Add --ignore-project to v2-build

[phadej]

E.g. to prevent looking cabal.project somewhere up in the directory tree

[cohomology]

This is great. Would be nice for other cabal subcommands as well, i.e. cabal v2-haddock

[mightybyte]

I'll throw in another vote of support for this feature.

[chessai]

+1

[ulysses4ever]

I looked a little bit into it. Quick notes.

• currently cabal build --ignore-project says unsupported option because this option is explicitly disabled here:

  cabal/cabal-install/src/Distribution/Client/CmdBuild.hs

  Lines 77 to 78 in dbfce81

  	, commandOptions = removeIgnoreProjectOption
  	. nixStyleOptions (\showOrParseArgs ->

• cabal build already uses withContextAndSelectors, which, in turn, uses withProjectOrGlobalConfig, which is aware about --ignore-project (it has a separate parameter for it).

So, it'd seem like an interesting experiment to remove the removeIgnoreProjectOption from the options definition. Well, if you try it, then cabal build --ignore-project gives you a baffling:

Error: cabal: There is no <pkgname>.cabal package file or cabal.project file.
To build packages locally you need at minimum a <pkgname>.cabal file. You can
use 'cabal init' to create one.

in a directory with both a cabal file and a project file. It seems like withContextAndSelectors builds a too simplistic of a context for the --ignore-project branch. It does it with establishDummyProjectBaseContext. Comments around the latter say that it's used to

-- | Create a dummy project context, without a .cabal or a .cabal.project file

No wonder we get the error about no .cabal file (see above). I guess, there has to be a less-dummy project context constructor...

[ulysses4ever]

@bacchanalia this code (#7057 (comment)) intersects with the parts you wrote in the scripts revamp (#7851). Do you think you could look into supporting --ignore-project for cabal build. I think, properly fixing it may also help in supporting project files for scripts -- something that some people think was a good thing before the revamp.

[bacchanalia]

@ulysses4ever what do you mean by supporting project files for scripts?

[ulysses4ever]

@bacchanalia see #8562 (comment) and the following discussion