Implemented #[skip] for the Add derive

[ErmitaVulpe]

Resolves #437

Synopsis

As outlined in #437 it isn't possible to derive Add for structs containing zero sized fields

Example:

#[derive(Add)] // method not found in `PhantomData<T>`
struct TupleWithZst<T>(i32, PhantomData<T>);

#[derive(Add)] // method not found in `PhantomData<T>`
struct StructWithZst<T> {
    x: i32
    _marker: PhantomData<T>,
}

Solution

Added a #[skip] attribute that allows to skip zero sized fields like so:

#[derive(Add)]
struct TupleWithZst<T>(i32, #[add(skip)] PhantomData<T>);

#[derive(Add)]
struct StructWithZst<T> {
    x: i32,
    #[add(skip)]
    _marker: PhantomData<T>,
}

Checklist

• Documentation is updated (if required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (if required)

[tyranron]

@ErmitaVulpe thanks!

However, as all our attributes are namespaced (e.g. #[from(skip)] and etc.), I would like to see this PR to stick with this style and have #[add(skip)] instead of just #[skip].

Also, it would be nice if the util::attr::Skip would be reused for this purpose.

[ErmitaVulpe]

To do this I needed to get a bit more familiar with the codebase and i noticed that Mul and MulAssign depended on add_helpers.rs for handling of the #[mul(forward)] attrib so I also implemented it for them :P

Also I changed the way that trait bounds are added to the generics so that the generics contained in the skipped fields do not require the derived trait to be implemented for them.