Make hash_result an Option.

src/base.rs

@@ -59,7 +59,13 @@ pub fn compile_codegen_unit<'tcx>(tcx: TyCtxt<'tcx>, cgu_name: Symbol) -> (Modul
     let start_time = Instant::now();
 
     let dep_node = tcx.codegen_unit(cgu_name).codegen_dep_node(tcx);
-    let (module, _) = tcx.dep_graph.with_task(dep_node, tcx, cgu_name, module_codegen, dep_graph::hash_result);
+    let (module, _) = tcx.dep_graph.with_task(
+        dep_node,
+        tcx,
+        cgu_name,
+        module_codegen,
+        Some(dep_graph::hash_result),
+    );
     let time_to_codegen = start_time.elapsed();
     drop(prof_timer);