Split out a separate feature gate for impl trait in associated types

src/librustdoc/lib.rs

@@ -15,6 +15,7 @@
 #![feature(type_ascription)]
 #![feature(iter_intersperse)]
 #![feature(type_alias_impl_trait)]
+#![cfg_attr(not(bootstrap), feature(impl_trait_in_assoc_type))]
 #![recursion_limit = "256"]
 #![warn(rustc::internal)]
 #![allow(clippy::collapsible_if, clippy::collapsible_else_if)]