Found in the README.md file (here):
CRAN has a policy:
packages should not write in the user’s home filespace (including clipboards), nor anywhere else on the file system
apart from the R session’s temporary directory
Not sure where this was taken from but FWIW there's the notable exception of folders obtained with tools::R_user_dir() where of course it's okay to write stuff. It would be very limiting (and sad) to not have a place to write stuff that is meant to persist across sessions.
Not sure how that helps in the context of the r-rust conversation though...