diff --git a/crates/hir-def/src/item_tree.rs b/crates/hir-def/src/item_tree.rs
index 85a43de6ff..5b9da17231 100644
--- a/crates/hir-def/src/item_tree.rs
+++ b/crates/hir-def/src/item_tree.rs
@@ -133,7 +133,8 @@ pub(crate) fn block_item_tree_query(db: &dyn DefDatabase, block: BlockId) -> Arc
 
     let ctx = lower::Ctx::new(db, loc.ast_id.file_id);
     let mut item_tree = ctx.lower_block(&block);
-    if item_tree.data.is_empty()
+    if item_tree.small_data.is_empty()
+        && item_tree.big_data.is_empty()
         && item_tree.top_level.is_empty()
         && item_tree.attrs.is_empty()
         && item_tree.top_attrs.is_empty()
@@ -143,7 +144,8 @@ pub(crate) fn block_item_tree_query(db: &dyn DefDatabase, block: BlockId) -> Arc
                 Arc::new(ItemTree {
                     top_level: Box::new([]),
                     attrs: FxHashMap::default(),
-                    data: FxHashMap::default(),
+                    small_data: FxHashMap::default(),
+                    big_data: FxHashMap::default(),
                     top_attrs: RawAttrs::EMPTY,
                     vis: ItemVisibilities { arena: ThinVec::new() },
                 })
@@ -162,7 +164,8 @@ pub struct ItemTree {
     attrs: FxHashMap<FileAstId<ast::Item>, RawAttrs>,
     vis: ItemVisibilities,
     // FIXME: They values store the key, turn this into a FxHashSet<ModItem> instead?
-    data: FxHashMap<FileAstId<ast::Item>, ModItem>,
+    big_data: FxHashMap<FileAstId<ast::Item>, BigModItem>,
+    small_data: FxHashMap<FileAstId<ast::Item>, SmallModItem>,
 }
 
 impl ItemTree {
@@ -204,13 +207,18 @@ impl ItemTree {
         let mut mods = 0;
         let mut macro_calls = 0;
         let mut macro_rules = 0;
-        for item in self.data.values() {
+        for item in self.small_data.values() {
             match item {
-                ModItem::Trait(_) => traits += 1,
-                ModItem::Impl(_) => impls += 1,
-                ModItem::Mod(_) => mods += 1,
-                ModItem::MacroCall(_) => macro_calls += 1,
-                ModItem::MacroRules(_) => macro_rules += 1,
+                SmallModItem::Trait(_) => traits += 1,
+                SmallModItem::Impl(_) => impls += 1,
+                SmallModItem::MacroRules(_) => macro_rules += 1,
+                _ => {}
+            }
+        }
+        for item in self.big_data.values() {
+            match item {
+                BigModItem::Mod(_) => mods += 1,
+                BigModItem::MacroCall(_) => macro_calls += 1,
                 _ => {}
             }
         }
@@ -222,9 +230,10 @@ impl ItemTree {
     }
 
     fn shrink_to_fit(&mut self) {
-        let ItemTree { top_level: _, attrs, data, vis: _, top_attrs: _ } = self;
+        let ItemTree { top_level: _, attrs, big_data, small_data, vis: _, top_attrs: _ } = self;
         attrs.shrink_to_fit();
-        data.shrink_to_fit();
+        big_data.shrink_to_fit();
+        small_data.shrink_to_fit();
     }
 }
 
@@ -234,35 +243,37 @@ struct ItemVisibilities {
 }
 
 #[derive(Debug, Clone, Eq, PartialEq)]
-enum ModItem {
+enum SmallModItem {
     Const(Const),
     Enum(Enum),
-    // 32
-    ExternBlock(ExternBlock),
-    // 40
-    ExternCrate(ExternCrate),
     Function(Function),
     Impl(Impl),
     Macro2(Macro2),
-    // 32
-    MacroCall(MacroCall),
     MacroRules(MacroRules),
-    // 40
-    Mod(Mod),
     Static(Static),
-    // 32
-    Struct(Struct),
     Trait(Trait),
     TraitAlias(TraitAlias),
     TypeAlias(TypeAlias),
     Union(Union),
-    // 40
+}
+
+#[derive(Debug, Clone, Eq, PartialEq)]
+enum BigModItem {
+    ExternBlock(ExternBlock),
+    ExternCrate(ExternCrate),
+    MacroCall(MacroCall),
+    Mod(Mod),
+    Struct(Struct),
     Use(Use),
 }
 
