Convert Span to proc_macro::Span

[antoyo]

Hi.
It would be useful to be able to convert a proc_macro2::Span to a proc_macro::Span.
For instance, this will allow to create Diagnostics with Span.
Thank you to add a From implementation for unstable only or something along these lines.

[alexcrichton]

Ah yeah currently this isn't done b/c this is functionality which can only be implemented with the unstable feature enabled, but seems reasonable to implement in that case to me!

[antoyo]

Either that or a function that returns Span::call_site() when the unstable feature is not enabled.
That will be very useful.
Thank you.

[mystor]

I have some WIP patches to add meaningful spans to the stable version of proc-macro2. It should allow us to expose those APIs from the span wrapper directly.

[alexcrichton]

Oh nice! Sounds good to me @mystor

[SergioBenitez]

I've actually made these changes in my fork: https://github.com/SergioBenitez/proc-macro2. If these seem reasonable, @alexcrichton, I'd be happy to submit a PR.

[alexcrichton]

There's some work for this at #36, with the conclusion at the end that what we should change is putting it behind an experimental feature.

[SergioBenitez]

The approach I've taken is a much simpler one: simply expose a method to retrieve a proc_macro::Span. This functionality is placed behind the "unstable" feature flag. Maybe we add another feature for this?

The reasoning here is that this approach relieves users of proc-macro2 from depending on the library being updated for each piece of new functionality introduced in the nightly proc-macro.

I don't have any issue with also exposing the shims in #36, however!

[alexcrichton]

Added in #41