Call to get dyn Any of contexts/attachment values for Dynamic reports and attachments.

[Kile-Asmussen]

Rootcause's support for type erasure/dynamic typing is pretty great, but also something of a walled garden. It would be potentially useful to have something like:

impl<'a, O, T> ReportRef<Dynamic, O, T> {
  fn current_context_dyn_any(&self) -> &dyn Any {
    /* implementation details */
  }
}

My proposal for an implementation is to include a field in the vtables of type:

  as_any: fn(&C /* or A*/) -> &dyn Any

which will get a value at creation time given as

  as_any: |c: &C /* or a: &A */| c as &dyn Any

and then this is used in the obvious manner to implement appropriate functions for RawReport(Ref/Mut) and RawAttachment(Ref/Mut), which are then used to implement according functions in Report(|Ref|Mut)<Dynamic> and ReportAttachment(|Ref|Mut)<Dynamic>.

Pros:

• Better integration with other libraries owing to using the 'lowest common denominator' for dynamic typing.

Cons:

• An extra field in the vtables.
• Functionality is technically already provided by current_context_type_id and downcast_current_context and the like.

Scope:

• Six new functions in public API (owned, ref, mut X report, attachments).
• Six new functions in rootcause internals raw API.
• Two extra function-typed fields in vtable API.
• An extra field in each of the two vtable structs.
• Probably little to no unsafe code (apart from access to the raw field.)
• Doc comment examples.

[TethysSvensson]

Sure, I would accept that PR!

The only open question in my mind is about the current_context_dyn_any name.

Do you think it might make sense to just call it current_context like on the non-dynamic ones? Or alternatively make the function available on the non-dynamic ones as well.

I'm also considering if this might replace downcast_current_context methods?