{"ast_node_type":"function","tokens":[{"end":81,"kind":"keyword","start":0,"text":"let"}]}
    let len_fn = get_impl_method(ctx, &impl_, &Name::new_symbol_root(sym::len))?;
    let impl_ = fn_node.syntax().ancestors().find_map(ast::Impl::cast)?;
    if !len_fn.ret_type(ctx.sema.db).is_usize() {