Stream the dep-graph to a file.

compiler/rustc_query_system/src/lib.rs

@@ -2,6 +2,7 @@
 #![feature(const_fn)]
 #![feature(const_panic)]
 #![feature(core_intrinsics)]
+#![feature(drain_filter)]
 #![feature(hash_raw_entry)]
 #![feature(iter_zip)]
 #![feature(min_specialization)]