diff --git a/crates/hir-def/src/body/lower.rs b/crates/hir-def/src/body/lower.rs
index a39d575a7a..d8ecde7102 100644
--- a/crates/hir-def/src/body/lower.rs
+++ b/crates/hir-def/src/body/lower.rs
@@ -1075,7 +1075,7 @@ impl ExprCollector<'_> {
             match block_id.map(|block_id| (self.db.block_def_map(block_id), block_id)) {
                 Some((def_map, block_id)) => {
                     self.body.block_scopes.push(block_id);
-                    (def_map.module_id(def_map.root()), def_map)
+                    (def_map.module_id(DefMap::ROOT), def_map)
                 }
                 None => (self.expander.module, self.def_map.clone()),
             };
diff --git a/crates/hir-def/src/body/tests/block.rs b/crates/hir-def/src/body/tests/block.rs
index 2899ca4337..6e77744f21 100644
--- a/crates/hir-def/src/body/tests/block.rs
+++ b/crates/hir-def/src/body/tests/block.rs
@@ -148,8 +148,8 @@ fn f() {
 }
     "#,
         expect![[r#"
-            BlockId(1) in ModuleId { krate: Idx::<CrateData>(0), block: Some(BlockId(0)), local_id: Idx::<ModuleData>(1) }
-            BlockId(0) in ModuleId { krate: Idx::<CrateData>(0), block: None, local_id: Idx::<ModuleData>(0) }
+            BlockId(1) in BlockRelativeModuleId { block: Some(BlockId(0)), local_id: Idx::<ModuleData>(1) }
+            BlockId(0) in BlockRelativeModuleId { block: None, local_id: Idx::<ModuleData>(0) }
             crate scope
         "#]],
     );
diff --git a/crates/hir-def/src/child_by_source.rs b/crates/hir-def/src/child_by_source.rs
index 21180fcbda..bb79e28f26 100644
--- a/crates/hir-def/src/child_by_source.rs
+++ b/crates/hir-def/src/child_by_source.rs
@@ -12,6 +12,7 @@ use crate::{
     db::DefDatabase,
     dyn_map::{keys, DynMap},
     item_scope::ItemScope,
+    nameres::DefMap,
     src::{HasChildSource, HasSource},
     AdtId, AssocItemId, DefWithBodyId, EnumId, EnumVariantId, FieldId, ImplId, Lookup, MacroId,
     ModuleDefId, ModuleId, TraitId, VariantId,
@@ -205,7 +206,7 @@ impl ChildBySource for DefWithBodyId {
         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, file_id);
+            def_map[DefMap::ROOT].scope.child_by_source_to(db, res, file_id);
         }
     }
 }
diff --git a/crates/hir-def/src/db.rs b/crates/hir-def/src/db.rs
index c3721a94f6..6d18e3f56c 100644
--- a/crates/hir-def/src/db.rs
+++ b/crates/hir-def/src/db.rs
@@ -198,15 +198,14 @@ pub trait DefDatabase: InternDatabase + ExpandDatabase + Upcast<dyn ExpandDataba
 
     // endregion:attrs
 
-    #[salsa::invoke(LangItems::crate_lang_items_query)]
-    fn crate_lang_items(&self, krate: CrateId) -> Arc<LangItems>;
-
     #[salsa::invoke(LangItems::lang_item_query)]
     fn lang_item(&self, start_crate: CrateId, item: LangItem) -> Option<LangItemTarget>;
 
     #[salsa::invoke(ImportMap::import_map_query)]
     fn import_map(&self, krate: CrateId) -> Arc<ImportMap>;
 
+    // region:visibilities
+
     #[salsa::invoke(visibility::field_visibilities_query)]
     fn field_visibilities(&self, var: VariantId) -> Arc<ArenaMap<LocalFieldId, Visibility>>;
 
@@ -217,8 +216,14 @@ pub trait DefDatabase: InternDatabase + ExpandDatabase + Upcast<dyn ExpandDataba
     #[salsa::invoke(visibility::const_visibility_query)]
     fn const_visibility(&self, def: ConstId) -> Visibility;
 
+    // endregion:visibilities
+
+    #[salsa::invoke(LangItems::crate_lang_items_query)]
+    fn crate_lang_items(&self, krate: CrateId) -> Arc<LangItems>;
+
     #[salsa::transparent]
     fn crate_limits(&self, crate_id: CrateId) -> CrateLimits;
+
     #[salsa::transparent]
     fn recursion_limit(&self, crate_id: CrateId) -> u32;
 
diff --git a/crates/hir-def/src/find_path.rs b/crates/hir-def/src/find_path.rs
index 1e299fecc9..e8cc2eab46 100644
--- a/crates/hir-def/src/find_path.rs
+++ b/crates/hir-def/src/find_path.rs
@@ -81,7 +81,7 @@ fn find_path_inner(
     }
 
     let def_map = from.def_map(db);
-    let crate_root = def_map.crate_root(db);
+    let crate_root = def_map.crate_root();
     // - if the item is a module, jump straight to module search
     if let ItemInNs::Types(ModuleDefId::ModuleId(module_id)) = item {
         let mut visited_modules = FxHashSet::default();
@@ -454,7 +454,7 @@ fn find_local_import_locations(
         worklist.push(ancestor);
     }
 
-    let def_map = def_map.crate_root(db).def_map(db);
+    let def_map = def_map.crate_root().def_map(db);
 
     let mut seen: FxHashSet<_> = FxHashSet::default();
 
diff --git a/crates/hir-def/src/import_map.rs b/crates/hir-def/src/import_map.rs
index 6ef2949ef5..ec150dc068 100644
--- a/crates/hir-def/src/import_map.rs
+++ b/crates/hir-def/src/import_map.rs
@@ -11,8 +11,8 @@ use rustc_hash::{FxHashSet, FxHasher};
 use triomphe::Arc;
 
 use crate::{
-    db::DefDatabase, item_scope::ItemInNs, visibility::Visibility, AssocItemId, ModuleDefId,
-    ModuleId, TraitId,
+    db::DefDatabase, item_scope::ItemInNs, nameres::DefMap, visibility::Visibility, AssocItemId,
+    ModuleDefId, ModuleId, TraitId,
 };
 
 type FxIndexMap<K, V> = IndexMap<K, V, BuildHasherDefault<FxHasher>>;
@@ -183,7 +183,7 @@ fn collect_import_map(db: &dyn DefDatabase, krate: CrateId) -> ImportMap {
 
     // We look only into modules that are public(ly reexported), starting with the crate root.
     let empty = ImportPath { segments: vec![] };
-    let root = def_map.module_id(def_map.root());
+    let root = def_map.module_id(DefMap::ROOT);
     let mut worklist = vec![(root, empty)];
     while let Some((module, mod_path)) = worklist.pop() {
         let ext_def_map;
diff --git a/crates/hir-def/src/macro_expansion_tests/mod.rs b/crates/hir-def/src/macro_expansion_tests/mod.rs
index 40849d4a66..4a62696df0 100644
--- a/crates/hir-def/src/macro_expansion_tests/mod.rs
+++ b/crates/hir-def/src/macro_expansion_tests/mod.rs
@@ -35,7 +35,7 @@ use tt::token_id::{Subtree, TokenId};
 use crate::{
     db::DefDatabase,
     macro_id_to_def_id,
-    nameres::{MacroSubNs, ModuleSource},
+    nameres::{DefMap, MacroSubNs, ModuleSource},
     resolver::HasResolver,
     src::HasSource,
     test_db::TestDB,
@@ -61,7 +61,7 @@ pub fn identity_when_valid(_attr: TokenStream, item: TokenStream) -> TokenStream
     let db = TestDB::with_files_extra_proc_macros(ra_fixture, extra_proc_macros);
     let krate = db.crate_graph().iter().next().unwrap();
     let def_map = db.crate_def_map(krate);
-    let local_id = def_map.root();
+    let local_id = DefMap::ROOT;
     let module = def_map.module_id(local_id);
     let resolver = module.resolver(&db);
     let source = def_map[local_id].definition_source(&db);
diff --git a/crates/hir-def/src/nameres.rs b/crates/hir-def/src/nameres.rs
index ccb9bed5c5..9b520bc303 100644
--- a/crates/hir-def/src/nameres.rs
+++ b/crates/hir-def/src/nameres.rs
@@ -94,7 +94,6 @@ use crate::{
 pub struct DefMap {
     _c: Count<Self>,
     block: Option<BlockInfo>,
-    root: LocalModuleId,
     modules: Arena<ModuleData>,
     krate: CrateId,
     /// The prelude module for this crate. This either comes from an import
@@ -104,20 +103,29 @@ pub struct DefMap {
     /// but that attribute is nightly and when used in a block, it affects resolution globally
     /// so we aren't handling this correctly anyways).
     prelude: Option<ModuleId>,
-    /// The extern prelude is only populated for non-block DefMaps
-    extern_prelude: FxHashMap<Name, ModuleId>,
     /// `macro_use` prelude that contains macros from `#[macro_use]`'d external crates. Note that
     /// this contains all kinds of macro, not just `macro_rules!` macro.
     macro_use_prelude: FxHashMap<Name, MacroId>,
 
