Rustdoc: remove ListAttributesIter and use impl Iterator instead

src/librustdoc/lib.rs

@@ -16,6 +16,8 @@
 #![feature(once_cell)]
 #![feature(type_ascription)]
 #![feature(iter_intersperse)]
+#![feature(type_alias_impl_trait)]
+#![feature(generic_associated_types)]
 #![recursion_limit = "256"]
 #![warn(rustc::internal)]
 #![allow(clippy::collapsible_if, clippy::collapsible_else_if)]