diff --git a/crates/ra_analysis/src/arena.rs b/crates/ra_analysis/src/arena.rs
new file mode 100644
index 0000000000..98ed892749
--- /dev/null
+++ b/crates/ra_analysis/src/arena.rs
@@ -0,0 +1,96 @@
+//! A simple id-based arena, similar to https://github.com/fitzgen/id-arena.
+//! We use our own version for more compact id's and to allow inherent impls
+//! on Ids.
+
+use std::{
+    fmt,
+    ops::{Index, IndexMut},
+    hash::{Hash, Hasher},
+    marker::PhantomData,
+};
+
+pub(crate) struct Id<T> {
+    idx: u32,
+    _ty: PhantomData<fn() -> T>,
+}
+
+impl<T> fmt::Debug for Id<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.debug_tuple("Id").field(&self.idx).finish()
+    }
+}
+impl<T> Copy for Id<T> {}
+impl<T> Clone for Id<T> {
+    fn clone(&self) -> Id<T> {
+        *self
+    }
+}
+
+impl<T> PartialEq for Id<T> {
+    fn eq(&self, other: &Id<T>) -> bool {
+        self.idx == other.idx
+    }
+}
+
+impl<T> Eq for Id<T> {}
+
+impl<T> Hash for Id<T> {
+    fn hash<H: Hasher>(&self, h: &mut H) {
+        self.idx.hash(h);
+    }
+}
+
+#[derive(Debug, PartialEq, Eq)]
+pub(crate) struct Arena<T> {
+    data: Vec<T>,
+}
+
+impl<T> Default for Arena<T> {
+    fn default() -> Arena<T> {
+        Arena { data: Vec::new() }
+    }
+}
+
+impl<T> Arena<T> {
+    pub(crate) fn push(&mut self, value: T) -> Id<T> {
+        let id = self.data.len() as u32;
+        self.data.push(value);
+        Id {
+            idx: id as u32,
+            _ty: PhantomData,
+        }
+    }
+
+    pub(crate) fn keys<'a>(&'a self) -> impl Iterator<Item = Id<T>> + 'a {
+        (0..(self.data.len() as u32)).into_iter().map(|idx| Id {
+            idx,
+            _ty: PhantomData,
+        })
+    }
+
+    pub(crate) fn items<'a>(&'a self) -> impl Iterator<Item = (Id<T>, &T)> + 'a {
+        self.data.iter().enumerate().map(|(idx, item)| {
+            let idx = idx as u32;
+            (
+                Id {
+                    idx,
+                    _ty: PhantomData,
+                },
+                item,
+            )
+        })
+    }
+}
+
+impl<T> Index<Id<T>> for Arena<T> {
+    type Output = T;
+    fn index(&self, id: Id<T>) -> &T {
+        &self.data[id.idx as usize]
+    }
+}
+
+impl<T> IndexMut<Id<T>> for Arena<T> {
+    fn index_mut(&mut self, id: Id<T>) -> &mut T {
+        &mut self.data[id.idx as usize]
+    }
+}
diff --git a/crates/ra_analysis/src/db.rs b/crates/ra_analysis/src/db.rs
index 7c28c9a2b1..6b56f99acb 100644
--- a/crates/ra_analysis/src/db.rs
+++ b/crates/ra_analysis/src/db.rs
@@ -1,5 +1,5 @@
 use std::sync::Arc;
-
+#[cfg(test)]
 use parking_lot::Mutex;
 use ra_editor::LineIndex;
 use ra_syntax::{SourceFileNode, SyntaxNode};
@@ -33,6 +33,7 @@ impl salsa::Database for RootDatabase {
         &self.runtime
     }
 
+    #[allow(unused)]
     fn salsa_event(&self, event: impl Fn() -> salsa::Event<RootDatabase>) {
         #[cfg(test)]
         {
diff --git a/crates/ra_analysis/src/descriptors/function/scope.rs b/crates/ra_analysis/src/descriptors/function/scope.rs
index bbe16947cc..54d7fa4568 100644
--- a/crates/ra_analysis/src/descriptors/function/scope.rs
+++ b/crates/ra_analysis/src/descriptors/function/scope.rs
@@ -6,15 +6,17 @@ use ra_syntax::{
     AstNode, SmolStr, SyntaxNodeRef,
 };
 
