diff --git a/Cargo.lock b/Cargo.lock
index 6ef51fed0cc..c84fa0c02d0 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -678,6 +678,7 @@ name = "ra_db"
 version = "0.1.0"
 dependencies = [
  "parking_lot 0.7.1 (registry+https://github.com/rust-lang/crates.io-index)",
+ "ra_arena 0.1.0",
  "ra_editor 0.1.0",
  "ra_syntax 0.1.0",
  "relative-path 0.4.0 (registry+https://github.com/rust-lang/crates.io-index)",
diff --git a/crates/ra_arena/src/lib.rs b/crates/ra_arena/src/lib.rs
index 040977dc472..43bfa925a7c 100644
--- a/crates/ra_arena/src/lib.rs
+++ b/crates/ra_arena/src/lib.rs
@@ -61,6 +61,9 @@ pub trait ArenaId {
 }
 
 impl<ID: ArenaId, T> Arena<ID, T> {
+    pub fn len(&self) -> usize {
+        self.data.len()
+    }
     pub fn alloc(&mut self, value: T) -> ID {
         let id = RawId(self.data.len() as u32);
         self.data.push(value);
diff --git a/crates/ra_db/Cargo.toml b/crates/ra_db/Cargo.toml
index c0e83a140e2..c43e6505174 100644
--- a/crates/ra_db/Cargo.toml
+++ b/crates/ra_db/Cargo.toml
@@ -9,6 +9,7 @@ relative-path = "0.4.0"
 salsa = "0.9.1"
 rustc-hash = "1.0"
 parking_lot = "0.7.0"
+ra_arena = { path = "../ra_arena" }
 ra_syntax = { path = "../ra_syntax" }
 ra_editor = { path = "../ra_editor" }
 test_utils = { path = "../test_utils" }
diff --git a/crates/ra_db/src/lib.rs b/crates/ra_db/src/lib.rs
index 3c41ee56daf..732899718ca 100644
--- a/crates/ra_db/src/lib.rs
+++ b/crates/ra_db/src/lib.rs
@@ -18,23 +18,9 @@
         FileTextQuery, FileSourceRootQuery, SourceRootQuery, LocalRootsQuery, LibraryRootsQuery, CrateGraphQuery,
         FileRelativePathQuery
     },
-    loc2id::{LocationIntener, NumericId},
+    loc2id::LocationIntener,
 };
 
-#[macro_export]
-macro_rules! impl_numeric_id {
-    ($id:ident) => {
-        impl $crate::NumericId for $id {
-            fn from_u32(id: u32) -> Self {
-                $id(id)
-            }
-            fn to_u32(self) -> u32 {
-                self.0
-            }
-        }
-    };
-}
-
 pub trait BaseDatabase: salsa::Database {
     fn check_canceled(&self) -> Cancelable<()> {
         if self.salsa_runtime().is_current_revision_canceled() {
diff --git a/crates/ra_db/src/loc2id.rs b/crates/ra_db/src/loc2id.rs
index 2dc7930d8e0..1d6761897e0 100644
--- a/crates/ra_db/src/loc2id.rs
+++ b/crates/ra_db/src/loc2id.rs
@@ -1,8 +1,8 @@
-use parking_lot::Mutex;
-
 use std::hash::Hash;
 
+use parking_lot::Mutex;
 use rustc_hash::FxHashMap;
+use ra_arena::{Arena, ArenaId};
 
 /// There are two principle ways to refer to things:
 ///   - by their locatinon (module in foo/bar/baz.rs at line 42)
@@ -17,33 +17,33 @@
 #[derive(Debug)]
 struct Loc2IdMap<LOC, ID>
 where
-    ID: NumericId,
+    ID: ArenaId + Clone,
     LOC: Clone + Eq + Hash,
 {
+    id2loc: Arena<ID, LOC>,
     loc2id: FxHashMap<LOC, ID>,
-    id2loc: FxHashMap<ID, LOC>,
 }
 
 impl<LOC, ID> Default for Loc2IdMap<LOC, ID>
 where
-    ID: NumericId,
+    ID: ArenaId + Clone,
     LOC: Clone + Eq + Hash,
 {
     fn default() -> Self {
         Loc2IdMap {
+            id2loc: Arena::default(),
             loc2id: FxHashMap::default(),
-            id2loc: FxHashMap::default(),
         }
     }
 }
 
 impl<LOC, ID> Loc2IdMap<LOC, ID>
 where
-    ID: NumericId,
+    ID: ArenaId + Clone,
     LOC: Clone + Eq + Hash,
 {
     pub fn len(&self) -> usize {
-        self.loc2id.len()
+        self.id2loc.len()
     }
 
     pub fn loc2id(&mut self, loc: &LOC) -> ID {
@@ -51,28 +51,20 @@ pub fn loc2id(&mut self, loc: &LOC) -> ID {
             Some(id) => return id.clone(),
             None => (),
         }
-        let id = self.loc2id.len();
-        assert!(id < u32::max_value() as usize);
-        let id = ID::from_u32(id as u32);
+        let id = self.id2loc.alloc(loc.clone());
         self.loc2id.insert(loc.clone(), id.clone());
-        self.id2loc.insert(id.clone(), loc.clone());
         id
     }
 
     pub fn id2loc(&self, id: ID) -> LOC {
-        self.id2loc[&id].clone()
+        self.id2loc[id].clone()
     }
 }
 
-pub trait NumericId: Clone + Eq + Hash {
-    fn from_u32(id: u32) -> Self;
-    fn to_u32(self) -> u32;
-}
-
 #[derive(Debug)]
 pub struct LocationIntener<LOC, ID>
 where
-    ID: NumericId,
+    ID: ArenaId + Clone,
     LOC: Clone + Eq + Hash,
 {
     map: Mutex<Loc2IdMap<LOC, ID>>,
@@ -80,7 +72,7 @@ pub struct LocationIntener<LOC, ID>
 
 impl<LOC, ID> Default for LocationIntener<LOC, ID>
 where
-    ID: NumericId,
+    ID: ArenaId + Clone,
     LOC: Clone + Eq + Hash,
 {
     fn default() -> Self {
@@ -92,7 +84,7 @@ fn default() -> Self {
 
 impl<LOC, ID> LocationIntener<LOC, ID>
 where
-    ID: NumericId,
+    ID: ArenaId + Clone,
     LOC: Clone + Eq + Hash,
 {
     pub fn len(&self) -> usize {
diff --git a/crates/ra_hir/src/ids.rs b/crates/ra_hir/src/ids.rs
index 730a3e542bc..624ab808fa0 100644
--- a/crates/ra_hir/src/ids.rs
+++ b/crates/ra_hir/src/ids.rs
@@ -93,8 +93,8 @@ fn from(macro_call_id: MacroCallId) -> HirFileId {
 /// `MacroCallId` identifies a particular macro invocation, like
 /// `println!("Hello, {}", world)`.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
-pub struct MacroCallId(u32);
-ra_db::impl_numeric_id!(MacroCallId);
+pub struct MacroCallId(RawId);
+impl_arena_id!(MacroCallId);
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct MacroCallLoc {
@@ -125,8 +125,8 @@ pub(crate) fn id(
 /// Def's are a core concept of hir. A `Def` is an Item (function, module, etc)
 /// in a specific module.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
-pub struct DefId(u32);
-ra_db::impl_numeric_id!(DefId);
+pub struct DefId(RawId);
+impl_arena_id!(DefId);
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub struct DefLoc {