+    /// Tracks which custom derives are in scope for an item, to allow resolution of derive helper
+    /// attributes.
+    derive_helpers_in_scope: FxHashMap<AstId<ast::Item>, Vec<(Name, MacroId, MacroCallId)>>,
+
+    diagnostics: Vec<DefDiagnostic>,
+
+    data: Arc<DefMapCrateData>,
+}
+
+/// Data that belongs to a crate which is shared between a crate's def map and all its block def maps.
+#[derive(Clone, Debug, PartialEq, Eq)]
+struct DefMapCrateData {
+    extern_prelude: FxHashMap<Name, ModuleId>,
+
     /// Side table for resolving derive helpers.
     exported_derives: FxHashMap<MacroDefId, Box<[Name]>>,
     fn_proc_macro_mapping: FxHashMap<FunctionId, ProcMacroId>,
     /// The error that occurred when failing to load the proc-macro dll.
     proc_macro_loading_error: Option<Box<str>>,
-    /// Tracks which custom derives are in scope for an item, to allow resolution of derive helper
-    /// attributes.
-    derive_helpers_in_scope: FxHashMap<AstId<ast::Item>, Vec<(Name, MacroId, MacroCallId)>>,
 
     /// Custom attributes registered with `#![register_attr]`.
     registered_attrs: Vec<SmolStr>,
@@ -132,7 +140,31 @@ pub struct DefMap {
 
     edition: Edition,
     recursion_limit: Option<u32>,
-    diagnostics: Vec<DefDiagnostic>,
+}
+
+impl DefMapCrateData {
+    fn shrink_to_fit(&mut self) {
+        let Self {
+            extern_prelude,
+            exported_derives,
+            fn_proc_macro_mapping,
+            registered_attrs,
+            registered_tools,
+            unstable_features,
+            proc_macro_loading_error: _,
+            rustc_coherence_is_core: _,
+            no_core: _,
+            no_std: _,
+            edition: _,
+            recursion_limit: _,
+        } = self;
+        extern_prelude.shrink_to_fit();
+        exported_derives.shrink_to_fit();
+        fn_proc_macro_mapping.shrink_to_fit();
+        registered_attrs.shrink_to_fit();
+        registered_tools.shrink_to_fit();
+        unstable_features.shrink_to_fit();
+    }
 }
 
 /// For `DefMap`s computed for a block expression, this stores its location in the parent map.
@@ -141,7 +173,23 @@ struct BlockInfo {
     /// The `BlockId` this `DefMap` was created from.
     block: BlockId,
     /// The containing module.
-    parent: ModuleId,
+    parent: BlockRelativeModuleId,
+}
+
+#[derive(Debug, PartialEq, Eq, Clone, Copy)]
+struct BlockRelativeModuleId {
+    block: Option<BlockId>,
+    local_id: LocalModuleId,
+}
+
+impl BlockRelativeModuleId {
+    fn def_map(self, db: &dyn DefDatabase, krate: CrateId) -> Arc<DefMap> {
+        self.into_module(krate).def_map(db)
+    }
+
+    fn into_module(self, krate: CrateId) -> ModuleId {
+        ModuleId { krate, block: self.block, local_id: self.local_id }
+    }
 }
 
 impl std::ops::Index<LocalModuleId> for DefMap {
@@ -231,6 +279,8 @@ pub struct ModuleData {
 }
 
 impl DefMap {
+    pub const ROOT: LocalModuleId = LocalModuleId::from_raw(la_arena::RawIdx::from_u32(0));
+
     pub(crate) fn crate_def_map_query(db: &dyn DefDatabase, krate: CrateId) -> Arc<DefMap> {
         let _p = profile::span("crate_def_map_query").detail(|| {
             db.crate_graph()[krate].display_name.as_deref().unwrap_or_default().to_string()
@@ -265,8 +315,15 @@ impl DefMap {
         let module_data =
             ModuleData::new(ModuleOrigin::BlockExpr { block: block.ast_id }, visibility);
 
-        let mut def_map = DefMap::empty(krate, parent_map.edition, module_data);
-        def_map.block = Some(BlockInfo { block: block_id, parent: block.module });
+        let mut def_map = DefMap::empty(krate, parent_map.data.edition, module_data);
+        def_map.data = parent_map.data.clone();
+        def_map.block = Some(BlockInfo {
+            block: block_id,
+            parent: BlockRelativeModuleId {
+                block: block.module.block,
+                local_id: block.module.local_id,
+            },
+        });
 
         let def_map = collector::collect_defs(db, def_map, tree_id);
         Arc::new(def_map)
@@ -275,29 +332,31 @@ impl DefMap {
     fn empty(krate: CrateId, edition: Edition, module_data: ModuleData) -> DefMap {
         let mut modules: Arena<ModuleData> = Arena::default();
         let root = modules.alloc(module_data);
+        assert_eq!(root, Self::ROOT);
 
         DefMap {
             _c: Count::new(),
             block: None,
-            krate,
-            edition,
-            recursion_limit: None,
-            extern_prelude: FxHashMap::default(),
-            macro_use_prelude: FxHashMap::default(),
-            exported_derives: FxHashMap::default(),
-            fn_proc_macro_mapping: FxHashMap::default(),
-            proc_macro_loading_error: None,
-            derive_helpers_in_scope: FxHashMap::default(),
-            prelude: None,
-            root,
             modules,
-            registered_attrs: Vec::new(),
-            registered_tools: Vec::new(),
-            unstable_features: FxHashSet::default(),
+            krate,
+            prelude: None,
+            macro_use_prelude: FxHashMap::default(),
+            derive_helpers_in_scope: FxHashMap::default(),
             diagnostics: Vec::new(),
-            rustc_coherence_is_core: false,
-            no_core: false,
-            no_std: false,
+            data: Arc::new(DefMapCrateData {
+                extern_prelude: FxHashMap::default(),
+                exported_derives: FxHashMap::default(),
+                fn_proc_macro_mapping: FxHashMap::default(),
+                proc_macro_loading_error: None,
+                registered_attrs: Vec::new(),
+                registered_tools: Vec::new(),
+                unstable_features: FxHashSet::default(),
+                rustc_coherence_is_core: false,
+                no_core: false,
+                no_std: false,
+                edition,
+                recursion_limit: None,
+            }),
         }
     }
 
