为什么serde_json :: from_reader取得了读者的所有权？

My code:

我的代码：

fn request_add<T>(request: &mut Request, collection_name: &'static str) -> Fallible<Fallible<String>>
where
    T: serde::Serialize + serde::de::DeserializeOwned,
{
    let add_dao = dao::MongoDao::new(collection_name);
    let obj = serde_json::from_reader::<Body, T>(request.body)?; //cannot move out of borrowed content
    Ok(add_dao.add(&obj))
}
fn request_add<T>(request: &