[Rd] help with rchk warnings on Rf_eval(Rf_lang2(...))

[Ben Bolker]

Dear r-devel folks,

  [if this is more appropriate for r-pkg-devel please let me know and
I'll repost it over there ...]

I'm writing to ask for help with some R/C++ integration idioms that are
used in a package I'm maintaining, that are unfamilar to me, and that
are now being flagged as problematic by Tomas Kalibera's 'rchk'
machinery (https://github.com/kalibera/rchk); results are here
https://raw.githubusercontent.com/kalibera/cran-checks/master/rchk/results/lme4.out

The problem is with constructions like

::Rf_eval(::Rf_lang2(fun, arg), d_rho)

I *think* this means "construct a two-element pairlist from fun and arg,
then evaluate it within expression d_rho"

This leads to warnings like

"calling allocating function Rf_eval with argument allocated using Rf_lang2"

Is this a false positive or ... ? Can anyone help interpret this?

Not sure why this idiom was used in the first place: speed? (e.g., see
https://stat.ethz.ch/pipermail/r-devel/2019-June/078020.html ) Should I
be rewriting to avoid Rf_eval entirely in favor of using a Function?
(i.e., as commented in
https://stackoverflow.com/questions/37845012/rcpp-function-slower-than-rf-eval
: "Also, calling Rf_eval() directly from a C++ context is dangerous as R
errors (ie, C longjmps) will bypass the destructors of C++ objects and
leak memory / cause undefined behavior in general. Rcpp::Function tries
to make sure that doesn't happen.")

 Any tips, corrections, pointers to further documentation, etc. would be
most welcome ... Web searching for this stuff hasn't gotten me very far,
and it seems to be deeper than most of the introductory material I can
find (including the Rcpp vignettes) ...

  cheers
   Ben Bolker