@@ -320,35 +379,31 @@ impl DefMap {
     }
 
     pub fn registered_tools(&self) -> &[SmolStr] {
-        &self.registered_tools
+        &self.data.registered_tools
     }
 
     pub fn registered_attrs(&self) -> &[SmolStr] {
-        &self.registered_attrs
+        &self.data.registered_attrs
     }
 
     pub fn is_unstable_feature_enabled(&self, feature: &str) -> bool {
-        self.unstable_features.contains(feature)
+        self.data.unstable_features.contains(feature)
     }
 
     pub fn is_rustc_coherence_is_core(&self) -> bool {
-        self.rustc_coherence_is_core
+        self.data.rustc_coherence_is_core
     }
 
     pub fn is_no_std(&self) -> bool {
-        self.no_std || self.no_core
-    }
-
-    pub fn root(&self) -> LocalModuleId {
-        self.root
+        self.data.no_std || self.data.no_core
     }
 
     pub fn fn_as_proc_macro(&self, id: FunctionId) -> Option<ProcMacroId> {
-        self.fn_proc_macro_mapping.get(&id).copied()
+        self.data.fn_proc_macro_mapping.get(&id).copied()
     }
 
     pub fn proc_macro_loading_error(&self) -> Option<&str> {
-        self.proc_macro_loading_error.as_deref()
+        self.data.proc_macro_loading_error.as_deref()
     }
 
     pub fn krate(&self) -> CrateId {
@@ -364,7 +419,7 @@ impl DefMap {
     }
 
     pub(crate) fn extern_prelude(&self) -> impl Iterator<Item = (&Name, ModuleId)> + '_ {
-        self.extern_prelude.iter().map(|(name, def)| (name, *def))
+        self.data.extern_prelude.iter().map(|(name, def)| (name, *def))
     }
 
     pub(crate) fn macro_use_prelude(&self) -> impl Iterator<Item = (&Name, MacroId)> + '_ {
@@ -376,15 +431,8 @@ impl DefMap {
         ModuleId { krate: self.krate, local_id, block }
     }
 
-    pub(crate) fn crate_root(&self, db: &dyn DefDatabase) -> ModuleId {
-        self.with_ancestor_maps(db, self.root, &mut |def_map, _module| {
-            if def_map.block.is_none() {
-                Some(def_map.module_id(def_map.root))
-            } else {
-                None
-            }
-        })
-        .expect("DefMap chain without root")
+    pub(crate) fn crate_root(&self) -> ModuleId {
+        ModuleId { krate: self.krate, block: None, local_id: DefMap::ROOT }
     }
 
     pub(crate) fn resolve_path(
@@ -439,7 +487,7 @@ impl DefMap {
         }
         let mut block = self.block;
         while let Some(block_info) = block {
-            let parent = block_info.parent.def_map(db);
+            let parent = block_info.parent.def_map(db, self.krate);
             if let Some(it) = f(&parent, block_info.parent.local_id) {
                 return Some(it);
             }
@@ -452,7 +500,8 @@ impl DefMap {
     /// If this `DefMap` is for a block expression, returns the module containing the block (which
     /// might again be a block, or a module inside a block).
     pub fn parent(&self) -> Option<ModuleId> {
-        Some(self.block?.parent)
+        let BlockRelativeModuleId { block, local_id } = self.block?.parent;
+        Some(ModuleId { krate: self.krate, block, local_id })
     }
 
     /// Returns the module containing `local_mod`, either the parent `mod`, or the module (or block) containing
@@ -460,7 +509,13 @@ impl DefMap {
     pub fn containing_module(&self, local_mod: LocalModuleId) -> Option<ModuleId> {
         match self[local_mod].parent {
             Some(parent) => Some(self.module_id(parent)),
-            None => self.block.map(|block| block.parent),
+            None => {
+                self.block.map(
+                    |BlockInfo { parent: BlockRelativeModuleId { block, local_id }, .. }| {
+                        ModuleId { krate: self.krate, block, local_id }
+                    },
+                )
+            }
         }
     }
 
@@ -471,12 +526,12 @@ impl DefMap {
         let mut arc;
         let mut current_map = self;
         while let Some(block) = current_map.block {
-            go(&mut buf, db, current_map, "block scope", current_map.root);
+            go(&mut buf, db, current_map, "block scope", Self::ROOT);
             buf.push('\n');
-            arc = block.parent.def_map(db);
+            arc = block.parent.def_map(db, self.krate);
             current_map = &arc;
         }
-        go(&mut buf, db, current_map, "crate", current_map.root);
+        go(&mut buf, db, current_map, "crate", Self::ROOT);
         return buf;
 
         fn go(
@@ -506,7 +561,7 @@ impl DefMap {
         let mut current_map = self;
         while let Some(block) = current_map.block {
             format_to!(buf, "{:?} in {:?}\n", block.block, block.parent);
-            arc = block.parent.def_map(db);
+            arc = block.parent.def_map(db, self.krate);
             current_map = &arc;
         }
 
@@ -518,38 +573,20 @@ impl DefMap {
         // Exhaustive match to require handling new fields.
         let Self {
             _c: _,
-            exported_derives,
-            extern_prelude,
             macro_use_prelude,
             diagnostics,
             modules,
-            registered_attrs,
-            registered_tools,
-            fn_proc_macro_mapping,
             derive_helpers_in_scope,
-            unstable_features,
-            proc_macro_loading_error: _,
             block: _,
-            edition: _,
-            recursion_limit: _,
             krate: _,
             prelude: _,
-            root: _,
-            rustc_coherence_is_core: _,
-            no_core: _,
-            no_std: _,
+            data: _,
         } = self;
 
-        extern_prelude.shrink_to_fit();
         macro_use_prelude.shrink_to_fit();
-        exported_derives.shrink_to_fit();
         diagnostics.shrink_to_fit();
         modules.shrink_to_fit();
-        registered_attrs.shrink_to_fit();
-        registered_tools.shrink_to_fit();
-        fn_proc_macro_mapping.shrink_to_fit();
         derive_helpers_in_scope.shrink_to_fit();
-        unstable_features.shrink_to_fit();
         for (_, module) in modules.iter_mut() {
             module.children.shrink_to_fit();
             module.scope.shrink_to_fit();
@@ -562,7 +599,7 @@ impl DefMap {
     }
 
     pub fn recursion_limit(&self) -> Option<u32> {
-        self.recursion_limit
+        self.data.recursion_limit
     }
 }
 
diff --git a/crates/hir-def/src/nameres/attr_resolution.rs b/crates/hir-def/src/nameres/attr_resolution.rs
index 6567bda709..a7abf44591 100644
--- a/crates/hir-def/src/nameres/attr_resolution.rs
+++ b/crates/hir-def/src/nameres/attr_resolution.rs
@@ -78,7 +78,7 @@ impl DefMap {
             let name = name.to_smol_str();
             let pred = |n: &_| *n == name;
 
-            let registered = self.registered_tools.iter().map(SmolStr::as_str);
+            let registered = self.data.registered_tools.iter().map(SmolStr::as_str);
             let is_tool = TOOL_MODULES.iter().copied().chain(registered).any(pred);
             // FIXME: tool modules can be shadowed by actual modules
             if is_tool {
@@ -86,7 +86,7 @@ impl DefMap {
             }
 
             if segments.len() == 1 {
-                let mut registered = self.registered_attrs.iter().map(SmolStr::as_str);
+                let mut registered = self.data.registered_attrs.iter().map(SmolStr::as_str);
                 let is_inert = find_builtin_attr_idx(&name).is_some() || registered.any(pred);
                 return is_inert;
             }
diff --git a/crates/hir-def/src/nameres/collector.rs b/crates/hir-def/src/nameres/collector.rs
index 64caf26299..06542b4b1e 100644
--- a/crates/hir-def/src/nameres/collector.rs
+++ b/crates/hir-def/src/nameres/collector.rs
@@ -5,7 +5,7 @@
 
 use std::{iter, mem};
 
-use base_db::{CrateId, Edition, FileId};
+use base_db::{CrateId, Dependency, Edition, FileId};
 use cfg::{CfgExpr, CfgOptions};
 use either::Either;
 use hir_expand::{
@@ -62,7 +62,7 @@ static GLOB_RECURSION_LIMIT: Limit = Limit::new(100);
 static EXPANSION_DEPTH_LIMIT: Limit = Limit::new(128);
 static FIXED_POINT_LIMIT: Limit = Limit::new(8192);
 
