When `derive` is already applied to struct, suggest extending it, not replacing it

The following suggestion to add derive(Debug) does not take into consideration that line 170 already has another derive. It should suggest extending the existing derive instead of a new line:

warning: you should consider deriving a `Default` implementation for `policy::Attributes`
   -->
    |
176 | /   pub fn new() -> Attributes {
177 | |     Attributes {
178 | |       map: HashMap::new(),
179 | |     }
180 | |   }
    | |___^
    |
    = note: #[warn(new_without_default_derive)] on by default
    = help: for further information visit https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#new_without_default_derive
help: try this
    |
170 | #[derive(Default)]
    |

Which when applying leads to having the following code:

169 | #[derive(Debug)]
170 | #[derive(Default)]
171 | struct Attributes {

Start by locating the new_without_default_derive lint and the code that builds its help suggestion. Check how an existing #[derive(...)] is represented and find the related suggestion tests, if present. Done means the suggestion extends an existing derive instead of adding a separate derive attribute.