-// `ModItem` is stored a bunch in `ItemTree`'s so we pay the max for each item. It should stay as small as possible.
+// `ModItem` is stored a bunch in `ItemTree`'s so we pay the max for each item. It should stay as
+// small as possible which is why we split them in two, most common ones are 3 usize but some rarer
+// ones are 5.
 #[cfg(target_pointer_width = "64")]
-const _: [(); std::mem::size_of::<ModItem>()] = [(); std::mem::size_of::<[usize; 5]>()];
+const _: [(); std::mem::size_of::<BigModItem>()] = [(); std::mem::size_of::<[usize; 5]>()];
+#[cfg(target_pointer_width = "64")]
+const _: [(); std::mem::size_of::<SmallModItem>()] = [(); std::mem::size_of::<[usize; 3]>()];
 
 #[derive(Default, Debug, Eq, PartialEq)]
 pub struct ItemTreeDataStats {
@@ -343,9 +354,12 @@ macro_rules! mod_items {
             impl Index<FileAstId<$ast>> for ItemTree {
                 type Output = $typ;
 
+                #[allow(unused_imports)]
                 fn index(&self, index: FileAstId<$ast>) -> &Self::Output {
-                    match &self.data[&index.upcast()] {
-                        ModItem::$typ(item) => item,
+                    use BigModItem::*;
+                    use SmallModItem::*;
+                    match &self.$fld[&index.upcast()] {
+                        $typ(item) => item,
                         _ => panic!("expected item of type `{}` at index `{:?}`", stringify!($typ), index),
                     }
                 }
@@ -356,23 +370,23 @@ macro_rules! mod_items {
 
 mod_items! {
 ModItemId ->
-    Use in uses -> ast::Use,
-    ExternCrate in extern_crates -> ast::ExternCrate,
-    ExternBlock in extern_blocks -> ast::ExternBlock,
-    Function in functions -> ast::Fn,
-    Struct in structs -> ast::Struct,
-    Union in unions -> ast::Union,
-    Enum in enums -> ast::Enum,
-    Const in consts -> ast::Const,
-    Static in statics -> ast::Static,
-    Trait in traits -> ast::Trait,
-    TraitAlias in trait_aliases -> ast::TraitAlias,
-    Impl in impls -> ast::Impl,
-    TypeAlias in type_aliases -> ast::TypeAlias,
-    Mod in mods -> ast::Module,
-    MacroCall in macro_calls -> ast::MacroCall,
-    MacroRules in macro_rules -> ast::MacroRules,
-    Macro2 in macro_defs -> ast::MacroDef,
+    Use in big_data -> ast::Use,
+    ExternCrate in big_data -> ast::ExternCrate,
+    ExternBlock in big_data -> ast::ExternBlock,
+    Function in small_data -> ast::Fn,
+    Struct in big_data -> ast::Struct,
+    Union in small_data -> ast::Union,
+    Enum in small_data -> ast::Enum,
+    Const in small_data -> ast::Const,
+    Static in small_data -> ast::Static,
+    Trait in small_data -> ast::Trait,
+    TraitAlias in small_data -> ast::TraitAlias,
+    Impl in small_data -> ast::Impl,
+    TypeAlias in small_data -> ast::TypeAlias,
+    Mod in big_data -> ast::Module,
+    MacroCall in big_data -> ast::MacroCall,
+    MacroRules in small_data -> ast::MacroRules,
+    Macro2 in small_data -> ast::MacroDef,
 }
 
 impl Index<RawVisibilityId> for ItemTree {
@@ -421,6 +435,7 @@ pub struct UseTree {
 }
 
