What is proc-macro2

[sgrif]

@alexcrichton @mystor

Did I do this right?

[alexcrichton]

Right now procedural methods have no method of preserving span information of expanded code or otherwise dealing with spans. That's all due to the usage of strings on both ends of the interface where you to_string a TokenStream and then parse back into a TokenStream. Clearly this isn't the best interface for all of time, so we'd like to fix that!

Upstream in rust-lang/rust the proc_macro crate is getting an enhanced API which will enable preserving and customizing span information of streams coming out of and going back into the compiler. This should be a great benefit to all procedural macros in terms of error messages and debugging usability.

All of those new APIs, however, are naturally unstable. They'll take some time to stabilize and the timeline for stabilization is unclear. To work around this the proc_macro2 crate is an attempt to sidestep this stabilization timeline. The proc_macro2 crate intends to provide the exact same API as proc_macro (if it were stable) yet also compile on stable Rust. To do that proc_macro2 by default will use the to_string and parse interfaces to convert to/from TokenStream. You can, however, enable the unstable feature of the proc_macro2 crate which actually compiles against the unstable interface of proc_macro. This requires a nightly compiler, but should be able to losslessly preserve all span information.

So basically tl;dr; proc_macro2 is future-proofing. Macros written against it have a turnkey solution for better spans on nightly today, and will have a turnkey solution for better spans in the future once the new proc_macro API is stabilized.

Does that make sense?

[sgrif]

It does make sense, yes. So how do I get one of these