add a new nightly flag

src/main.rs

@@ -1,4 +1,9 @@
-#![feature(generators, generator_trait, type_alias_impl_trait)]
+#![feature(
+    generators,
+    generator_trait,
+    type_alias_impl_trait,
+    impl_trait_in_assoc_type
+)]
 
 mod core;
 mod prelude;