Merge #11513

11513: internal: Expand the derive attribute into a pseudo expansion r=Veykril a=Veykril

Quoting my comment:

> We generate a very specific expansion here, as we do not actually expand the `#[derive]` attribute
> itself in name res, but we do want to expand it to something for the IDE layer, so that the input
> derive attributes can be downmapped, and resolved as proper paths.
> This is basically a hack, that simplifies the hacks we need in a lot of ide layer places to
> somewhat inconsistently resolve derive attributes.
> 
> As such, we expand `#[derive(Foo, bar::Bar)]` into
> ```
>  #[Foo]
>  #[bar::Bar]
>  ();
> ```
> which allows fallback path resolution in hir::Semantics to properly identify our derives.
> Since we do not expand the attribute in nameres though, we keep the original item.
> 
> The ideal expansion here would be for the `#[derive]` to re-emit the annotated item and somehow
> use the input paths in its output as well.
> But that would bring two problems with it, for one every derive would duplicate the item token tree
> wasting a lot of memory, and it would also require some way to use a path in a way that makes it
> always resolve as a derive without nameres recollecting them.
> So this hacky approach is a lot more friendly for us, though it does require a bit of support in
> [`hir::Semantics`] to make this work.



Co-authored-by: Lukas Wirth <lukastw97@gmail.com>

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