`type_ascription` and `box_syntax` are not required to compile the crate (and `box_syntax` has been removed in any case).
@@ -35,7 +35,7 @@
//! combinator patterns to express the parse.
#![no_std]
-#![feature(decl_macro, type_ascription, box_syntax)]
+#![feature(decl_macro)]
extern crate alloc;