{"ast_node_type":"variable","tokens":[{"end":81,"kind":"keyword","start":0,"text":"let"}]}
    let param = impl_trait_type.syntax().ancestors().find_map(ast::Param::cast)?;
    let impl_trait_type = ctx.find_node_at_offset::<ast::ImplTraitType>()?;
    let fn_ = param.syntax().ancestors().nth(2).and_then(ast::Fn::cast)?;