libproc_macro: get crate for span or support $crate metavar in quote!{}

[abonander]

One Reddit user was asking how to reference the invoking crate in a proc-macro, suggesting that quote!{} should support the $crate metavar. I don't think this is too unreasonable since $ is already used for unquoting variables but crate couldn't be a user variable since it's a keyword.

Alternately, I think Span could have a method telling us what crate it comes from as a String.

cc @alexcrichton @dtolnay

[Badel2]

I don't know if my use case can be resolved this way: I have a crate enum-vec but when I want to use it I have to include 3 crates:

#[macro_use]
extern crate enum_like_derive;
extern crate enum_like;
extern crate enum_vec;

I thought that maybe if I reexport EnumLike from enum_vec (since enum_vec already depends on enum_like) I could do use enum_vec::EnumLike; instead of including the enum_like crate, saving one precious line of code. But my proc macro enum_like_derive is hardcoded to use ::enum_like::EnumLike, so I was wondering which is the correct way to refer to EnumLike.

I currently do impl ::enum_like::EnumLike on the proc macro, should I just do impl EnumLike if I want to support reexporting the EnumLike trait under other names? What if someone does extern crate enum_like as enl; or something like that? My proc macro is forced to fail?

[alexcrichton]

Yes this definitely isn't great today unfortunately. The "real solution" is hygiene and until we get that a derive/macro basically needs to pick how it should be invoked and document that. For exampel @Badel2 you can use reexports yeah but you'd have to only do that effectively

[safinaskar]

This is duplicate of #54363