Feature gate auto trait and ignore it in rustdoc

src/librustdoc/visit_ast.rs

@@ -481,7 +481,7 @@ pub fn visit_item(&mut self, item: &hir::Item,
                 };
                 om.constants.push(s);
             },
-            hir::ItemTrait(unsafety, ref gen, ref b, ref item_ids) => {
+            hir::ItemTrait(_, unsafety, ref gen, ref b, ref item_ids) => {
                 let items = item_ids.iter()
                                     .map(|ti| self.cx.tcx.hir.trait_item(ti.id).clone())
                                     .collect();

src/libsyntax/feature_gate.rs

@@ -1389,6 +1389,12 @@ fn visit_item(&mut self, i: &'a ast::Item) {
                 }
             }
 
+            ast::ItemKind::Trait(ast::IsAuto::Yes, ..) => {
+                gate_feature_post!(&self, optin_builtin_traits,
+                                   i.span,
+                                   "auto traits are experimental and possibly buggy");
+            }
+            
             ast::ItemKind::MacroDef(ast::MacroDef { legacy: false, .. }) => {
                 let msg = "`macro` is experimental";
                 gate_feature_post!(&self, decl_macro, i.span, msg);