Turn features() into a query.

src/librustc_incremental/assert_dep_graph.rs

@@ -69,7 +69,7 @@ pub fn assert_dep_graph<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
         // if the `rustc_attrs` feature is not enabled, then the
         // attributes we are interested in cannot be present anyway, so
         // skip the walk.
-        if !tcx.sess.features.borrow().rustc_attrs {
+        if !tcx.features().rustc_attrs {
             return;
         }
 

src/librustc_incremental/persist/dirty_clean.rs

@@ -219,7 +219,7 @@ impl Assertion {
 
 pub fn check_dirty_clean_annotations<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
     // can't add `#[rustc_dirty]` etc without opting in to this feature
-    if !tcx.sess.features.borrow().rustc_attrs {
+    if !tcx.features().rustc_attrs {
         return;
     }