Don't requery the param_env of a union

Union fields have the ParamEnv of the union.

Author: matthewjasper

src/librustc_typeck/check/mod.rs

@@ -1545,11 +1545,11 @@ fn check_union_fields(tcx: TyCtxt<'_>, span: Span, item_def_id: DefId) -> bool {
     if let ty::Adt(def, substs) = item_type.kind {
         assert!(def.is_union());
         let fields = &def.non_enum_variant().fields;
+        let param_env = tcx.param_env(item_def_id);
         for field in fields {
             let field_ty = field.ty(tcx, substs);
             // We are currently checking the type this field came from, so it must be local.
             let field_span = tcx.hir().span_if_local(field.did).unwrap();
-            let param_env = tcx.param_env(field.did);
             if field_ty.needs_drop(tcx, param_env) {
                 struct_span_err!(
                     tcx.sess,