`question_mark` feature stabilized.

src/lib.rs

@@ -16,8 +16,7 @@
 #![no_std]
 
 #![feature(allocator, const_fn, core_intrinsics, stmt_expr_attributes, drop_types_in_const,
-           nonzero, optin_builtin_traits, type_ascription, question_mark, thread_local, linkage,
-           try_from)]
+           nonzero, optin_builtin_traits, type_ascription, thread_local, linkage, try_from)]
 #![warn(missing_docs, missing_docs_in_private_items, cast_precision_loss, cast_sign_loss,
         cast_possible_wrap, cast_possible_truncation, filter_map, if_not_else,
         items_after_statements, invalid_upcast_comparisons, mutex_integer, nonminimal_bool,