Switch from Rf_findVarInFrame to R_getVarEx

[eddelbuettel]

Closes #1421

As described in #1421, this is another upcoming R API tightening. This fairly narrow PR does this, keeping the old code til we can remove it a few years from now and switches to the new functions for R 4.5.0 or newer.

We should leave this open until the reverse dependency checks give us a view into what the impact may be.

Checklist

• Code compiles correctly
• R CMD check still passes all tests
• Preferably, new tests were added which fail without the change
• Document the changes by file in ChangeLog

[kevinushey]

Some nits around the value we pass for inherits.

[kevinushey]

Suggested change

	Rcpp_cache = R_getVar(Rf_install(".rcpp_cache"), RCPP, TRUE);
	Rcpp_cache = R_getVar(Rf_install(".rcpp_cache"), RCPP, FALSE);

[eddelbuettel]

This one I was more torn about. Note the comment just above the code:

        /**
         * Get an object from the environment or one of its
         * parents
         *
         * @param name name of the object
         *
         */
        SEXP find( const std::string& name) const{

So does is walk through the other envs?

Also note that lines 139 to 162 (for find(std::string)) and 164 to 188 (for find(Symbol)) should be symmetric. Both TRUE, or both FALSE?

[kevinushey]

I'm not sure if I'm missing something, but isn't this code just trying to get a reference to the .rcpp_cache variable in the Rcpp namespace?

[kevinushey]

I think the prior edits are preserving that behavior you suggest, though -- get => inherits = FALSE; find => inherits = TRUE.

[eddelbuettel]

I think that is where we are at right now (i.e.: get has a FALSE, find has a TRUE) and that is due to one correction made after your comment made days ago (as noted above). And that seems fine.

(The 'just look for .rcpp_cache' is in a different file and seemingly a debugging-only helper.)

[eddelbuettel]

Rebased once more

[eddelbuettel]

@kevinushey Do you want to take another look an Environment.h ? We now have the two get() with FALSE and not descending, and the two find() have TRUE. That seems to make sense and work in the (still-running) reverse-depends check.

[eddelbuettel]

Reverse-depends checks turned up no new issues so all good from there too