map_identity: false negative with map_err

[TonalidadeHidrica]

What it does

It detects verbose identity conversions like iterator.map(|a| a), result.map(|a| a), result.map_err(|a| a), option.map(|a| a) and so on.

Lint Name

needless_identity_map

Category

No response

Advantage

• Removes unnecessary code.

Drawbacks

• Possibly eliminates readability originally intended by verbose code...? I doubt it.

Example

  let r: Result<u32, u32> = Ok(1);
  let u = r.map_err(|a| a);

Could be written as:

  let r: Result<u32, u32> = Ok(1);
  let u = r;

[matthiaskrgr]

Isn't this what the map_identity lint is about? 🤔

pub fn main() {
    let _ = vec![1, 2, 3].into_iter().map(|x| x).next();
}

warning: unnecessary map of the identity function
 [--> src/main.rs:2:38](https://play.rust-lang.org/#)  |
2 |     let _ = vec![1, 2, 3].into_iter().map(|x| x).next();
  |                                      ^^^^^^^^^^^ help: remove the call to `map`
  |
  = note: `#[warn(clippy::map_identity)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#map_identity

[TonalidadeHidrica]

@matthiaskrgr Sorry that I didn't check thoroughly. I realized that it won't work for Result::map_err and submitted an over-generalized request. I now confirmed that Result::map_err is not covered by map_identity lint. Should I open a new issue?

[matthiaskrgr]

Nah, I can simply reopen this one. 😄
I think linting on map_err makes sense for the lint so that's a false negative 🙃

[dswij]

@rustbot claim