diff --git a/crates/ra_analysis/src/descriptors/module/mod.rs b/crates/ra_analysis/src/descriptors/module/mod.rs
index 8a75e11be4..16faea94e9 100644
--- a/crates/ra_analysis/src/descriptors/module/mod.rs
+++ b/crates/ra_analysis/src/descriptors/module/mod.rs
@@ -6,6 +6,7 @@ use std::sync::Arc;
 use ra_editor::find_node_at_offset;
 
 use ra_syntax::{
+    algo::generate,
     ast::{self, AstNode, NameOwner},
     SmolStr, SyntaxNode, SyntaxNodeRef,
 };
@@ -27,6 +28,16 @@ pub(crate) struct ModuleDescriptor {
 }
 
 impl ModuleDescriptor {
+    /// Lookup `ModuleDescriptor` by `FileId`. Note that this is inherently
+    /// lossy transformation: in general, a single source might correspond to
+    /// several modules.
+    pub fn guess_from_file_id(
+        db: &impl DescriptorDatabase,
+        file_id: FileId,
+    ) -> Cancelable<Option<ModuleDescriptor>> {
+        ModuleDescriptor::guess_from_source(db, file_id, ModuleSource::SourceFile(file_id))
+    }
+
     /// Lookup `ModuleDescriptor` by position in the source code. Note that this
     /// is inherently lossy transformation: in general, a single source might
     /// correspond to several modules.
@@ -34,14 +45,23 @@ impl ModuleDescriptor {
         db: &impl DescriptorDatabase,
         position: FilePosition,
     ) -> Cancelable<Option<ModuleDescriptor>> {
-        let source_root = db.file_source_root(position.file_id);
-        let module_tree = db.module_tree(source_root)?;
         let file = db.file_syntax(position.file_id);
         let module_source = match find_node_at_offset::<ast::Module>(file.syntax(), position.offset)
         {
             Some(m) if !m.has_semi() => ModuleSource::new_inline(position.file_id, m),
             _ => ModuleSource::SourceFile(position.file_id),
         };
+        ModuleDescriptor::guess_from_source(db, position.file_id, module_source)
+    }
+
+    fn guess_from_source(
+        db: &impl DescriptorDatabase,
+        file_id: FileId,
+        module_source: ModuleSource,
+    ) -> Cancelable<Option<ModuleDescriptor>> {
+        let source_root = db.file_source_root(file_id);
+        let module_tree = db.module_tree(source_root)?;
+
         let res = match module_tree.any_module_for_source(module_source) {
             None => None,
             Some(module_id) => Some(ModuleDescriptor {
@@ -64,6 +84,11 @@ impl ModuleDescriptor {
         Some((file_id, src))
     }
 
+    pub fn source(&self) -> ModuleSource {
+        self.module_id.source(&self.tree)
+    }
+
+    /// Parent module. Returns `None` if this is a root module.
     pub fn parent(&self) -> Option<ModuleDescriptor> {
         let parent_id = self.module_id.parent(&self.tree)?;
         Some(ModuleDescriptor {
@@ -71,6 +96,14 @@ impl ModuleDescriptor {
             module_id: parent_id,
         })
     }
+
+    /// The root of the tree this module is part of
+    pub fn crate_root(&self) -> ModuleDescriptor {
+        generate(Some(self.clone()), |it| it.parent())
+            .last()
+            .unwrap()
+    }
+
     /// `name` is `None` for the crate's root module
     pub fn name(&self) -> Option<SmolStr> {
         let link = self.module_id.parent_link(&self.tree)?;
diff --git a/crates/ra_analysis/src/imp.rs b/crates/ra_analysis/src/imp.rs
index c0bed04bfa..49b8637564 100644
--- a/crates/ra_analysis/src/imp.rs
+++ b/crates/ra_analysis/src/imp.rs
@@ -220,6 +220,8 @@ impl AnalysisImpl {
         let source_root = self.db.file_source_root(file_id);
         self.db.module_tree(source_root)
     }
+    /// This return `Vec`: a module may be inclucded from several places.
+    /// We don't handle this case yet though, so the Vec has length at most one.
     pub fn parent_module(&self, position: FilePosition) -> Cancelable<Vec<(FileId, FileSymbol)>> {
         let descr = match ModuleDescriptor::guess_from_position(&*self.db, position)? {
             None => return Ok(Vec::new()),
@@ -238,18 +240,21 @@ impl AnalysisImpl {
         };
         Ok(vec![(file_id, sym)])
     }
+    /// Returns `Vec` for the same reason as `parent_module`
     pub fn crate_for(&self, file_id: FileId) -> Cancelable<Vec<CrateId>> {
-        let module_tree = self.module_tree(file_id)?;
-        let crate_graph = self.db.crate_graph();
-        let res = module_tree
-            .modules_for_source(ModuleSource::SourceFile(file_id))
-            .into_iter()
-            .map(|it| it.root(&module_tree))
-            .filter_map(|it| it.source(&module_tree).as_file())
-            .filter_map(|it| crate_graph.crate_id_for_crate_root(it))
-            .collect();
+        let descr = match ModuleDescriptor::guess_from_file_id(&*self.db, file_id)? {
+            None => return Ok(Vec::new()),
+            Some(it) => it,
+        };
+        let root = descr.crate_root();
+        let file_id = root
+            .source()
+            .as_file()
+            .expect("root module always has a file as a source");
 
-        Ok(res)
+        let crate_graph = self.db.crate_graph();
+        let crate_id = crate_graph.crate_id_for_crate_root(file_id);
+        Ok(crate_id.into_iter().collect())
     }
     pub fn crate_root(&self, crate_id: CrateId) -> FileId {
         self.db.crate_graph().crate_roots[&crate_id]