-use crate::syntax_ptr::LocalSyntaxPtr;
+use crate::{
+    syntax_ptr::LocalSyntaxPtr,
+    arena::{Arena, Id},
+};
 
-#[derive(Clone, Copy, PartialEq, Eq, Debug)]
-pub(crate) struct ScopeId(u32);
+pub(crate) type ScopeId = Id<ScopeData>;
 
 #[derive(Debug, PartialEq, Eq)]
 pub struct FnScopes {
     pub(crate) self_param: Option<LocalSyntaxPtr>,
-    scopes: Vec<ScopeData>,
+    scopes: Arena<ScopeData>,
     scope_for: FxHashMap<LocalSyntaxPtr, ScopeId>,
 }
 
@@ -25,7 +27,7 @@ pub struct ScopeEntry {
 }
 
 #[derive(Debug, PartialEq, Eq)]
-struct ScopeData {
+pub(crate) struct ScopeData {
     parent: Option<ScopeId>,
     entries: Vec<ScopeEntry>,
 }
@@ -37,7 +39,7 @@ impl FnScopes {
                 .param_list()
                 .and_then(|it| it.self_param())
                 .map(|it| LocalSyntaxPtr::new(it.syntax())),
-            scopes: Vec::new(),
+            scopes: Arena::default(),
             scope_for: FxHashMap::default(),
         };
         let root = scopes.root_scope();
@@ -48,26 +50,24 @@ impl FnScopes {
         scopes
     }
     pub(crate) fn entries(&self, scope: ScopeId) -> &[ScopeEntry] {
-        &self.get(scope).entries
+        &self.scopes[scope].entries
     }
     pub fn scope_chain<'a>(&'a self, node: SyntaxNodeRef) -> impl Iterator<Item = ScopeId> + 'a {
-        generate(self.scope_for(node), move |&scope| self.get(scope).parent)
+        generate(self.scope_for(node), move |&scope| {
+            self.scopes[scope].parent
+        })
     }
     fn root_scope(&mut self) -> ScopeId {
-        let res = ScopeId(self.scopes.len() as u32);
         self.scopes.push(ScopeData {
             parent: None,
             entries: vec![],
-        });
-        res
+        })
     }
     fn new_scope(&mut self, parent: ScopeId) -> ScopeId {
-        let res = ScopeId(self.scopes.len() as u32);
         self.scopes.push(ScopeData {
             parent: Some(parent),
             entries: vec![],
-        });
-        res
+        })
     }
     fn add_bindings(&mut self, scope: ScopeId, pat: ast::Pat) {
         let entries = pat
@@ -75,7 +75,7 @@ impl FnScopes {
             .descendants()
             .filter_map(ast::BindPat::cast)
             .filter_map(ScopeEntry::new);
-        self.get_mut(scope).entries.extend(entries);
+        self.scopes[scope].entries.extend(entries);
     }
     fn add_params_bindings(&mut self, scope: ScopeId, params: Option<ast::ParamList>) {
         params
@@ -93,12 +93,6 @@ impl FnScopes {
             .filter_map(|it| self.scope_for.get(&it).map(|&scope| scope))
             .next()
     }
-    fn get(&self, scope: ScopeId) -> &ScopeData {
-        &self.scopes[scope.0 as usize]
-    }
-    fn get_mut(&mut self, scope: ScopeId) -> &mut ScopeData {
-        &mut self.scopes[scope.0 as usize]
-    }
 }
 
 impl ScopeEntry {
diff --git a/crates/ra_analysis/src/descriptors/module/imp.rs b/crates/ra_analysis/src/descriptors/module/imp.rs
index d4dce861fa..80892acb74 100644
--- a/crates/ra_analysis/src/descriptors/module/imp.rs
+++ b/crates/ra_analysis/src/descriptors/module/imp.rs
@@ -94,10 +94,7 @@ fn create_module_tree<'a>(
     db: &impl DescriptorDatabase,
     source_root: SourceRootId,
 ) -> Cancelable<ModuleTree> {
-    let mut tree = ModuleTree {
-        mods: Vec::new(),
-        links: Vec::new(),
-    };
+    let mut tree = ModuleTree::default();
 