 // FIXME: Would be nice to encode `None` into this
+// We could just use a `Name` where `_` well means `_` ..
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum ImportAlias {
     /// Unnamed alias, as in `use Foo as _;`
diff --git a/crates/hir-def/src/item_tree/lower.rs b/crates/hir-def/src/item_tree/lower.rs
index ab60b7ad70..6ae745bb9a 100644
--- a/crates/hir-def/src/item_tree/lower.rs
+++ b/crates/hir-def/src/item_tree/lower.rs
@@ -20,10 +20,11 @@ use triomphe::Arc;
 use crate::{
     db::DefDatabase,
     item_tree::{
-        Const, Enum, ExternBlock, ExternCrate, FieldsShape, Function, Impl, ImportAlias, Interned,
-        ItemTree, ItemTreeAstId, Macro2, MacroCall, MacroRules, Mod, ModItem, ModItemId, ModKind,
-        ModPath, RawAttrs, RawVisibility, RawVisibilityId, Static, Struct, StructKind, Trait,
-        TraitAlias, TypeAlias, Union, Use, UseTree, UseTreeKind, VisibilityExplicitness,
+        BigModItem, Const, Enum, ExternBlock, ExternCrate, FieldsShape, Function, Impl,
+        ImportAlias, Interned, ItemTree, ItemTreeAstId, Macro2, MacroCall, MacroRules, Mod,
+        ModItemId, ModKind, ModPath, RawAttrs, RawVisibility, RawVisibilityId, SmallModItem,
+        Static, Struct, StructKind, Trait, TraitAlias, TypeAlias, Union, Use, UseTree, UseTreeKind,
+        VisibilityExplicitness,
     },
 };
 
@@ -168,7 +169,7 @@ impl<'a> Ctx<'a> {
         let ast_id = self.source_ast_id_map.ast_id(strukt);
         let shape = adt_shape(strukt.kind());
         let res = Struct { name, visibility, shape, ast_id };
-        self.tree.data.insert(ast_id.upcast(), ModItem::Struct(res));
+        self.tree.big_data.insert(ast_id.upcast(), BigModItem::Struct(res));
 
         Some(ast_id)
     }
@@ -178,7 +179,7 @@ impl<'a> Ctx<'a> {
         let name = union.name()?.as_name();
         let ast_id = self.source_ast_id_map.ast_id(union);
         let res = Union { name, visibility, ast_id };
-        self.tree.data.insert(ast_id.upcast(), ModItem::Union(res));
+        self.tree.small_data.insert(ast_id.upcast(), SmallModItem::Union(res));
         Some(ast_id)
     }
 
@@ -187,7 +188,7 @@ impl<'a> Ctx<'a> {
         let name = enum_.name()?.as_name();
         let ast_id = self.source_ast_id_map.ast_id(enum_);
         let res = Enum { name, visibility, ast_id };
-        self.tree.data.insert(ast_id.upcast(), ModItem::Enum(res));
+        self.tree.small_data.insert(ast_id.upcast(), SmallModItem::Enum(res));
         Some(ast_id)
     }
 
@@ -199,7 +200,7 @@ impl<'a> Ctx<'a> {
 
         let res = Function { name, visibility, ast_id };
 
-        self.tree.data.insert(ast_id.upcast(), ModItem::Function(res));
+        self.tree.small_data.insert(ast_id.upcast(), SmallModItem::Function(res));
         Some(ast_id)
     }
 
@@ -211,7 +212,7 @@ impl<'a> Ctx<'a> {
         let visibility = self.lower_visibility(type_alias);
         let ast_id = self.source_ast_id_map.ast_id(type_alias);
         let res = TypeAlias { name, visibility, ast_id };
-        self.tree.data.insert(ast_id.upcast(), ModItem::TypeAlias(res));
+        self.tree.small_data.insert(ast_id.upcast(), SmallModItem::TypeAlias(res));
         Some(ast_id)
     }
 
@@ -220,7 +221,7 @@ impl<'a> Ctx<'a> {
         let visibility = self.lower_visibility(static_);
         let ast_id = self.source_ast_id_map.ast_id(static_);
         let res = Static { name, visibility, ast_id };
-        self.tree.data.insert(ast_id.upcast(), ModItem::Static(res));
+        self.tree.small_data.insert(ast_id.upcast(), SmallModItem::Static(res));
         Some(ast_id)
     }
 
@@ -229,7 +230,7 @@ impl<'a> Ctx<'a> {
         let visibility = self.lower_visibility(konst);
         let ast_id = self.source_ast_id_map.ast_id(konst);
         let res = Const { name, visibility, ast_id };
-        self.tree.data.insert(ast_id.upcast(), ModItem::Const(res));
+        self.tree.small_data.insert(ast_id.upcast(), SmallModItem::Const(res));
         ast_id
     }
 
@@ -251,7 +252,7 @@ impl<'a> Ctx<'a> {
         };
         let ast_id = self.source_ast_id_map.ast_id(module);
         let res = Mod { name, visibility, kind, ast_id };
-        self.tree.data.insert(ast_id.upcast(), ModItem::Mod(res));
+        self.tree.big_data.insert(ast_id.upcast(), BigModItem::Mod(res));
         Some(ast_id)
     }
 
