clone())
     }
 
     pub fn from_segments(kind: PathKind, segments: impl IntoIterator<Item = Name>) -> ModPath {
@@ -122,8 +122,8 @@ impl ModPath {
 pub struct Path {
     /// Type based path like `<T>::foo`.
     /// Note that paths like `<Type as Trait>::foo` are desugard to `Trait::<Self=Type>::foo`.
-    type_anchor: Option<Box<TypeRef>>,
-    mod_path: ModPath,
+    type_anchor: Option<Interned<TypeRef>>,
+    mod_path: Interned<ModPath>,
     /// Invariant: the same len as `self.mod_path.segments`
     generic_args: Vec<Option<Arc<GenericArgs>>>,
 }
@@ -176,7 +176,7 @@ impl Path {
         path: ModPath,
         generic_args: Vec<Option<Arc<GenericArgs>>>,
     ) -> Path {
-        Path { type_anchor: None, mod_path: path, generic_args }
+        Path { type_anchor: None, mod_path: Interned::new(path), generic_args }
     }
 
     pub fn kind(&self) -> &PathKind {
@@ -204,10 +204,10 @@ impl Path {
         }
         let res = Path {
             type_anchor: self.type_anchor.clone(),
-            mod_path: ModPath::from_segments(
+            mod_path: Interned::new(ModPath::from_segments(
                 self.mod_path.kind.clone(),
                 self.mod_path.segments[..self.mod_path.segments.len() - 1].iter().cloned(),
-            ),
+            )),
             generic_args: self.generic_args[..self.generic_args.len() - 1].to_vec(),
         };
         Some(res)
@@ -283,7 +283,7 @@ impl From<Name> for Path {
     fn from(name: Name) -> Path {
         Path {
             type_anchor: None,
-            mod_path: ModPath::from_segments(PathKind::Plain, iter::once(name)),
+            mod_path: Interned::new(ModPath::from_segments(PathKind::Plain, iter::once(name))),
             generic_args: vec![None],
         }
     }
diff --git a/crates/hir_def/src/path/lower.rs b/crates/hir_def/src/path/lower.rs
index 505493a74..7b29d9d4f 100644
--- a/crates/hir_def/src/path/lower.rs
+++ b/crates/hir_def/src/path/lower.rs
@@ -2,6 +2,7 @@
 
 mod lower_use;
 
+use crate::intern::Interned;
 use std::sync::Arc;
 
 use either::Either;
@@ -68,15 +69,17 @@ pub(super) fn lower_path(mut path: ast::Path, hygiene: &Hygiene) -> Option<Path>
                 match trait_ref {
                     // <T>::foo
                     None => {
-                        type_anchor = Some(Box::new(self_type));
+                        type_anchor = Some(Interned::new(self_type));
                         kind = PathKind::Plain;
                     }
                     // <T as Trait<A>>::Foo desugars to Trait<Self=T, A>::Foo
                     Some(trait_ref) => {
                         let path = Path::from_src(trait_ref.path()?, hygiene)?;
-                        kind = path.mod_path.kind;
+                        let mod_path = (*path.mod_path).clone();
+                        let num_segments = path.mod_path.segments.len();
+                        kind = mod_path.kind;
 
-                        let mut prefix_segments = path.mod_path.segments;
+                        let mut prefix_segments = mod_path.segments;
                         prefix_segments.reverse();
                         segments.extend(prefix_segments);
 
@@ -85,7 +88,8 @@ pub(super) fn lower_path(mut path: ast::Path, hygiene: &Hygiene) -> Option<Path>
                         generic_args.extend(prefix_args);
 
                         // Insert the type reference (T in the above example) as Self parameter for the trait
-                        let last_segment = generic_args.last_mut()?;
+                        let last_segment =
+                            generic_args.iter_mut().rev().nth(num_segments.saturating_sub(1))?;
                         if last_segment.is_none() {
                             *last_segment = Some(Arc::new(GenericArgs::empty()));
                         };
@@ -138,7 +142,7 @@ pub(super) fn lower_path(mut path: ast::Path, hygiene: &Hygiene) -> Option<Path>
         }
     }
 
