track the root UseTree in addition to the leaf

src/librustc_resolve/build_reduced_graph.rs

@@ -100,6 +100,8 @@ impl<'a> Resolver<'a> {
     }
 
     fn build_reduced_graph_for_use_tree(&mut self,
+                                        root_use_tree: &ast::UseTree,
+                                        root_id: NodeId,
                                         use_tree: &ast::UseTree,
                                         id: NodeId,
                                         vis: ty::Visibility,
@@ -182,7 +184,14 @@ impl<'a> Resolver<'a> {
                     type_ns_only,
                 };
                 self.add_import_directive(
-                    module_path, subclass, use_tree.span, id, vis, expansion,
+                    module_path,
+                    subclass,
+                    use_tree.span,
+                    id,
+                    root_use_tree.span,
+                    root_id,
+                    vis,
+                    expansion,
                 );
             }
             ast::UseTreeKind::Glob => {
@@ -191,7 +200,14 @@ impl<'a> Resolver<'a> {
                     max_vis: Cell::new(ty::Visibility::Invisible),
                 };
                 self.add_import_directive(
-                    module_path, subclass, use_tree.span, id, vis, expansion,
+                    module_path,
+                    subclass,
+                    use_tree.span,
+                    id,
+                    root_use_tree.span,
+                    root_id,
+                    vis,
+                    expansion,
                 );
             }
             ast::UseTreeKind::Nested(ref items) => {
@@ -226,7 +242,7 @@ impl<'a> Resolver<'a> {
 
                 for &(ref tree, id) in items {
                     self.build_reduced_graph_for_use_tree(
-                        tree, id, vis, &prefix, true, item, expansion
+                        root_use_tree, root_id, tree, id, vis, &prefix, true, item, expansion
                     );
                 }
             }
@@ -249,7 +265,7 @@ impl<'a> Resolver<'a> {
                 };
 
                 self.build_reduced_graph_for_use_tree(
-                    use_tree, item.id, vis, &prefix, false, item, expansion,
+                    use_tree, item.id, use_tree, item.id, vis, &prefix, false, item, expansion,
                 );
             }
 
@@ -266,10 +282,12 @@ impl<'a> Resolver<'a> {
                 let binding =
                     (module, ty::Visibility::Public, sp, expansion).to_name_binding(self.arenas);
                 let directive = self.arenas.alloc_import_directive(ImportDirective {
+                    root_id: item.id,
                     id: item.id,
                     parent,
                     imported_module: Cell::new(Some(module)),
                     subclass: ImportDirectiveSubclass::ExternCrate(orig_name),
+                    root_span: item.span,
                     span: item.span,
                     module_path: Vec::new(),
                     vis: Cell::new(vis),
@@ -640,10 +658,12 @@ impl<'a> Resolver<'a> {
 
         let (graph_root, arenas) = (self.graph_root, self.arenas);
         let macro_use_directive = |span| arenas.alloc_import_directive(ImportDirective {
+            root_id: item.id,
             id: item.id,
             parent: graph_root,
             imported_module: Cell::new(Some(module)),
             subclass: ImportDirectiveSubclass::MacroUse,
+            root_span: span,
             span,
             module_path: Vec::new(),
             vis: Cell::new(ty::Visibility::Restricted(DefId::local(CRATE_DEF_INDEX))),

src/librustc_resolve/resolve_imports.rs

@@ -55,12 +55,36 @@ pub enum ImportDirectiveSubclass<'a> {
 /// One import directive.
 #[derive(Debug,Clone)]
 pub struct ImportDirective<'a> {
+    /// The id of the `extern crate`, `UseTree` etc that imported this `ImportDirective`.
+    ///
+    /// In the case where the `ImportDirective` was expanded from a "nested" use tree,
+    /// this id is the id of the leaf tree. For example:
+    ///
+    /// ```rust,ignore
+    /// use foo::bar::{a, b}
+    /// ```
+    ///
+    /// If this is the import directive for `foo::bar::a`, we would have the id of the `UseTree`
+    /// for `a` in this field.
     pub id: NodeId,
+
+    /// The `id` of the "root" use-kind -- this is always the same as
+    /// `id` except in the case of "nested" use trees, in which case
+    /// it will be the `id` of the root use tree. e.g., in the example
+    /// from `id`, this would be the id of the `use foo::bar`
+    /// `UseTree` node.
+    pub root_id: NodeId,
+
+    /// Span of this use tree.
+    pub span: Span,
+
+    /// Span of the *root* use tree (see `root_id`).
+    pub root_span: Span,
+
     pub parent: Module<'a>,
     pub module_path: Vec<Ident>,
     pub imported_module: Cell<Option<Module<'a>>>, // the resolution of `module_path`
     pub subclass: ImportDirectiveSubclass<'a>,
-    pub span: Span,
     pub vis: Cell<ty::Visibility>,
     pub expansion: Mark,
     pub used: Cell<bool>,
@@ -296,6 +320,8 @@ impl<'a> Resolver<'a> {
                                 subclass: ImportDirectiveSubclass<'a>,
                                 span: Span,
                                 id: NodeId,
+                                root_span: Span,
+                                root_id: NodeId,
                                 vis: ty::Visibility,
                                 expansion: Mark) {
         let current_module = self.current_module;
@@ -306,6 +332,8 @@ impl<'a> Resolver<'a> {
             subclass,
             span,
             id,
+            root_span,
+            root_id,
             vis: Cell::new(vis),
             expansion,
             used: Cell::new(false),