Remove support for gen arg

src/libsyntax/fold.rs

@@ -1304,7 +1304,6 @@ pub fn noop_fold_expr<T: Folder>(Expr {id, node, span, attrs}: Expr, folder: &mu
                 };
             }
             ExprKind::Yield(ex) => ExprKind::Yield(ex.map(|x| folder.fold_expr(x))),
-            ExprKind::ImplArg => ExprKind::ImplArg,
             ExprKind::Try(ex) => ExprKind::Try(folder.fold_expr(ex)),
             ExprKind::Catch(body) => ExprKind::Catch(folder.fold_block(body)),
         },