Cache param_env on disk.

Author: cjgillot

compiler/rustc_middle/src/query/mod.rs

@@ -1635,6 +1635,7 @@ rustc_queries! {
     /// then you should take a `TypingEnv` to ensure that you handle opaque types correctly.
     query param_env(def_id: DefId) -> ty::ParamEnv<'tcx> {
         desc { |tcx| "computing normalized predicates of `{}`", tcx.def_path_str(def_id) }
+        cache_on_disk_if { true }
         feedable
     }