### What it does
Checks for usage of `.to_string()` on an `&&T` where
`T` implements `ToString` directly (like `&&str` or `&&String`).

### Why is this bad?
This bypasses the specialized implementation of
`ToString` and instead goes through the more expensive string formatting
facilities.

### Example
```
// Generic implementation for `T: Display` is used (slow)
["foo", "bar"].iter().map(|s| s.to_string());

// OK, the specialized impl is used
["foo", "bar"].iter().map(|&s| s.to_string());
```