-pub(super) fn collect_defs(db: &dyn DefDatabase, mut def_map: DefMap, tree_id: TreeId) -> DefMap {
+pub(super) fn collect_defs(db: &dyn DefDatabase, def_map: DefMap, tree_id: TreeId) -> DefMap {
     let crate_graph = db.crate_graph();
 
     let mut deps = FxHashMap::default();
@@ -70,14 +70,8 @@ pub(super) fn collect_defs(db: &dyn DefDatabase, mut def_map: DefMap, tree_id: T
     let krate = &crate_graph[def_map.krate];
     for dep in &krate.dependencies {
         tracing::debug!("crate dep {:?} -> {:?}", dep.name, dep.crate_id);
-        let dep_def_map = db.crate_def_map(dep.crate_id);
-        let dep_root = dep_def_map.module_id(dep_def_map.root);
 
-        deps.insert(dep.as_name(), dep_root);
-
-        if dep.is_prelude() && !tree_id.is_block() {
-            def_map.extern_prelude.insert(dep.as_name(), dep_root);
-        }
+        deps.insert(dep.as_name(), dep.clone());
     }
 
     let cfg_options = &krate.cfg_options;
@@ -86,7 +80,7 @@ pub(super) fn collect_defs(db: &dyn DefDatabase, mut def_map: DefMap, tree_id: T
     let proc_macros = if is_proc_macro {
         match db.proc_macros().get(&def_map.krate) {
             Some(Ok(proc_macros)) => {
-                proc_macros
+                Ok(proc_macros
                     .iter()
                     .enumerate()
                     .map(|(idx, it)| {
@@ -95,20 +89,13 @@ pub(super) fn collect_defs(db: &dyn DefDatabase, mut def_map: DefMap, tree_id: T
                             tt::Ident { text: it.name.clone(), span: tt::TokenId::unspecified() };
                         (name.as_name(), ProcMacroExpander::new(base_db::ProcMacroId(idx as u32)))
                     })
-                    .collect()
-            }
-            Some(Err(e)) => {
-                def_map.proc_macro_loading_error = Some(e.clone().into_boxed_str());
-                Vec::new()
-            }
-            None => {
-                def_map.proc_macro_loading_error =
-                    Some("No proc-macros present for crate".to_owned().into_boxed_str());
-                Vec::new()
+                    .collect())
             }
+            Some(Err(e)) => Err(e.clone().into_boxed_str()),
+            None => Err("No proc-macros present for crate".to_owned().into_boxed_str()),
         }
     } else {
-        vec![]
+        Ok(vec![])
     };
 
     let mut collector = DefCollector {
@@ -252,7 +239,7 @@ enum MacroDirectiveKind {
 struct DefCollector<'a> {
     db: &'a dyn DefDatabase,
     def_map: DefMap,
-    deps: FxHashMap<Name, ModuleId>,
+    deps: FxHashMap<Name, Dependency>,
     glob_imports: FxHashMap<LocalModuleId, Vec<(LocalModuleId, Visibility)>>,
     unresolved_imports: Vec<ImportDirective>,
     indeterminate_imports: Vec<ImportDirective>,
@@ -263,7 +250,7 @@ struct DefCollector<'a> {
     /// built by the build system, and is the list of proc. macros we can actually expand. It is
     /// empty when proc. macro support is disabled (in which case we still do name resolution for
     /// them).
-    proc_macros: Vec<(Name, ProcMacroExpander)>,
+    proc_macros: Result<Vec<(Name, ProcMacroExpander)>, Box<str>>,
     is_proc_macro: bool,
     from_glob_import: PerNsGlobImports,
     /// If we fail to resolve an attribute on a `ModItem`, we fall back to ignoring the attribute.
@@ -287,9 +274,23 @@ impl DefCollector<'_> {
 
         let file_id = self.db.crate_graph()[self.def_map.krate].root_file_id;
         let item_tree = self.db.file_item_tree(file_id.into());
-        let module_id = self.def_map.root;
+        let module_id = DefMap::ROOT;
 
         let attrs = item_tree.top_level_attrs(self.db, self.def_map.krate);
+        let crate_data = Arc::get_mut(&mut self.def_map.data).unwrap();
+
+        if let Err(e) = &self.proc_macros {
+            crate_data.proc_macro_loading_error = Some(e.clone());
+        }
+
+        for (name, dep) in &self.deps {
+            if dep.is_prelude() {
+                crate_data.extern_prelude.insert(
+                    name.clone(),
+                    ModuleId { krate: dep.crate_id, block: None, local_id: DefMap::ROOT },
+                );
+            }
+        }
 
         // Process other crate-level attributes.
         for attr in &*attrs {
@@ -306,7 +307,7 @@ impl DefCollector<'_> {
             if *attr_name == hir_expand::name![recursion_limit] {
                 if let Some(limit) = attr.string_value() {
                     if let Ok(limit) = limit.parse() {
-                        self.def_map.recursion_limit = Some(limit);
+                        crate_data.recursion_limit = Some(limit);
                     }
                 }
                 continue;
@@ -320,17 +321,17 @@ impl DefCollector<'_> {
             }
 
             if *attr_name == hir_expand::name![no_core] {
-                self.def_map.no_core = true;
+                crate_data.no_core = true;
                 continue;
             }
 
             if *attr_name == hir_expand::name![no_std] {
-                self.def_map.no_std = true;
+                crate_data.no_std = true;
                 continue;
             }
 
             if attr_name.as_text().as_deref() == Some("rustc_coherence_is_core") {
-                self.def_map.rustc_coherence_is_core = true;
+                crate_data.rustc_coherence_is_core = true;
                 continue;
             }
 
@@ -344,7 +345,7 @@ impl DefCollector<'_> {
                         [name] => Some(name.to_smol_str()),
                         _ => None,
                     });
-                self.def_map.unstable_features.extend(features);
+                crate_data.unstable_features.extend(features);
             }
 
             let attr_is_register_like = *attr_name == hir_expand::name![register_attr]
@@ -359,14 +360,15 @@ impl DefCollector<'_> {
             };
 
             if *attr_name == hir_expand::name![register_attr] {
-                self.def_map.registered_attrs.push(registered_name.to_smol_str());
+                crate_data.registered_attrs.push(registered_name.to_smol_str());
                 cov_mark::hit!(register_attr);
             } else {
-                self.def_map.registered_tools.push(registered_name.to_smol_str());
+                crate_data.registered_tools.push(registered_name.to_smol_str());
                 cov_mark::hit!(register_tool);
             }
         }
 
+        crate_data.shrink_to_fit();
         self.inject_prelude();
 
         ModCollector {
@@ -382,7 +384,7 @@ impl DefCollector<'_> {
 
     fn seed_with_inner(&mut self, tree_id: TreeId) {
         let item_tree = tree_id.item_tree(self.db);
-        let module_id = self.def_map.root;
+        let module_id = DefMap::ROOT;
 
         let is_cfg_enabled = item_tree
             .top_level_attrs(self.db, self.def_map.krate)
@@ -464,7 +466,7 @@ impl DefCollector<'_> {
             // Additionally, while the proc macro entry points must be `pub`, they are not publicly
             // exported in type/value namespace. This function reduces the visibility of all items
             // in the crate root that aren't proc macros.
-            let root = self.def_map.root;
+            let root = DefMap::ROOT;
             let module_id = self.def_map.module_id(root);
             let root = &mut self.def_map.modules[root];
             root.scope.censor_non_proc_macros(module_id);
@@ -530,12 +532,12 @@ impl DefCollector<'_> {
     fn inject_prelude(&mut self) {
         // See compiler/rustc_builtin_macros/src/standard_library_imports.rs
 
-        if self.def_map.no_core {
+        if self.def_map.data.no_core {
             // libcore does not get a prelude.
             return;
         }
 
