Warn about unused flags in Cabal file

[bos]

(Imported from Trac #763, reported by @tibbe on 2010-11-11)

Cabal should output a warning during configure if a flag is defined but never used. I almost forgot to delete an old flag due to removing its use sites but not its definition.

[BardurArantsson]

Suggest closing. Non-trivial to implement AFAICT and no apparent interest in a long time.

[23Skidoo]

Not exactly trivial, but not impossible either. Walk the tree of conditionals and collect all flags that are mentioned, then do a set difference against the set of all flags.

[BardurArantsson]

Oh, sorry. I completely misread "flags" -- I was thinking of language extensions because I'd been doing an extension-related bulk update in my own code :).

Nevertheless, is this likely to actually ever get implemented because it's in the tracker? If not, I'd still suggest closing. Anyone wanting to scratch this particular itch is still absolutely allowed to come along with a PR at any point :).

[23Skidoo]

I was thinking of language extensions

For this, see https://ghc.haskell.org/trac/ghc/ticket/3085.