Entry `builddir: <dir>` silently ignored from `cabal.project`

[andreasabel]

Entries builddir: <dir> are silently dropped from cabal.project. (Context: #7368 (comment))

Whether they should be supported or not is discussed in #5271.

In the meantime, while that issue cooks, cabal should give a warning that builddir: entries are ignored/unknown/unsupported.

[ulysses4ever]

Historical reference: the --builddir option (and the docs saying that it won't work from inside cabal.project) comes from #4124 where the description reads:

It's not too easy to make the value of 'dist-dir' (currently builddir -- u4e) saveable
to the cabal.project file, as we'd prefer to load directly
from the cache if cabal.project is not updated.

I should confess I don't understand what it means though.

[Kleidukos]

@mixphix didn't you stumble on this by any chance?

[ulysses4ever]

@Kleidukos you may be thinking of #8947

[mixphix]

Yeah, which I learned was a duplicate of #5271. It turns out that HLS puts things into dist-newstyle anyway, so I've given up on the tractability of what I was trying to do there.