`borrowed_box` incorrectly assumes mutable boxed slice allocation won't change

[okready]

Defining a function that has a &mut Box<[T]> parameter triggers the borrowed_box lint, recommending it be replaced with a &mut [T]. It's possible to have a function that replaces the boxed slice with a different allocation (e.g. a slice with a different size), which cannot be done if the parameter is just a slice reference.

Example:

pub fn maybe_sets_box(boxed_slice: &mut Box<[i32]>) {
    if boxed_slice.is_empty() {
        let mut data = Vec::with_capacity(1);
        data.push(12);
        *boxed_slice = data.into_boxed_slice();
    }
}

[EFanZh]

I think Clippy should allow the usage of &mut Box<T> but disallow the usage of &Box<T> since we might want to change the box pointer instead of the object it points to.

[jmaargh]

A (perhaps a little contrived) example: https://godbolt.org/z/LFRvd7

[ebroto]

I think this issue can be closed given that #5491 was merged