diff --git a/Cargo.lock b/Cargo.lock
index 18aac79ab75d..c6ed5107cb4b 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -600,6 +600,7 @@ version = "0.1.0"
 dependencies = [
  "fst 0.3.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "log 0.4.6 (registry+https://github.com/rust-lang/crates.io-index)",
+ "parking_lot 0.6.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "ra_editor 0.1.0",
  "ra_syntax 0.1.0",
  "rayon 1.0.3 (registry+https://github.com/rust-lang/crates.io-index)",
diff --git a/crates/ra_analysis/Cargo.toml b/crates/ra_analysis/Cargo.toml
index 908ee1c812a7..b4a1a09b522c 100644
--- a/crates/ra_analysis/Cargo.toml
+++ b/crates/ra_analysis/Cargo.toml
@@ -11,6 +11,7 @@ rayon = "1.0.2"
 fst = "0.3.1"
 salsa = "0.8.0"
 rustc-hash = "1.0"
+parking_lot = "0.6.4"
 ra_syntax = { path = "../ra_syntax" }
 ra_editor = { path = "../ra_editor" }
 test_utils = { path = "../test_utils" }
diff --git a/crates/ra_analysis/src/db.rs b/crates/ra_analysis/src/db.rs
index 194f1a6b082f..d78b6afb944b 100644
--- a/crates/ra_analysis/src/db.rs
+++ b/crates/ra_analysis/src/db.rs
@@ -12,12 +12,14 @@ use crate::{
     },
     symbol_index::SymbolIndex,
     syntax_ptr::SyntaxPtr,
+    loc2id::{IdMaps, IdDatabase},
     Cancelable, Canceled, FileId,
 };
 
 #[derive(Debug)]
 pub(crate) struct RootDatabase {
     runtime: salsa::Runtime<RootDatabase>,
+    id_maps: IdMaps,
 }
 
 impl salsa::Database for RootDatabase {
@@ -29,7 +31,8 @@ impl salsa::Database for RootDatabase {
 impl Default for RootDatabase {
     fn default() -> RootDatabase {
         let mut db = RootDatabase {
-            runtime: Default::default(),
+            runtime: salsa::Runtime::default(),
+            id_maps: IdMaps::default(),
         };
         db.query_mut(crate::input::SourceRootQuery)
             .set(crate::input::WORKSPACE, Default::default());
@@ -53,10 +56,17 @@ impl salsa::ParallelDatabase for RootDatabase {
     fn snapshot(&self) -> salsa::Snapshot<RootDatabase> {
         salsa::Snapshot::new(RootDatabase {
             runtime: self.runtime.snapshot(self),
+            id_maps: self.id_maps.clone(),
         })
     }
 }
 
+impl IdDatabase for RootDatabase {
+    fn id_maps(&self) -> &IdMaps {
+        &self.id_maps
+    }
+}
+
 salsa::database_storage! {
     pub(crate) struct RootDatabaseStorage for RootDatabase {
         impl crate::input::FilesDatabase {
diff --git a/crates/ra_analysis/src/descriptors/function/imp.rs b/crates/ra_analysis/src/descriptors/function/imp.rs
index a989a04cd034..a7257acf9a16 100644
--- a/crates/ra_analysis/src/descriptors/function/imp.rs
+++ b/crates/ra_analysis/src/descriptors/function/imp.rs
@@ -8,9 +8,9 @@ use crate::descriptors::{
 };
 
 /// Resolve `FnId` to the corresponding `SyntaxNode`
-/// TODO: this should return something more type-safe then `SyntaxNode`
 pub(crate) fn fn_syntax(db: &impl DescriptorDatabase, fn_id: FnId) -> FnDefNode {
-    let syntax = db.resolve_syntax_ptr(fn_id.0);
+    let ptr = db.id_maps().fn_ptr(fn_id);
+    let syntax = db.resolve_syntax_ptr(ptr);
     FnDef::cast(syntax.borrowed()).unwrap().owned()
 }
 
diff --git a/crates/ra_analysis/src/descriptors/function/mod.rs b/crates/ra_analysis/src/descriptors/function/mod.rs
index d5db28a64006..86eee5e93c2c 100644
--- a/crates/ra_analysis/src/descriptors/function/mod.rs
+++ b/crates/ra_analysis/src/descriptors/function/mod.rs
@@ -8,17 +8,18 @@ use ra_syntax::{
     TextRange, TextUnit,
 };
 
