### What it does
Checks for instances of `map(f)` where `f` is the identity function.

### Why is this bad?
It can be written more concisely without the call to `map`.

### Example
```
let x = [1, 2, 3];
let y: Vec<_> = x.iter().map(|x| x).map(|x| 2*x).collect();
```
Use instead:
```
let x = [1, 2, 3];
let y: Vec<_> = x.iter().map(|x| 2*x).collect();
```