use_data(): match R dependency to version

[dpprdan]

version = 3 is only supported by R >= 3.5.0, so this should be the minimal dependency version, IMHO.

Are there tests for use_dependency() in use_data()? I haven't found any (but possibly not looked hard enough?).

Should version default to NULL or 3 now or anytime soon? 2 feels outdated for current packages. I guess there is something to be said for a fixed version as default (instead of NULL = default depending on (local) R version), but the downside is that it gets outdated. It's probably better to nudge users into setting the desired version themselves (via use_data_raw()) instead of relying on the default?

[hadley]

Agreed that we should do better here, but I don't think setting to 3.5 like this is the right approach. I think it would be to just check the current version and tell the user if they need to increment it. Do you want to try implementing that?

[dpprdan]

I think it would be to just check the current version and tell the user if they need to increment it.

What's your motivation? The "when you use version 3 you get an R v3.5 dependency" gotcha? Or something else?

You're probably aware, but use_dependency() only upgrades if min_version > existing_version and issues a message if it sets or upgrades R in Depends.

> use_data(mtcars, version = 2)
✔ Adding 'R' to Depends field in DESCRIPTION
✔ Creating 'data/'
✔ Setting LazyData to 'true' in 'DESCRIPTION'
✔ Saving 'mtcars' to 'data/mtcars.rda'
• Document your data (see 'https://r-pkgs.org/data.html')
> use_data(mtcars, overwrite = TRUE, version = 3)
✔ Increasing 'R' version to '>= 3.5.0' in DESCRIPTION
✔ Saving 'mtcars' to 'data/mtcars.rda'
• Document your data (see 'https://r-pkgs.org/data.html')

I don't fully grasp yet in what scenario setting 3.5 (vs. 2.10 I suppose?) like this is problematic, I suppose.

[hadley]

Oh I didn't know that. This approach should be fine then.

[hadley]

So to finish off this PR, can you please add a bullet to the top of NEWS.md? It should briefly describe the change and end with (@yourname, #issuenumber).

[dpprdan]

Done, thanks!