save-analysis: document `process_use_tree`

src/librustc_save_analysis/dump_visitor.rs

@@ -1231,6 +1231,14 @@ impl<'l, 'tcx: 'l, 'll, O: DumpOutput + 'll> DumpVisitor<'l, 'tcx, 'll, O> {
         }
     }
 
+    /// Dumps imports in a use tree recursively.
+    ///
+    /// A use tree is an import that may contain nested braces (RFC 2128). The `use_tree` parameter
+    /// is the current use tree under scrutiny, while `id` and `prefix` are its corresponding node
+    /// id and path. `root_item` is the topmost use tree in the hierarchy.
+    ///
+    /// If `use_tree` is a simple or glob import, it is dumped into the analysis data. Otherwise,
+    /// each child use tree is dumped recursively.
     fn process_use_tree(&mut self,
                          use_tree: &'l ast::UseTree,
                          id: NodeId,
@@ -1238,6 +1246,8 @@ impl<'l, 'tcx: 'l, 'll, O: DumpOutput + 'll> DumpVisitor<'l, 'tcx, 'll, O> {
                          prefix: &ast::Path) {
         let path = &use_tree.prefix;
         let access = access_from!(self.save_ctxt, root_item);
+
+        // The parent def id of a given use tree is always the enclosing item.
         let parent = self.save_ctxt.tcx.hir.opt_local_def_id(id)
             .and_then(|id| self.save_ctxt.tcx.parent_def_id(id))
             .map(::id_from_def_id);