我有下面的export_directory函数。

// For handling multiple error type
type BoxResult<T> = Result<T, Box<std::error::Error>>;

fn export_directory(dir: &Directory, export_path: &str) -> BoxResult<String> {
  let mut file = OpenOptions::new().write(true).truncate(true).create(true).open(export_path)?;
  serde_json::to_string(dir).or_else(|err| Err(Box::new(err)))
}

fn main() {
  let mut dir = Directory::new();
  export_directory(&dir, "export_path.json");
}

当我编译代码的时候，它给了我

error[E0308]: mismatched types
  --> src/main.rs:98:3
   |
96 | fn export_directory(dir: &Directory, export_path: &str) -> BoxResult<String> {
   |                                                            ----------------- expected `std::result::Result<std::string::String, std::boxed::Box<(dyn std::error::Error + 'static)>>` because of return type
97 |   let mut file = OpenOptions::new().write(true).truncate(true).create(true).open(export_path)?;
98 |   serde_json::to_string(dir).or_else(|err| Err(Box::new(err)))
   |   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected trait std::error::Error, found struct `serde_json::error::Error`
   |
   = note: expected type `std::result::Result<_, std::boxed::Box<(dyn std::error::Error + 'static)>>`
              found type `std::result::Result<_, std::boxed::Box<serde_json::error::Error>>`

但是当引用serde documentation时，serde_json::error::Error已经实现了Error trait，这里出了什么问题？