Add feature gate for inherent associate types.

compiler/rustc_feature/src/active.rs

@@ -641,6 +641,9 @@ declare_features! (
     /// Allows `pub` on `macro_rules` items.
     (active, pub_macro_rules, "1.52.0", Some(78855), None),
 
+    /// Allows associated types in inherent impls.
+    (active, inherent_associated_types, "1.52.0", Some(8995), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------
@@ -666,6 +669,7 @@ pub const INCOMPLETE_FEATURES: &[Symbol] = &[
     sym::unsized_locals,
     sym::capture_disjoint_fields,
     sym::const_generics_defaults,
+    sym::inherent_associated_types,
 ];
 
 /// Some features are not allowed to be used together at the same time, if