performance tests error with R-devel

[tdhock]

In atime I do a test of data.table on GHA with R-devel. https://github.com/tdhock/atime/actions/runs/23857902892/job/69556180249
Bad news: all of the performance tests error in R-devel, because they require compiling old versions of data.table, and these old versions use parts of the C API which have been removed in R-devel.

What should we do about this?

• option 1: we could hack a pkg.edit.fun which modifies the old versions to compile under R-devel. seems very complicated! (impossible?) and also not an accurate estimation of performance of old versions of data.table.
• option 2: currently performance tests use whatever version of R is the most recent release. we could make the performance test suite always use R-4.5.3 which is likely the last release of R which will support compiling old versions of data.table. will work in short term, but maybe one day this old version of R will no longer be easily installable?

both options seem not great, but option 2 seems more reasonable to me.

[MichaelChirico]

Option 1 seems more feasible to me. It entails maintaining a backports.c file with some copy-pasted definitions from R, right? Might be fussy to get set up initially but then maintenance should be minimal. (I think)