-        let krate = if self.def_map.no_std {
+        let krate = if self.def_map.data.no_std {
             name![core]
         } else {
             let std = name![std];
@@ -548,25 +550,20 @@ impl DefCollector<'_> {
             }
         };
 
-        let edition = match self.def_map.edition {
+        let edition = match self.def_map.data.edition {
             Edition::Edition2015 => name![rust_2015],
             Edition::Edition2018 => name![rust_2018],
             Edition::Edition2021 => name![rust_2021],
         };
 
-        let path_kind = match self.def_map.edition {
+        let path_kind = match self.def_map.data.edition {
             Edition::Edition2015 => PathKind::Plain,
             _ => PathKind::Abs,
         };
         let path = ModPath::from_segments(path_kind, [krate, name![prelude], edition]);
 
-        let (per_ns, _) = self.def_map.resolve_path(
-            self.db,
-            self.def_map.root,
-            &path,
-            BuiltinShadowMode::Other,
-            None,
-        );
+        let (per_ns, _) =
+            self.def_map.resolve_path(self.db, DefMap::ROOT, &path, BuiltinShadowMode::Other, None);
 
         match per_ns.types {
             Some((ModuleDefId::ModuleId(m), _)) => {
@@ -603,23 +600,29 @@ impl DefCollector<'_> {
         def: ProcMacroDef,
         id: ItemTreeId<item_tree::Function>,
         fn_id: FunctionId,
-        module_id: ModuleId,
     ) {
+        if self.def_map.block.is_some() {
+            return;
+        }
+        let crate_root = self.def_map.module_id(DefMap::ROOT);
+
         let kind = def.kind.to_basedb_kind();
-        let (expander, kind) = match self.proc_macros.iter().find(|(n, _)| n == &def.name) {
-            Some(&(_, expander)) => (expander, kind),
-            None => (ProcMacroExpander::dummy(), kind),
-        };
+        let (expander, kind) =
+            match self.proc_macros.as_ref().map(|it| it.iter().find(|(n, _)| n == &def.name)) {
+                Ok(Some(&(_, expander))) => (expander, kind),
+                _ => (ProcMacroExpander::dummy(), kind),
+            };
 
         let proc_macro_id =
-            ProcMacroLoc { container: module_id, id, expander, kind }.intern(self.db);
+            ProcMacroLoc { container: crate_root, id, expander, kind }.intern(self.db);
         self.define_proc_macro(def.name.clone(), proc_macro_id);
+        let crate_data = Arc::get_mut(&mut self.def_map.data).unwrap();
         if let ProcMacroKind::CustomDerive { helpers } = def.kind {
-            self.def_map
+            crate_data
                 .exported_derives
                 .insert(macro_id_to_def_id(self.db, proc_macro_id.into()), helpers);
         }
-        self.def_map.fn_proc_macro_mapping.insert(fn_id, proc_macro_id);
+        crate_data.fn_proc_macro_mapping.insert(fn_id, proc_macro_id);
     }
 
     /// Define a macro with `macro_rules`.
@@ -661,7 +664,7 @@ impl DefCollector<'_> {
         // In Rust, `#[macro_export]` macros are unconditionally visible at the
         // crate root, even if the parent modules is **not** visible.
         if export {
-            let module_id = self.def_map.root;
+            let module_id = DefMap::ROOT;
             self.def_map.modules[module_id].scope.declare(macro_.into());
             self.update(
                 module_id,
@@ -712,7 +715,7 @@ impl DefCollector<'_> {
     /// A proc macro is similar to normal macro scope, but it would not visible in legacy textual scoped.
     /// And unconditionally exported.
     fn define_proc_macro(&mut self, name: Name, macro_: ProcMacroId) {
-        let module_id = self.def_map.root;
+        let module_id = DefMap::ROOT;
         self.def_map.modules[module_id].scope.declare(macro_.into());
         self.update(
             module_id,
@@ -732,7 +735,7 @@ impl DefCollector<'_> {
         let def_map = self.db.crate_def_map(krate);
         // `#[macro_use]` brings macros into macro_use prelude. Yes, even non-`macro_rules!`
         // macros.
-        let root_scope = &def_map[def_map.root].scope;
+        let root_scope = &def_map[DefMap::ROOT].scope;
         if let Some(names) = names {
             for name in names {
                 // FIXME: Report diagnostic on 404.
@@ -778,7 +781,7 @@ impl DefCollector<'_> {
     fn resolve_import(&self, module_id: LocalModuleId, import: &Import) -> PartialResolvedImport {
         let _p = profile::span("resolve_import")
             .detail(|| format!("{}", import.path.display(self.db.upcast())));
-        tracing::debug!("resolving import: {:?} ({:?})", import, self.def_map.edition);
+        tracing::debug!("resolving import: {:?} ({:?})", import, self.def_map.data.edition);
         if import.is_extern_crate {
             let name = import
                 .path
@@ -831,16 +834,13 @@ impl DefCollector<'_> {
     fn resolve_extern_crate(&self, name: &Name) -> Option<ModuleId> {
         if *name == name!(self) {
             cov_mark::hit!(extern_crate_self_as);
-            let root = match self.def_map.block {
-                Some(_) => {
-                    let def_map = self.def_map.crate_root(self.db).def_map(self.db);
-                    def_map.module_id(def_map.root())
-                }
-                None => self.def_map.module_id(self.def_map.root()),
-            };
-            Some(root)
+            Some(self.def_map.crate_root())
         } else {
-            self.deps.get(name).copied()
+            self.deps.get(name).map(|dep| ModuleId {
+                krate: dep.crate_id,
+                block: None,
+                local_id: DefMap::ROOT,
+            })
         }
     }
 
@@ -879,11 +879,14 @@ impl DefCollector<'_> {
                 // extern crates in the crate root are special-cased to insert entries into the extern prelude: rust-lang/rust#54658
                 if import.is_extern_crate
                     && self.def_map.block.is_none()
-                    && module_id == self.def_map.root
+                    && module_id == DefMap::ROOT
                 {
                     if let (Some(ModuleDefId::ModuleId(def)), Some(name)) = (def.take_types(), name)
                     {
-                        self.def_map.extern_prelude.insert(name.clone(), def);
+                        Arc::get_mut(&mut self.def_map.data)
+                            .unwrap()
+                            .extern_prelude
+                            .insert(name.clone(), def);
                     }
                 }
 
@@ -1158,7 +1161,7 @@ impl DefCollector<'_> {
                         // Record its helper attributes.
                         if def_id.krate != self.def_map.krate {
                             let def_map = self.db.crate_def_map(def_id.krate);
-                            if let Some(helpers) = def_map.exported_derives.get(&def_id) {
+                            if let Some(helpers) = def_map.data.exported_derives.get(&def_id) {
                                 self.def_map
                                     .derive_helpers_in_scope
                                     .entry(ast_id.ast_id.map(|it| it.upcast()))
@@ -1525,7 +1528,7 @@ impl ModCollector<'_, '_> {
 
     fn collect(&mut self, items: &[ModItem], container: ItemContainerId) {
         let krate = self.def_collector.def_map.krate;
-        let is_crate_root = self.module_id == self.def_collector.def_map.root;
+        let is_crate_root = self.module_id == DefMap::ROOT;
 
         // Note: don't assert that inserted value is fresh: it's simply not true
         // for macros.
@@ -1641,14 +1644,12 @@ impl ModCollector<'_, '_> {
                         FunctionLoc { container, id: ItemTreeId::new(self.tree_id, id) }.intern(db);
 
                     let vis = resolve_vis(def_map, &self.item_tree[it.visibility]);
-                    if self.def_collector.is_proc_macro && self.module_id == def_map.root {
+                    if self.def_collector.is_proc_macro && self.module_id == DefMap::ROOT {
                         if let Some(proc_macro) = attrs.parse_proc_macro_decl(&it.name) {
-                            let crate_root = def_map.module_id(def_map.root);
                             self.def_collector.export_proc_macro(
                                 proc_macro,
                                 ItemTreeId::new(self.tree_id, id),
                                 fn_id,
-                                crate_root,
                             );
                         }
                     }
@@ -2169,10 +2170,12 @@ impl ModCollector<'_, '_> {
             &self.item_tree[mac.visibility],
         );
         if let Some(helpers) = helpers_opt {
-            self.def_collector
-                .def_map
-                .exported_derives
-                .insert(macro_id_to_def_id(self.def_collector.db, macro_id.into()), helpers);
+            if self.def_collector.def_map.block.is_none() {
+                Arc::get_mut(&mut self.def_collector.def_map.data)
+                    .unwrap()
+                    .exported_derives
+                    .insert(macro_id_to_def_id(self.def_collector.db, macro_id.into()), helpers);
+            }
         }
     }
 
