Support declaring extra dependencies

[jeroen]

As discussed on slack, we want to formalize declaring several types of development dependencies in the description file. These are packages that are not needed to pass check, but used for development. E.g:

• pkgdown: packages used by pkgdown articles
• document: packages such as roxygen2 needed to generate manuals

By declaring these, we can automate rendering docs without having to do custom stuff in the travis file, or making these packages a requirement for running CMD check (as Suggests does).

The proposal is to prefix the fields with Config/Depends/ such that they do not trigger a NOTE.

Imports: foo, bar
Suggest: baz
Config/Depends/pkgdown: tidyverse, bench
Config/Depends/coverage: covr
Config/Depends/document: roxygen2

Fields may contain remotes syntax like cran::dplyr@0.8.2, r-lib/roxygen@branch.

We generalize the dependencies parameter from the remotes::install_...() functions to support these custom dependencies such as "pkgdown".

One open question is the dependency relationship between the dependency types :) For example:

remotes::install_github("dplyr", dependencies = "pkgdown")

This should probably dependencies in Config/Depends/pkgdown plus hard dependencies (Depends, Imports, LinkingTo), but not maybe Suggests (which are mostly required to run tests)?

[gaborcsardi]

I think "pkgdown" should only install Config/Depends/pkgdown, to be able to express everything. In general, if it is a character vector, then it should only install the specified stuff.

So for your site you would typically do c("hard", "soft", "pkgdown").

We can also have "all", to install absolutely everything (non-recursively of course), and TRUE would mean c("hard", "soft"), like it already does.

[maelle]

I was wondering why usethis::use_coverage() added covr to Suggests (even if covr is only needed on Travis), and I suppose the proposal by Jeroen would mean that could be avoided?

[hadley]

I wonder if Config/Needs/... would be slightly better than Config/Depends/... since it avoids the existing meaning of the "Depends" field.

[jimhester]

Closed by #483