diff --git a/lib/compiler/State.ml b/lib/compiler/State.ml
--- a/lib/compiler/State.ml
+++ b/lib/compiler/State.ml
@@ -21,7 +21,7 @@ config: Config.t;
   index: Tree.t URI.Tbl.t;
   diagnostics: Error.t list URI.Tbl.t;
   graphs: (module Forest_graphs.S);
-  mutable import_graph: Forest_graph.t;
+  import_graph: Forest_graph.t;
   search_index: Forester_search.Index.t;
   history: Action.t list;
   hosts: (string, unit) Hashtbl.t;