@@ -261,7 +262,7 @@ impl<'a> Ctx<'a> {
         let ast_id = self.source_ast_id_map.ast_id(trait_def);
 
         let def = Trait { name, visibility, ast_id };
-        self.tree.data.insert(ast_id.upcast(), ModItem::Trait(def));
+        self.tree.small_data.insert(ast_id.upcast(), SmallModItem::Trait(def));
         Some(ast_id)
     }
 
@@ -274,7 +275,7 @@ impl<'a> Ctx<'a> {
         let ast_id = self.source_ast_id_map.ast_id(trait_alias_def);
 
         let alias = TraitAlias { name, visibility, ast_id };
-        self.tree.data.insert(ast_id.upcast(), ModItem::TraitAlias(alias));
+        self.tree.small_data.insert(ast_id.upcast(), SmallModItem::TraitAlias(alias));
         Some(ast_id)
     }
 
@@ -283,7 +284,7 @@ impl<'a> Ctx<'a> {
         // Note that trait impls don't get implicit `Self` unlike traits, because here they are a
         // type alias rather than a type parameter, so this is handled by the resolver.
         let res = Impl { ast_id };
-        self.tree.data.insert(ast_id.upcast(), ModItem::Impl(res));
+        self.tree.small_data.insert(ast_id.upcast(), SmallModItem::Impl(res));
         ast_id
     }
 
@@ -295,7 +296,7 @@ impl<'a> Ctx<'a> {
         })?;
 
         let res = Use { visibility, ast_id, use_tree };
-        self.tree.data.insert(ast_id.upcast(), ModItem::Use(res));
+        self.tree.big_data.insert(ast_id.upcast(), BigModItem::Use(res));
         Some(ast_id)
     }
 
@@ -311,7 +312,7 @@ impl<'a> Ctx<'a> {
         let ast_id = self.source_ast_id_map.ast_id(extern_crate);
 
         let res = ExternCrate { name, alias, visibility, ast_id };
-        self.tree.data.insert(ast_id.upcast(), ModItem::ExternCrate(res));
+        self.tree.big_data.insert(ast_id.upcast(), BigModItem::ExternCrate(res));
         Some(ast_id)
     }
 
@@ -325,7 +326,7 @@ impl<'a> Ctx<'a> {
         let ast_id = self.source_ast_id_map.ast_id(m);
         let expand_to = hir_expand::ExpandTo::from_call_site(m);
         let res = MacroCall { path, ast_id, expand_to, ctxt: span_map.span_for_range(range).ctx };
-        self.tree.data.insert(ast_id.upcast(), ModItem::MacroCall(res));
+        self.tree.big_data.insert(ast_id.upcast(), BigModItem::MacroCall(res));
         Some(ast_id)
     }
 
@@ -334,7 +335,7 @@ impl<'a> Ctx<'a> {
         let ast_id = self.source_ast_id_map.ast_id(m);
 
         let res = MacroRules { name: name.as_name(), ast_id };
-        self.tree.data.insert(ast_id.upcast(), ModItem::MacroRules(res));
+        self.tree.small_data.insert(ast_id.upcast(), SmallModItem::MacroRules(res));
         Some(ast_id)
     }
 
@@ -345,7 +346,7 @@ impl<'a> Ctx<'a> {
         let visibility = self.lower_visibility(m);
 
         let res = Macro2 { name: name.as_name(), ast_id, visibility };
-        self.tree.data.insert(ast_id.upcast(), ModItem::Macro2(res));
+        self.tree.small_data.insert(ast_id.upcast(), SmallModItem::Macro2(res));
         Some(ast_id)
     }
 
@@ -372,7 +373,7 @@ impl<'a> Ctx<'a> {
         });
 
         let res = ExternBlock { ast_id, children };
-        self.tree.data.insert(ast_id.upcast(), ModItem::ExternBlock(res));
+        self.tree.big_data.insert(ast_id.upcast(), BigModItem::ExternBlock(res));
         ast_id
     }
 
diff --git a/crates/intern/src/symbol.rs b/crates/intern/src/symbol.rs
index 3b962a86f1..8b2d6e8717 100644
--- a/crates/intern/src/symbol.rs
+++ b/crates/intern/src/symbol.rs
@@ -112,7 +112,6 @@ impl TaggedArcPtr {
     }
 }
 
-// FIXME: This should have more than one niche
 #[derive(PartialEq, Eq, Hash)]
 pub struct Symbol {
     repr: TaggedArcPtr,