Migration plan for v2 -> v3

[hansl]

There is a lack of examples mixing both imperative arg declaration (using App and Arg::with_name()) and the declarative proc_macro v3 style (using #[derive(Clap)]).

We want to use the beta for v3 proc_macro style declaration (because it's awesome), and all our code is currently using v2.

We wanted to know if there was any code we could use as example that uses both. If it is possible at all, of course.

[pksunkara]

That is a good idea. Currently proc_macro style examples live at https://github.com/clap-rs/clap/tree/master/clap_derive/examples. Each struct/enum will become a clap::App. You can also use cargo-expand cli to test how the program expands to. Hope that would guide you to create a minimal example.

[CreepySkeleton]

@hansl You can very well use the proc_macro style with 2.x right now via structopt 😉

That reminded me - the clap's derive will not be 100% back compatible with structopt. Simple #[structopt] ->#[clap] won't be enough in some (rare, I hope) cases.

[pickfire]

Is auto-migration or compat layer planned?

[pksunkara]

Not really, but me and @CreepySkeleton were thinking about a migration script. And I have been thinking if we can make something like that rust in general. So, if I build a crate A and I want to release breaking version v2, I can write a small script to help people upgrading.

[pickfire]

Nice, it's like angular update. Hopefully as easy as rustfmt but we don't have something like that in rust IIRC yet.

[pksunkara]

Yeah, definitely need to make it easy as rustfmt. It would also need syntax tree traversal. I want to work on it but I am already involved in too many projects 😞

[CreepySkeleton]

I'll be definitely looking into it, but beta & bugfixes come first.

I've been thinking about rustc-plugins. In short, we need to traverse the AST before macros have been expanded. Probably clippy code could teach me something.