MultiItemDecorator should take &Annotatable

[Manishearth]

pub trait ItemDecorator {
    fn expand(&self, ecx: &mut ExtCtxt, sp: Span, meta_item: &MetaItem, item: &Item, push: &mut FnMut(P<Item>));
}

is deprecated in favor of

pub trait MultiItemDecorator {
    fn expand(&self, ecx: &mut ExtCtxt, sp: Span, meta_item: &MetaItem, item: Annotatable, push: &mut FnMut(Annotatable));
}

The new version takes the thing being decorated by-move, and this is achieved by a clone. The clone is totally unnecessary; a decorator doesn't modify the original item and should be fine with a simple reference to it.

This probably causes a lot of unnecessary clones since MultiDecorators are used everywhere in the form of #[derive].

Should we make this &Annotatable and remove the clone?

cc @nrc @huonw

[bluss]

How large is annotatable? It only matters if it's expensive to clone.

[Manishearth]

An Annotatable is generally an Item. In this case, mostly structs and enums. Cloning a struct in the AST isn't too expensive, but when you do it multiple times for almost every struct it adds up. I'm going to profile this for the heck of it and see.

[nrc]

An Annotatable is just an enum of pointers (a pointer to an item, not actually an item), so it is only one word, cloning it shouldn't make any difference. Think of it as a smart pointer. Still interested to see a profile.

[Manishearth]

It contains P which does a deep clone