9bd4e5469e
Boxes are a bit magic in that they need to use `*` to get an owned value out of the box. They implement `Deref` but that only returns a reference. This means an easy way to convert an `Option<Box<T>>` to an `<Option<T>` is: ``` box_option.map(|b| *b) ``` However, since b36bb0a6 the `map_clone` lint is detecting this as an attempt to copy the box. Fix by excluding boxes completely from the deref part of this lint. Fixes #3274