     let mut roots = FxHashMap::default();
     let mut visited = FxHashSet::default();
@@ -154,7 +151,7 @@ fn build_subtree(
                     .into_iter()
                     .map(|file_id| match roots.remove(&file_id) {
                         Some(module_id) => {
-                            tree.module_mut(module_id).parent = Some(link);
+                            tree.mods[module_id].parent = Some(link);
                             Ok(module_id)
                         }
                         None => build_subtree(
@@ -184,8 +181,8 @@ fn build_subtree(
             }
         };
 
-        tree.link_mut(link).points_to = points_to;
-        tree.link_mut(link).problem = problem;
+        tree.links[link].points_to = points_to;
+        tree.links[link].problem = problem;
     }
     Ok(id)
 }
diff --git a/crates/ra_analysis/src/descriptors/module/mod.rs b/crates/ra_analysis/src/descriptors/module/mod.rs
index acc6c1c5a1..54ff95b66b 100644
--- a/crates/ra_analysis/src/descriptors/module/mod.rs
+++ b/crates/ra_analysis/src/descriptors/module/mod.rs
@@ -15,7 +15,8 @@ use relative_path::RelativePathBuf;
 use crate::{
     db::SyntaxDatabase, syntax_ptr::SyntaxPtr, FileId, FilePosition, Cancelable,
     descriptors::{Path, PathKind, DescriptorDatabase},
-    input::SourceRootId
+    input::SourceRootId,
+    arena::{Arena, Id},
 };
 
 pub(crate) use self::nameres::ModuleScope;
@@ -157,26 +158,22 @@ impl ModuleDescriptor {
 /// Module encapsulate the logic of transitioning from the fuzzy world of files
 /// (which can have multiple parents) to the precise world of modules (which
 /// always have one parent).
-#[derive(Debug, PartialEq, Eq, Hash)]
+#[derive(Default, Debug, PartialEq, Eq)]
 pub(crate) struct ModuleTree {
-    mods: Vec<ModuleData>,
-    links: Vec<LinkData>,
+    mods: Arena<ModuleData>,
+    links: Arena<LinkData>,
 }
 
 impl ModuleTree {
     fn modules<'a>(&'a self) -> impl Iterator<Item = ModuleId> + 'a {
-        self.mods
-            .iter()
-            .enumerate()
-            .map(|(idx, _)| ModuleId(idx as u32))
+        self.mods.keys()
     }
 
     fn modules_for_source(&self, source: ModuleSource) -> Vec<ModuleId> {
         self.mods
-            .iter()
-            .enumerate()
+            .items()
             .filter(|(_idx, it)| it.source == source)
-            .map(|(idx, _)| ModuleId(idx as u32))
+            .map(|(idx, _)| idx)
             .collect()
     }
 
@@ -201,11 +198,8 @@ enum ModuleSourceNode {
     Module(ast::ModuleNode),
 }
 
-#[derive(Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord, Debug)]
-pub(crate) struct ModuleId(u32);
-
-#[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
-struct LinkId(u32);
+pub(crate) type ModuleId = Id<ModuleData>;
+type LinkId = Id<LinkData>;
 
 #[derive(Clone, Debug, Hash, PartialEq, Eq)]
 pub enum Problem {
@@ -220,14 +214,14 @@ pub enum Problem {
 
