nightly feature tracking: get rid of the per-feature bool fields

compiler/rustc_feature/src/builtin_attrs.rs

@@ -14,7 +14,7 @@ type GateFn = fn(&Features) -> bool;
 
 macro_rules! cfg_fn {
     ($field: ident) => {
-        (|features| features.$field) as GateFn
+        Features::$field as GateFn
     };
 }
 
@@ -1193,7 +1193,7 @@ pub static BUILTIN_ATTRIBUTE_MAP: LazyLock<FxHashMap<Symbol, &BuiltinAttribute>>
 pub fn is_stable_diagnostic_attribute(sym: Symbol, features: &Features) -> bool {
     match sym {
         sym::on_unimplemented => true,
-        sym::do_not_recommend => features.do_not_recommend,
+        sym::do_not_recommend => features.do_not_recommend(),
         _ => false,
     }
 }