Automatically skip missing packages on CRAN

[hadley]

Practically, this should mean that you never need to add skip_if_not_installed() for suggested packages. It's already the tidyverse style to not do this, but now this works everywhere.

Fixes #1598

@MichaelChirico is there anyway we could make this more convenient for google infrastructure? Maybe consult another env var?

[MichaelChirico]

Maybe sync it exactly with the skip_if_not_installed() message?

testthat/R/skip.R

Line 107 in 6099802

error = function(e) skip(paste0("{", pkg, "} is not installed"))

[MichaelChirico]

this looks like a great & reasonable compromise -- minimal implementation & fits well with existing approach that always sets !on_cran in package GHA.

one possible concern is that it doesn't make any effort to distinguish "package missing in current environment" from "you forgot to add a package to your Suggests". I guess R CMD check is inspecting tests/ files for pkg:: usage so it should be caught anyway.

[MichaelChirico]

@MichaelChirico is there anyway we could make this more convenient for google infrastructure? Maybe consult another env var?

no I think it's fine. thanks for the consideration! AIUI it will now work exactly as skip_if_not_installed() does (since we test on_cran()) -- the test will pass and it's the importer's responsibility to add requirements as needed.

One advantage of the skip_if_not_installed() approach is we can easily do static analysis of testthat/ files and pull out which packages are needed. With this, we'll need to pull that information out from the test logs, but that's basically how it goes in the current status quo.