Rust 为结果返回'map\u err'，失败

我正在找出返回

map\u err

时以下操作失败的原因：

Compiling playground v0.0.1 (/playground)
error[E0308]: mismatched types
  --> src/main.rs:37:5
   |
36 | fn R() -> Result<i32, MyError> {
   |           -------------------- expected `std::result::Result<i32, MyError>` because of return type
37 |     decode().map_err(|e| Err(MyError {v: "changed".to_string() }))
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `MyError`, found enum `std::result::Result`
   |
   = note: expected enum `std::result::Result<_, MyError>`
              found enum `std::result::Result<_, std::result::Result<_, MyError>>`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground`.

如果您查看，您将看到它期望一个采用错误类型并返回错误类型的闭包，而不是

结果

decode().map_err(|e| MyError {v: "changed".to_string() })