Remove calls to `Rf_findVarInFrame`

[t-kalinowski]

S7 currently has a check Note:

Version: 0.2.1
Check: compiled code
Result: NOTE 
  File ‘S7/libs/S7.so’:
    Found non-API call to R: ‘Rf_findVarInFrame’
  
  Compiled code should not call non-API entry points in R.
  
  See ‘Writing portable packages’ in the ‘Writing R Extensions’ manual,
  and section ‘Moving into C API compliance’ for issues with the use of
  non-API entry points.

We also received an email with a deadline for when these should be addressed:

The NOTEs about calls to Rf_findVar and Rf_findVarInFrame will soon turn
into WARNINGs, and we would like to remove these entry points in the
4.6.0 release, which will be released in late April.

Can you please change to no longer use these entry points as quickly as
possible?  They have replacements documented in "Some API replacements
for non-API entry points" of "Writing R Extensions": please change to
use these replacements.

Please correct before 2026-04-09 to safely retain your package on CRAN.

Best wishes,
The CRAN Team

[t-kalinowski]

Most of the usage sites in S7 are trivial to update, but there is one for which there is no clear path forward: method dispatch.

With the new C API, some existing S7 behavior is hard to preserve.

Today, S7 dispatch has the following behavior:

• dispatch arguments are evaluated once
• missing() works as expected in methods
• substitute() works as expected in methods, including for dispatch arguments

S7 is able to provide this behavior by forwarding the already-evaluated promise object from the generic call to the method call.

With the new public package C API, this no longer appears to be possible. There are many entry points for binding inspection and creation, but there seems to be no way to forward an existing promise.

I can think of some extremely hacky ways to approach this, but they all introduce tradeoffs or other breakages, and I do not see a clean solution with the current API.

@ltierney, do you have any guidance on how to proceed?