Ignore zero-width fields like `PhantomData`

[raklaptudirm]

Ignore zero-width fields to allow complier hinting with types like PhantomData while deriving trait imlementations.

// This should work.
#[derive(derive_more::Add)]
struct A<B>(u16, PhantomData<B>);

// Equivalent to:
impl<B> Add for A<B> {
    type Output = Self;

    fn add(self, rhs: Self) -> Self::Output {
        Self(self.0 + rhs.0, PhantomData)
    }
}

This behavior can be seen in #[repr(transparent)], where the annotation will work as long as there is only one non-zero-width type.

[tyranron]

@raklaptudirm since Rust doesn't have any compile-time reflection, we cannot know whether a type is zero-sized during macro expansion. Of course, we can try to match them by name, but that will break shortly if the type is aliased by an unknown name for us.

What we can do instead, is to allow user specifying that:

#[derive(derive_more::Add)]
struct A<B>(u16, #[add(skip)] PhantomData<B>);

// will expand to:
impl<B> Add for A<B> {
    type Output = Self;

    fn add(self, rhs: Self) -> Self::Output {
        Self(self.0 + rhs.0, self.1)
    }
}

[raklaptudirm]

Makes sense, #[add(skip)] or even a more general #[skip] would be perfect.