Fix kind for associated types in rustdoc JSON output for trait implementations

Author: GuillaumeGomez

src/librustdoc/json/conversions.rs

@@ -252,8 +252,11 @@ fn from_clean_item(item: clean::Item, tcx: TyCtxt<'_>) -> ItemEnum {
             bounds: b.into_iter().map(|x| x.into_tcx(tcx)).collect(),
             default: None,
         },
-        // FIXME: do not map to Typedef but to a custom variant
-        AssocTypeItem(t, _) => ItemEnum::Typedef(t.into_tcx(tcx)),
+        AssocTypeItem(t, b) => ItemEnum::AssocType {
+            generics: t.generics.into_tcx(tcx),
+            bounds: b.into_iter().map(|x| x.into_tcx(tcx)).collect(),
+            default: t.item_type.map(|ty| ty.into_tcx(tcx)),
+        },
         // `convert_item` early returns `None` for striped items and keywords.
         StrippedItem(_) | KeywordItem(_) => unreachable!(),
         ExternCrateItem { ref src } => ItemEnum::ExternCrate {