-    let mod_path = ModPath::from_segments(kind, segments);
+    let mod_path = Interned::new(ModPath::from_segments(kind, segments));
     return Some(Path { type_anchor, mod_path, generic_args });
 
     fn qualifier(path: &ast::Path) -> Option<ast::Path> {
diff --git a/crates/hir_def/src/resolver.rs b/crates/hir_def/src/resolver.rs
index a73585ee7..0391cc49b 100644
--- a/crates/hir_def/src/resolver.rs
+++ b/crates/hir_def/src/resolver.rs
@@ -14,6 +14,7 @@ use crate::{
     db::DefDatabase,
     expr::{ExprId, LabelId, PatId},
     generics::GenericParams,
+    intern::Interned,
     item_scope::{BuiltinShadowMode, BUILTIN_SCOPE},
     nameres::DefMap,
     path::{ModPath, PathKind},
@@ -50,7 +51,7 @@ 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: Arc<GenericParams> },
+    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
diff --git a/crates/hir_def/src/test_db.rs b/crates/hir_def/src/test_db.rs
index 10977761c..dd36106f8 100644
--- a/crates/hir_def/src/test_db.rs
+++ b/crates/hir_def/src/test_db.rs
@@ -265,4 +265,17 @@ impl TestDB {
 
         assert_eq!(annotations, actual);
     }
+
+    pub(crate) fn check_no_diagnostics(&self) {
+        let db: &TestDB = self;
+        let annotations = db.extract_annotations();
+        assert!(annotations.is_empty());
+
+        let mut has_diagnostics = false;
+        db.diagnostics(|_| {
+            has_diagnostics = true;
+        });
+
+        assert!(!has_diagnostics);
+    }
 }
diff --git a/crates/hir_def/src/type_ref.rs b/crates/hir_def/src/type_ref.rs
index 049b2e462..4c24aae94 100644
--- a/crates/hir_def/src/type_ref.rs
+++ b/crates/hir_def/src/type_ref.rs
@@ -51,6 +51,23 @@ impl Rawness {
     }
 }
 
+#[derive(Clone, PartialEq, Eq, Hash, Debug)]
+pub struct TraitRef {
+    pub path: Path,
+}
+
+impl TraitRef {
+    /// Converts an `ast::PathType` to a `hir::TraitRef`.
+    pub(crate) fn from_ast(ctx: &LowerCtx, node: ast::Type) -> Option<Self> {
+        // FIXME: Use `Path::from_src`
+        match node {
+            ast::Type::PathType(path) => {
+                path.path().and_then(|it| ctx.lower_path(it)).map(|path| TraitRef { path })
+            }
+            _ => None,
+        }
+    }
+}
 /// Compare ty::Ty
 #[derive(Clone, PartialEq, Eq, Hash, Debug)]
 pub enum TypeRef {
diff --git a/crates/hir_def/src/visibility.rs b/crates/hir_def/src/visibility.rs
index 7d00a37c4..9908cd926 100644
--- a/crates/hir_def/src/visibility.rs
+++ b/crates/hir_def/src/visibility.rs
@@ -11,7 +11,7 @@ use crate::{
     nameres::DefMap,
     path::{ModPath, PathKind},
     resolver::HasResolver,
-    FunctionId, HasModule, LocalFieldId, ModuleDefId, ModuleId, VariantId,
+    FunctionId, HasModule, LocalFieldId, ModuleId, VariantId,
 };
 
 /// Visibility of an item, not yet resolved.
@@ -25,7 +25,7 @@ pub enum RawVisibility {
 }
 
 impl RawVisibility {
-    pub(crate) const fn private() -> RawVisibility {
+    pub(crate) fn private() -> RawVisibility {
         RawVisibility::Module(ModPath::from_kind(PathKind::Super(0)))
     }
 
@@ -217,6 +217,6 @@ pub(crate) fn field_visibilities_query(
 
 /// Resolve visibility of a function.
 pub(crate) fn function_visibility_query(db: &dyn DefDatabase, def: FunctionId) -> Visibility {
-    let resolver = ModuleDefId::from(def).module(db).unwrap().resolver(db);
+    let resolver = def.resolver(db);
     db.function_data(def).visibility.resolve(db, &resolver)
 }