[Rd] Suggested dependencies in context of R CMD check

[Jan Górecki]

Dear R team,

Are suggested dependencies mandatory in context of `R CMD check` when
using env var `_R_CHECK_FORCE_SUGGESTS_=FALSE`?

Suggested dependencies are nice because are optional.
But that feature often isn't valid when trying to run `R CMD check` on them.
I would like to use `export _R_CHECK_FORCE_SUGGESTS_=FALSE`, which
according to the comment in `tools/R/check.R`:

> The suggested packages are required for a complete check.
 Checking can be attempted without them by setting the environment
variable _R_CHECK_FORCE_SUGGESTS_ to a false value.

should be sufficient to run *check* process without installing suggested deps.
If it is not true, then how to differentiate the suggested packages
which are optional from those mandatory?
It would be helpful to have kind of `vignetteBuilder` DESCRIPTION
field called `testChecker`, so DESCRIPTION file could precisely manage
packages dependencies.
That way batch checking packages would be easier, as required dep
metadata would be at hand in description file. Batch checking pkgs
with all their suggests will simply result into testing whole CRAN.

In a single package it can be handled with `if (requireNamespace(.))
test_package(.)` for `testthat` and `knitr`, also with mocking up
`.Rout` files.

But I'm interested into canonical design of a suggested package and
`_R_CHECK_FORCE_SUGGESTS_` env var.
Is there any R core dev team recommendation/suggestion on that? and
don't you thing new field `testChecker` in DESCRIPTION could help for
batch checking pkgs? Installing all suggested packages of all reverse
dependencies doesn't scale.

Jan Gorecki