 impl ModuleId {
     fn source(self, tree: &ModuleTree) -> ModuleSource {
-        tree.module(self).source
+        tree.mods[self].source
     }
     fn parent_link(self, tree: &ModuleTree) -> Option<LinkId> {
-        tree.module(self).parent
+        tree.mods[self].parent
     }
     fn parent(self, tree: &ModuleTree) -> Option<ModuleId> {
         let link = self.parent_link(tree)?;
-        Some(tree.link(link).owner)
+        Some(tree.links[link].owner)
     }
     fn crate_root(self, tree: &ModuleTree) -> ModuleId {
         generate(Some(self), move |it| it.parent(tree))
@@ -235,27 +229,26 @@ impl ModuleId {
             .unwrap()
     }
     fn child(self, tree: &ModuleTree, name: &str) -> Option<ModuleId> {
-        let link = tree
-            .module(self)
+        let link = tree.mods[self]
             .children
             .iter()
-            .map(|&it| tree.link(it))
+            .map(|&it| &tree.links[it])
             .find(|it| it.name == name)?;
         Some(*link.points_to.first()?)
     }
     fn children<'a>(self, tree: &'a ModuleTree) -> impl Iterator<Item = (SmolStr, ModuleId)> + 'a {
-        tree.module(self).children.iter().filter_map(move |&it| {
-            let link = tree.link(it);
+        tree.mods[self].children.iter().filter_map(move |&it| {
+            let link = &tree.links[it];
             let module = *link.points_to.first()?;
             Some((link.name.clone(), module))
         })
     }
     fn problems(self, tree: &ModuleTree, db: &impl SyntaxDatabase) -> Vec<(SyntaxNode, Problem)> {
-        tree.module(self)
+        tree.mods[self]
             .children
             .iter()
             .filter_map(|&it| {
-                let p = tree.link(it).problem.clone()?;
+                let p = tree.links[it].problem.clone()?;
                 let s = it.bind_source(tree, db);
                 let s = s.borrowed().name().unwrap().syntax().owned();
                 Some((s, p))
@@ -266,17 +259,17 @@ impl ModuleId {
 
 impl LinkId {
     fn owner(self, tree: &ModuleTree) -> ModuleId {
-        tree.link(self).owner
+        tree.links[self].owner
     }
     fn name(self, tree: &ModuleTree) -> SmolStr {
-        tree.link(self).name.clone()
+        tree.links[self].name.clone()
     }
     fn bind_source<'a>(self, tree: &ModuleTree, db: &impl SyntaxDatabase) -> ast::ModuleNode {
         let owner = self.owner(tree);
         match owner.source(tree).resolve(db) {
             ModuleSourceNode::SourceFile(root) => {
                 let ast = imp::modules(root.borrowed())
-                    .find(|(name, _)| name == &tree.link(self).name)
+                    .find(|(name, _)| name == &tree.links[self].name)
                     .unwrap()
                     .1;
                 ast.owned()
@@ -287,7 +280,7 @@ impl LinkId {
 }
 
 #[derive(Debug, PartialEq, Eq, Hash)]
-struct ModuleData {
+pub(crate) struct ModuleData {
     source: ModuleSource,
     parent: Option<LinkId>,
     children: Vec<LinkId>,
@@ -339,28 +332,13 @@ struct LinkData {
 }
 
 impl ModuleTree {
-    fn module(&self, id: ModuleId) -> &ModuleData {
-        &self.mods[id.0 as usize]
-    }
-    fn module_mut(&mut self, id: ModuleId) -> &mut ModuleData {
-        &mut self.mods[id.0 as usize]
-    }
-    fn link(&self, id: LinkId) -> &LinkData {
-        &self.links[id.0 as usize]
-    }
-    fn link_mut(&mut self, id: LinkId) -> &mut LinkData {
-        &mut self.links[id.0 as usize]
-    }
-
     fn push_mod(&mut self, data: ModuleData) -> ModuleId {
-        let id = ModuleId(self.mods.len() as u32);
-        self.mods.push(data);
-        id
+        self.mods.push(data)
     }
     fn push_link(&mut self, data: LinkData) -> LinkId {
-        let id = LinkId(self.links.len() as u32);
-        self.mods[data.owner.0 as usize].children.push(id);
-        self.links.push(data);
+        let owner = data.owner;
+        let id = self.links.push(data);
+        self.mods[owner].children.push(id);
         id
     }
 }
diff --git a/crates/ra_analysis/src/lib.rs b/crates/ra_analysis/src/lib.rs
index eccda84a7f..cedbd1fc8d 100644
--- a/crates/ra_analysis/src/lib.rs
+++ b/crates/ra_analysis/src/lib.rs
@@ -9,6 +9,7 @@ extern crate relative_path;
 extern crate rustc_hash;
 extern crate salsa;
 
+mod arena;
 mod db;
 mod loc2id;
 mod input;