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