@@ -2280,7 +2283,7 @@ mod tests {
             unresolved_macros: Vec::new(),
             mod_dirs: FxHashMap::default(),
             cfg_options: &CfgOptions::default(),
-            proc_macros: Default::default(),
+            proc_macros: Ok(vec![]),
             from_glob_import: Default::default(),
             skip_attrs: Default::default(),
             is_proc_macro: false,
diff --git a/crates/hir-def/src/nameres/path_resolution.rs b/crates/hir-def/src/nameres/path_resolution.rs
index 24dc4e243b..5f6163175a 100644
--- a/crates/hir-def/src/nameres/path_resolution.rs
+++ b/crates/hir-def/src/nameres/path_resolution.rs
@@ -20,7 +20,7 @@ use crate::{
     path::{ModPath, PathKind},
     per_ns::PerNs,
     visibility::{RawVisibility, Visibility},
-    AdtId, CrateId, EnumVariantId, LocalModuleId, ModuleDefId, ModuleId,
+    AdtId, CrateId, EnumVariantId, LocalModuleId, ModuleDefId,
 };
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -74,17 +74,6 @@ impl PerNs {
 }
 
 impl DefMap {
-    pub(super) fn resolve_name_in_extern_prelude(
-        &self,
-        db: &dyn DefDatabase,
-        name: &Name,
-    ) -> Option<ModuleId> {
-        match self.block {
-            Some(_) => self.crate_root(db).def_map(db).extern_prelude.get(name).copied(),
-            None => self.extern_prelude.get(name).copied(),
-        }
-    }
-
     pub(crate) fn resolve_visibility(
         &self,
         db: &dyn DefDatabase,
@@ -121,7 +110,7 @@ impl DefMap {
             // ...unless we're resolving visibility for an associated item in an impl.
             if self.block_id() != m.block && !within_impl {
                 cov_mark::hit!(adjust_vis_in_block_def_map);
-                vis = Visibility::Module(self.module_id(self.root()));
+                vis = Visibility::Module(self.module_id(Self::ROOT));
                 tracing::debug!("visibility {:?} points outside DefMap, adjusting to {:?}", m, vis);
             }
         }
@@ -173,7 +162,7 @@ impl DefMap {
             match &current_map.block {
                 Some(block) => {
                     original_module = block.parent.local_id;
-                    arc = block.parent.def_map(db);
+                    arc = block.parent.def_map(db, current_map.krate);
                     current_map = &*arc;
                 }
                 None => return result,
@@ -204,21 +193,21 @@ impl DefMap {
             PathKind::DollarCrate(krate) => {
                 if krate == self.krate {
                     cov_mark::hit!(macro_dollar_crate_self);
-                    PerNs::types(self.crate_root(db).into(), Visibility::Public)
+                    PerNs::types(self.crate_root().into(), Visibility::Public)
                 } else {
                     let def_map = db.crate_def_map(krate);
-                    let module = def_map.module_id(def_map.root);
+                    let module = def_map.module_id(Self::ROOT);
                     cov_mark::hit!(macro_dollar_crate_other);
                     PerNs::types(module.into(), Visibility::Public)
                 }
             }
-            PathKind::Crate => PerNs::types(self.crate_root(db).into(), Visibility::Public),
+            PathKind::Crate => PerNs::types(self.crate_root().into(), Visibility::Public),
             // plain import or absolute path in 2015: crate-relative with
             // fallback to extern prelude (with the simplification in
             // rust-lang/rust#57745)
             // FIXME there must be a nicer way to write this condition
             PathKind::Plain | PathKind::Abs
-                if self.edition == Edition::Edition2015
+                if self.data.edition == Edition::Edition2015
                     && (path.kind == PathKind::Abs || mode == ResolveMode::Import) =>
             {
                 let (_, segment) = match segments.next() {
@@ -268,14 +257,17 @@ impl DefMap {
                                     path.display(db.upcast()),
                                     new_path.display(db.upcast())
                                 );
-                                return block.parent.def_map(db).resolve_path_fp_with_macro(
-                                    db,
-                                    mode,
-                                    block.parent.local_id,
-                                    &new_path,
-                                    shadow,
-                                    expected_macro_subns,
-                                );
+                                return block
+                                    .parent
+                                    .def_map(db, self.krate)
+                                    .resolve_path_fp_with_macro(
+                                        db,
+                                        mode,
+                                        block.parent.local_id,
+                                        &new_path,
+                                        shadow,
+                                        expected_macro_subns,
+                                    );
                             }
                             None => {
                                 tracing::debug!("super path in root module");
@@ -301,7 +293,7 @@ impl DefMap {
                     Some((_, segment)) => segment,
                     None => return ResolvePathResult::empty(ReachedFixedPoint::Yes),
                 };
-                if let Some(&def) = self.extern_prelude.get(segment) {
+                if let Some(&def) = self.data.extern_prelude.get(segment) {
                     tracing::debug!("absolute path {:?} resolved to crate {:?}", path, def);
                     PerNs::types(def.into(), Visibility::Public)
                 } else {
@@ -450,7 +442,12 @@ impl DefMap {
         };
 
         let extern_prelude = || {
-            self.extern_prelude
+            if self.block.is_some() {
+                // Don't resolve extern prelude in block `DefMap`s.
+                return PerNs::none();
+            }
+            self.data
+                .extern_prelude
                 .get(name)
                 .map_or(PerNs::none(), |&it| PerNs::types(it.into(), Visibility::Public))
         };
@@ -475,13 +472,20 @@ impl DefMap {
     ) -> PerNs {
         let from_crate_root = match self.block {
             Some(_) => {
-                let def_map = self.crate_root(db).def_map(db);
-                def_map[def_map.root].scope.get(name)
+                let def_map = self.crate_root().def_map(db);
+                def_map[Self::ROOT].scope.get(name)
             }
-            None => self[self.root].scope.get(name),
+            None => self[Self::ROOT].scope.get(name),
         };
         let from_extern_prelude = || {
-            self.resolve_name_in_extern_prelude(db, name)
+            if self.block.is_some() {
+                // Don't resolve extern prelude in block `DefMap`s.
+                return PerNs::none();
+            }
+            self.data
+                .extern_prelude
+                .get(name)
+                .copied()
                 .map_or(PerNs::none(), |it| PerNs::types(it.into(), Visibility::Public))
         };
 
diff --git a/crates/hir-def/src/nameres/tests/macros.rs b/crates/hir-def/src/nameres/tests/macros.rs
index 57f0233607..f4cca8d68d 100644
--- a/crates/hir-def/src/nameres/tests/macros.rs
+++ b/crates/hir-def/src/nameres/tests/macros.rs
@@ -750,7 +750,7 @@ macro_rules! foo {
 pub use core::clone::Clone;
 "#,
     );
-    assert_eq!(map.modules[map.root].scope.impls().len(), 1);
+    assert_eq!(map.modules[DefMap::ROOT].scope.impls().len(), 1);
 }
 
 #[test]
@@ -772,7 +772,7 @@ pub macro Copy {}
 pub macro Clone {}
 "#,
     );
-    assert_eq!(map.modules[map.root].scope.impls().len(), 2);
+    assert_eq!(map.modules[DefMap::ROOT].scope.impls().len(), 2);
 }
 