-use crate::{syntax_ptr::SyntaxPtr, FileId};
+use crate::{
+    syntax_ptr::SyntaxPtr, FileId,
+    loc2id::IdDatabase,
+};
 
 pub(crate) use self::scope::{resolve_local_name, FnScopes};
-
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
-pub(crate) struct FnId(SyntaxPtr);
+pub(crate) use crate::loc2id::FnId;
 
 impl FnId {
-    pub(crate) fn new(file_id: FileId, fn_def: ast::FnDef) -> FnId {
+    pub(crate) fn get(db: &impl IdDatabase, file_id: FileId, fn_def: ast::FnDef) -> FnId {
         let ptr = SyntaxPtr::new(file_id, fn_def.syntax());
-        FnId(ptr)
+        db.id_maps().fn_id(ptr)
     }
 }
 
diff --git a/crates/ra_analysis/src/descriptors/mod.rs b/crates/ra_analysis/src/descriptors/mod.rs
index 56bde38492cf..d602c4e04ea2 100644
--- a/crates/ra_analysis/src/descriptors/mod.rs
+++ b/crates/ra_analysis/src/descriptors/mod.rs
@@ -13,12 +13,13 @@ use crate::{
     descriptors::function::{resolve_local_name, FnId, FnScopes},
     descriptors::module::{ModuleId, ModuleScope, ModuleTree, ModuleSource},
     input::SourceRootId,
+    loc2id::IdDatabase,
     syntax_ptr::LocalSyntaxPtr,
     Cancelable,
 };
 
 salsa::query_group! {
-    pub(crate) trait DescriptorDatabase: SyntaxDatabase {
+    pub(crate) trait DescriptorDatabase: SyntaxDatabase + IdDatabase {
         fn module_tree(source_root_id: SourceRootId) -> Cancelable<Arc<ModuleTree>> {
             type ModuleTreeQuery;
             use fn module::imp::module_tree;
diff --git a/crates/ra_analysis/src/descriptors/module/mod.rs b/crates/ra_analysis/src/descriptors/module/mod.rs
index bc1148b22842..055a56b54de2 100644
--- a/crates/ra_analysis/src/descriptors/module/mod.rs
+++ b/crates/ra_analysis/src/descriptors/module/mod.rs
@@ -58,6 +58,15 @@ enum ModuleSourceNode {
 #[derive(Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord, Debug)]
 pub(crate) struct ModuleId(u32);
 
+impl crate::loc2id::NumericId for ModuleId {
+    fn from_u32(id: u32) -> Self {
+        ModuleId(id)
+    }
+    fn to_u32(self) -> u32 {
+        self.0
+    }
+}
+
 #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug)]
 pub(crate) struct LinkId(u32);
 
diff --git a/crates/ra_analysis/src/imp.rs b/crates/ra_analysis/src/imp.rs
index 74c248a96c71..812fed32d93d 100644
--- a/crates/ra_analysis/src/imp.rs
+++ b/crates/ra_analysis/src/imp.rs
@@ -621,7 +621,7 @@ fn resolve_local_name(
     name_ref: ast::NameRef,
 ) -> Option<(SmolStr, TextRange)> {
     let fn_def = name_ref.syntax().ancestors().find_map(ast::FnDef::cast)?;
-    let fn_id = FnId::new(file_id, fn_def);
+    let fn_id = FnId::get(db, file_id, fn_def);
     let scopes = db.fn_scopes(fn_id);
     let scope_entry = crate::descriptors::function::resolve_local_name(name_ref, &scopes)?;
     let syntax = db.resolve_syntax_ptr(scope_entry.ptr().into_global(file_id));
diff --git a/crates/ra_analysis/src/lib.rs b/crates/ra_analysis/src/lib.rs
index ab0e3cb0cd16..eccda84a7f23 100644
--- a/crates/ra_analysis/src/lib.rs
+++ b/crates/ra_analysis/src/lib.rs
@@ -10,6 +10,7 @@ extern crate rustc_hash;
 extern crate salsa;
 
