syn VS case-studies

[2] https://github.com/dtolnay/syn/issues/782

It looks to me like you are trying to make a syntax tree generic over arities. I suggest you take a look at the Expr enum in syn, its variant structs, and the ast_enum_of_structs_impl macro, which is a macro that implements a trait over different Expr variants with different fields.

Rust makes this project easy by providing the syn crate (parsing and formatting) and WASM (for interactivity). Source code lives at https://github.com/CarlKCarlK/rust-ast-explorer. Contributions welcome.

Perhaps there should, but if you look at the commit history you'll see that syn is maintained almost entirely by dtolnay (https://github.com/dtolnay/syn/graphs/contributors). Furthermore, with Rust continually adding new syntax that needs to be supported, syn can't be easy to maintain and most of the effort probably goes into making it work at all rather than optimisations.

Luckily there is a workaround to emulate such a definition, which dtolnay discovered and explained here: https://github.com/dtolnay/case-studies/tree/b9802f6df8dc8e54970b83fb9af6df923b46abf5/unit-type-parameters.

I think they are talking about this one trick which the devs don't want you to know about. Note that while it looks like specialization, it works only in a few very limited cases and is quite fragile, so it's a hack, not a substitute for the real feature.

Also I suggest his case studies repo since you are looking at what is possible: https://github.com/dtolnay/case-studies

/u/dtolnay has a great case studies repository.

Once you have the basics down, read dtolnay's case studies. They show how to do advanced stuff with easy macros.

If you were writing your own println macro, you might be able to get away with this kind of hack: https://github.com/dtolnay/case-studies/blob/master/autoref-specialization/README.md

You can use "Autoref-based stable specialization" or use/mimic the impls crate.

With a subset of specialization likely riding the trains soon and a workaround available, why would you give up?

this works since values and types are in different namespaces (see: Rusts Universes or dtolnay's Case Study about "Unit struct with type parameters")

To do this with traits you would need specialization but since you are using macros you should be able to use "Autoref-based stable specialization". Here is a playground which uses the latter approach to implement the wanted macro without using any nightly features.