 #[test]
@@ -815,7 +815,7 @@ pub macro derive($item:item) {}
 pub macro Clone {}
 "#,
     );
-    assert_eq!(map.modules[map.root].scope.impls().len(), 1);
+    assert_eq!(map.modules[DefMap::ROOT].scope.impls().len(), 1);
 }
 
 #[test]
@@ -1094,8 +1094,8 @@ pub fn derive_macro_2(_item: TokenStream) -> TokenStream {
     let krate = db.crate_graph().iter().next().unwrap();
     let def_map = db.crate_def_map(krate);
 
-    assert_eq!(def_map.exported_derives.len(), 1);
-    match def_map.exported_derives.values().next() {
+    assert_eq!(def_map.data.exported_derives.len(), 1);
+    match def_map.data.exported_derives.values().next() {
         Some(helpers) => match &**helpers {
             [attr] => assert_eq!(attr.display(&db).to_string(), "helper_attr"),
             _ => unreachable!(),
@@ -1286,7 +1286,7 @@ fn proc_attr(a: TokenStream, b: TokenStream) -> TokenStream { a }
     let krate = db.crate_graph().iter().next().unwrap();
     let def_map = db.crate_def_map(krate);
 
-    let root_module = &def_map[def_map.root()].scope;
+    let root_module = &def_map[DefMap::ROOT].scope;
     assert!(
         root_module.legacy_macros().count() == 0,
         "`#[macro_use]` shouldn't bring macros into textual macro scope",
@@ -1392,7 +1392,7 @@ macro_rules! derive { () => {} }
 struct S;
     "#,
     );
-    assert_eq!(map.modules[map.root].scope.impls().len(), 1);
+    assert_eq!(map.modules[DefMap::ROOT].scope.impls().len(), 1);
 }
 
 #[test]
diff --git a/crates/hir-def/src/resolver.rs b/crates/hir-def/src/resolver.rs
index 3eaff61b15..06f5b2526a 100644
--- a/crates/hir-def/src/resolver.rs
+++ b/crates/hir-def/src/resolver.rs
@@ -586,8 +586,9 @@ impl Resolver {
             }));
             if let Some(block) = expr_scopes.block(scope_id) {
                 let def_map = db.block_def_map(block);
-                let root = def_map.root();
-                resolver.scopes.push(Scope::BlockScope(ModuleItemMap { def_map, module_id: root }));
+                resolver
+                    .scopes
+                    .push(Scope::BlockScope(ModuleItemMap { def_map, module_id: DefMap::ROOT }));
                 // FIXME: This adds as many module scopes as there are blocks, but resolving in each
                 // already traverses all parents, so this is O(n²). I think we could only store the
                 // innermost module scope instead?
@@ -753,8 +754,7 @@ fn resolver_for_scope_(
     for scope in scope_chain.into_iter().rev() {
         if let Some(block) = scopes.block(scope) {
             let def_map = db.block_def_map(block);
-            let root = def_map.root();
-            r = r.push_block_scope(def_map, root);
+            r = r.push_block_scope(def_map, DefMap::ROOT);
             // FIXME: This adds as many module scopes as there are blocks, but resolving in each
             // already traverses all parents, so this is O(n²). I think we could only store the
             // innermost module scope instead?
diff --git a/crates/hir-def/src/test_db.rs b/crates/hir-def/src/test_db.rs
index d4b4031364..a6befc8a81 100644
--- a/crates/hir-def/src/test_db.rs
+++ b/crates/hir-def/src/test_db.rs
@@ -110,7 +110,7 @@ impl TestDB {
                 }
                 _ => {
                     // FIXME: handle `mod` inside block expression
-                    return def_map.module_id(def_map.root());
+                    return def_map.module_id(DefMap::ROOT);
                 }
             }
         }
@@ -119,7 +119,7 @@ impl TestDB {
     /// Finds the smallest/innermost module in `def_map` containing `position`.
     fn mod_at_position(&self, def_map: &DefMap, position: FilePosition) -> LocalModuleId {
         let mut size = None;
-        let mut res = def_map.root();
+        let mut res = DefMap::ROOT;
         for (module, data) in def_map.modules() {
             let src = data.definition_source(self);
             if src.file_id != position.file_id.into() {
diff --git a/crates/hir-ty/src/display.rs b/crates/hir-ty/src/display.rs
index 058d5059b1..32f50cb051 100644
--- a/crates/hir-ty/src/display.rs
+++ b/crates/hir-ty/src/display.rs
@@ -13,6 +13,7 @@ use hir_def::{
     generics::{TypeOrConstParamData, TypeParamProvenance},
     item_scope::ItemInNs,
     lang_item::{LangItem, LangItemTarget},
+    nameres::DefMap,
     path::{Path, PathKind},
     type_ref::{TraitBoundModifier, TypeBound, TypeRef},
     visibility::Visibility,
@@ -1488,7 +1489,7 @@ pub fn write_visibility(
         Visibility::Public => write!(f, "pub "),
         Visibility::Module(vis_id) => {
             let def_map = module_id.def_map(f.db.upcast());
-            let root_module_id = def_map.module_id(def_map.root());
+            let root_module_id = def_map.module_id(DefMap::ROOT);
             if vis_id == module_id {
                 // pub(self) or omitted
                 Ok(())
diff --git a/crates/hir/src/display.rs b/crates/hir/src/display.rs
index 0c8793c6df..9a2090ab79 100644
--- a/crates/hir/src/display.rs
+++ b/crates/hir/src/display.rs
@@ -548,7 +548,7 @@ impl HirDisplay for Module {
         // FIXME: Module doesn't have visibility saved in data.
         match self.name(f.db) {
             Some(name) => write!(f, "mod {}", name.display(f.db.upcast())),
-            None if self.is_crate_root(f.db) => match self.krate(f.db).display_name(f.db) {
+            None if self.is_crate_root() => match self.krate(f.db).display_name(f.db) {
                 Some(name) => write!(f, "extern crate {name}"),
                 None => f.write_str("extern crate {unknown}"),
             },
diff --git a/crates/hir/src/lib.rs b/crates/hir/src/lib.rs
index 7c432197a6..19709bb44a 100644
--- a/crates/hir/src/lib.rs
+++ b/crates/hir/src/lib.rs
@@ -118,7 +118,7 @@ pub use {
         find_path::PrefixKind,
         import_map,
         lang_item::LangItem,
-        nameres::ModuleSource,
+        nameres::{DefMap, ModuleSource},
         path::{ModPath, PathKind},
         type_ref::{Mutability, TypeRef},
         visibility::Visibility,
@@ -202,7 +202,7 @@ impl Crate {
 
     pub fn root_module(self, db: &dyn HirDatabase) -> Module {
         let def_map = db.crate_def_map(self.id);
-        Module { id: def_map.module_id(def_map.root()) }
+        Module { id: def_map.module_id(DefMap::ROOT) }
     }
 
     pub fn modules(self, db: &dyn HirDatabase) -> Vec<Module> {
@@ -475,12 +475,11 @@ impl Module {
     /// in the module tree of any target in `Cargo.toml`.
     pub fn crate_root(self, db: &dyn HirDatabase) -> Module {
         let def_map = db.crate_def_map(self.id.krate());
-        Module { id: def_map.module_id(def_map.root()) }
+        Module { id: def_map.module_id(DefMap::ROOT) }
     }
 
-    pub fn is_crate_root(self, db: &dyn HirDatabase) -> bool {
-        let def_map = db.crate_def_map(self.id.krate());
-        def_map.root() == self.id.local_id
+    pub fn is_crate_root(self) -> bool {
+        DefMap::ROOT == self.id.local_id
     }
 