 mod db;
+mod loc2id;
 mod input;
 mod imp;
 mod completion;
diff --git a/crates/ra_analysis/src/loc2id.rs b/crates/ra_analysis/src/loc2id.rs
new file mode 100644
index 000000000000..8eaa249971f9
--- /dev/null
+++ b/crates/ra_analysis/src/loc2id.rs
@@ -0,0 +1,108 @@
+use parking_lot::Mutex;
+
+use std::{
+    hash::Hash,
+    sync::Arc,
+};
+
+use rustc_hash::FxHashMap;
+
+use crate::{
+    syntax_ptr::SyntaxPtr,
+};
+
+/// There are two principle ways to refer to things:
+///   - by their locatinon (module in foo/bar/baz.rs at line 42)
+///   - by their numeric id (module `ModuleId(42)`)
+///
+/// The first one is more powerful (you can actually find the thing in question
+/// by id), but the second one is so much more compact.
+///
+/// `Loc2IdMap` allows us to have a cake an eat it as well: by maintaining a
+/// bidirectional mapping between positional and numeric ids, we can use compact
+/// representation wich still allows us to get the actual item
+#[derive(Debug)]
+pub(crate) struct Loc2IdMap<L, ID>
+where
+    ID: NumericId,
+    L: Clone + Eq + Hash,
+{
+    loc2id: FxHashMap<L, ID>,
+    id2loc: FxHashMap<ID, L>,
+}
+
+impl<L, ID> Default for Loc2IdMap<L, ID>
+where
+    ID: NumericId,
+    L: Clone + Eq + Hash,
+{
+    fn default() -> Self {
+        Loc2IdMap {
+            loc2id: FxHashMap::default(),
+            id2loc: FxHashMap::default(),
+        }
+    }
+}
+
+impl<L, ID> Loc2IdMap<L, ID>
+where
+    ID: NumericId,
+    L: Clone + Eq + Hash,
+{
+    pub fn loc2id(&mut self, loc: &L) -> ID {
+        match self.loc2id.get(loc) {
+            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);
+        self.loc2id.insert(loc.clone(), id.clone());
+        self.id2loc.insert(id.clone(), loc.clone());
+        id
+    }
+
+    pub fn id2loc(&self, id: ID) -> L {
+        self.id2loc[&id].clone()
+    }
+}
+
+pub(crate) trait NumericId: Clone + Eq + Hash {
+    fn from_u32(id: u32) -> Self;
+    fn to_u32(self) -> u32;
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub(crate) struct FnId(u32);
+
+impl NumericId for FnId {
+    fn from_u32(id: u32) -> FnId {
+        FnId(id)
+    }
+    fn to_u32(self) -> u32 {
+        self.0
+    }
+}
+
+pub(crate) trait IdDatabase: salsa::Database {
+    fn id_maps(&self) -> &IdMaps;
+}
+
+#[derive(Debug, Default, Clone)]
+pub(crate) struct IdMaps {
+    inner: Arc<IdMapsInner>,
+}
+
+impl IdMaps {
+    pub(crate) fn fn_id(&self, ptr: SyntaxPtr) -> FnId {
+        self.inner.fns.lock().loc2id(&ptr)
+    }
+    pub(crate) fn fn_ptr(&self, fn_id: FnId) -> SyntaxPtr {
+        self.inner.fns.lock().id2loc(fn_id)
+    }
+}
+
+#[derive(Debug, Default)]
+struct IdMapsInner {
+    fns: Mutex<Loc2IdMap<SyntaxPtr, FnId>>,
+}