enum variants should have modifiers

as I mentioned on the thread asking for what new features to add to syn on rust internals, [enum variant/field visibility has been proposed](https://internals.rust-lang.org/t/crowdsourcing-request-what-syntax-changes-that-will-affect-the-ast-are-currently-in-flight/24361/10?u=programmerjake). is it too late to adjust 3.x to allow adding a non-exhaustive modifiers struct to enum variants, just like the other modifiers structs?

maybe publish 4.0 soon? it would be nice to not have to wait another 3 years.

The issue names no files or tests. Start by reviewing how syn represents enum variants and its existing modifier structs, then compare that design with the linked Rust syntax proposal. Done means reaching a maintainer-approved scope for supporting non-exhaustive variant modifiers and clarifying the compatible release target.

Written by the indexing model from the issue text.