     /// Iterates over all child modules.
diff --git a/crates/ide-completion/src/completions.rs b/crates/ide-completion/src/completions.rs
index d53e9e2fa9..480cb77b4f 100644
--- a/crates/ide-completion/src/completions.rs
+++ b/crates/ide-completion/src/completions.rs
@@ -158,7 +158,7 @@ impl Completions {
         path_ctx: &PathCompletionCtx,
     ) {
         ctx.process_all_names(&mut |name, res, doc_aliases| match res {
-            ScopeDef::ModuleDef(hir::ModuleDef::Module(m)) if m.is_crate_root(ctx.db) => {
+            ScopeDef::ModuleDef(hir::ModuleDef::Module(m)) if m.is_crate_root() => {
                 self.add_module(ctx, path_ctx, m, name, doc_aliases);
             }
             _ => (),
diff --git a/crates/ide-db/src/helpers.rs b/crates/ide-db/src/helpers.rs
index 8e3b1eef15..eba9d8afc4 100644
--- a/crates/ide-db/src/helpers.rs
+++ b/crates/ide-db/src/helpers.rs
@@ -77,7 +77,7 @@ pub fn visit_file_defs(
     }
     module.impl_defs(db).into_iter().for_each(|impl_| cb(impl_.into()));
 
-    let is_root = module.is_crate_root(db);
+    let is_root = module.is_crate_root();
     module
         .legacy_macros(db)
         .into_iter()
diff --git a/crates/ide-db/src/rename.rs b/crates/ide-db/src/rename.rs
index 5b2558ea8f..52a23b4b8f 100644
--- a/crates/ide-db/src/rename.rs
+++ b/crates/ide-db/src/rename.rs
@@ -178,7 +178,7 @@ fn rename_mod(
 
     let mut source_change = SourceChange::default();
 
-    if module.is_crate_root(sema.db) {
+    if module.is_crate_root() {
         return Ok(source_change);
     }
 
diff --git a/crates/ide-db/src/search.rs b/crates/ide-db/src/search.rs
index 9d00c71709..73cd5dcaf2 100644
--- a/crates/ide-db/src/search.rs
+++ b/crates/ide-db/src/search.rs
@@ -225,7 +225,7 @@ impl Definition {
         // def is crate root
         // FIXME: We don't do searches for crates currently, as a crate does not actually have a single name
         if let &Definition::Module(module) = self {
-            if module.is_crate_root(db) {
+            if module.is_crate_root() {
                 return SearchScope::reverse_dependencies(db, module.krate());
             }
         }
@@ -392,7 +392,7 @@ impl<'a> FindUsages<'a> {
 
         let name = match self.def {
             // special case crate modules as these do not have a proper name
-            Definition::Module(module) if module.is_crate_root(self.sema.db) => {
+            Definition::Module(module) if module.is_crate_root() => {
                 // FIXME: This assumes the crate name is always equal to its display name when it really isn't
                 module
                     .krate()
@@ -500,7 +500,7 @@ impl<'a> FindUsages<'a> {
             let scope =
                 search_scope.intersection(&SearchScope::module_and_children(self.sema.db, module));
 
-            let is_crate_root = module.is_crate_root(self.sema.db).then(|| Finder::new("crate"));
+            let is_crate_root = module.is_crate_root().then(|| Finder::new("crate"));
             let finder = &Finder::new("super");
 
             for (text, file_id, search_range) in scope_files(sema, &scope) {
diff --git a/crates/ide-diagnostics/src/handlers/unlinked_file.rs b/crates/ide-diagnostics/src/handlers/unlinked_file.rs
index b9c5384ea1..271e7ce73b 100644
--- a/crates/ide-diagnostics/src/handlers/unlinked_file.rs
+++ b/crates/ide-diagnostics/src/handlers/unlinked_file.rs
@@ -2,7 +2,7 @@
 
 use std::iter;
 
-use hir::{db::DefDatabase, InFile, ModuleSource};
+use hir::{db::DefDatabase, DefMap, InFile, ModuleSource};
 use ide_db::{
     base_db::{FileId, FileLoader, SourceDatabase, SourceDatabaseExt},
     source_change::SourceChange,
@@ -74,7 +74,7 @@ fn fixes(ctx: &DiagnosticsContext<'_>, file_id: FileId) -> Option<Vec<Assist>> {
     'crates: for &krate in &*ctx.sema.db.relevant_crates(file_id) {
         let crate_def_map = ctx.sema.db.crate_def_map(krate);
 
-        let root_module = &crate_def_map[crate_def_map.root()];
+        let root_module = &crate_def_map[DefMap::ROOT];
         let Some(root_file_id) = root_module.origin.file_id() else { continue };
         let Some(crate_root_path) = source_root.path_for_file(&root_file_id) else { continue };
         let Some(rel) = parent.strip_prefix(&crate_root_path.parent()?) else { continue };
diff --git a/crates/ide/src/status.rs b/crates/ide/src/status.rs
index 70bff41121..d2c77e2dc7 100644
--- a/crates/ide/src/status.rs
+++ b/crates/ide/src/status.rs
@@ -1,7 +1,7 @@
 use std::{fmt, marker::PhantomData};
 
 use hir::{
-    db::{AstIdMapQuery, AttrsQuery, ParseMacroExpansionQuery},
+    db::{AstIdMapQuery, AttrsQuery, BlockDefMapQuery, ParseMacroExpansionQuery},
     Attr, Attrs, ExpandResult, MacroFile, Module,
 };
 use ide_db::{
@@ -51,6 +51,7 @@ pub(crate) fn status(db: &RootDatabase, file_id: Option<FileId>) -> String {
     format_to!(buf, "\nDebug info:\n");
     format_to!(buf, "{}\n", collect_query(AttrsQuery.in_db(db)));
     format_to!(buf, "{} ast id maps\n", collect_query_count(AstIdMapQuery.in_db(db)));
+    format_to!(buf, "{} block def maps\n", collect_query_count(BlockDefMapQuery.in_db(db)));
 
     if let Some(file_id) = file_id {
         format_to!(buf, "\nFile info:\n");
diff --git a/crates/ide/src/syntax_highlighting/highlight.rs b/crates/ide/src/syntax_highlighting/highlight.rs
index 925057ffaa..3c40246a69 100644
--- a/crates/ide/src/syntax_highlighting/highlight.rs
+++ b/crates/ide/src/syntax_highlighting/highlight.rs
@@ -340,7 +340,7 @@ fn highlight_def(
         Definition::Field(_) => Highlight::new(HlTag::Symbol(SymbolKind::Field)),
         Definition::Module(module) => {
             let mut h = Highlight::new(HlTag::Symbol(SymbolKind::Module));
-            if module.is_crate_root(db) {
+            if module.is_crate_root() {
                 h |= HlMod::CrateRoot;
             }
             h
diff --git a/lib/la-arena/src/lib.rs b/lib/la-arena/src/lib.rs
index 1f8ef01a5b..5107f29439 100644
--- a/lib/la-arena/src/lib.rs
+++ b/lib/la-arena/src/lib.rs
@@ -18,6 +18,18 @@ pub use map::{ArenaMap, Entry, OccupiedEntry, VacantEntry};
 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct RawIdx(u32);
 
+impl RawIdx {
+    /// Constructs a [`RawIdx`] from a u32.
+    pub const fn from_u32(u32: u32) -> Self {
+        RawIdx(u32)
+    }
+
+    /// Deconstructs a [`RawIdx`] into the underlying u32.
+    pub const fn into_u32(self) -> u32 {
+        self.0
+    }
+}
+
 impl From<RawIdx> for u32 {
     #[inline]
     fn from(raw: RawIdx) -> u32 {
@@ -94,12 +106,12 @@ impl<T> fmt::Debug for Idx<T> {
 
 impl<T> Idx<T> {
     /// Creates a new index from a [`RawIdx`].
-    pub fn from_raw(raw: RawIdx) -> Self {
+    pub const fn from_raw(raw: RawIdx) -> Self {
         Idx { raw, _ty: PhantomData }
     }
 
     /// Converts this index into the underlying [`RawIdx`].
-    pub fn into_raw(self) -> RawIdx {
+    pub const fn into_raw(self) -> RawIdx {
         self.raw
     }
 }