[nix-local-build] Allow specifying freeze file for other commands

[acfoltzer]

Right now the constraints file generated by new-freeze is called cabal.project.freeze, and the D.C.ProjectConfig code looks for it by that name. This makes it difficult to support multiple compiler versions in a project, though. The current workaround is to have CI symlink to the appropriate freeze file based on an environment variable, but this is pretty clunky.

Would it be possible to add a --freeze-file=foo.freeze option to the relevant commands? Would it make more sense to generalize to --extra-config-file, since these the freeze file is just mappended to the other config files?

[gbaz]

We actually have an --extra-config-file type option added for the standard build setup already as per: #2820

The only question I have (haven't tested) is if this gets handled by the nix-local-build stuff or if it needs to be plugged in. I suspect the latter?

[dcoutts]

@acfoltzer I was thinking of making the includes into the main cabal.project fully explicit, which would 1. make it clearer what files are used and how they're combined, 2. make it possible to use more complex setups. In that case it's not needed to have a freeze-file option except for when generating one, to produce one with the non-default name.

[phadej]

Includes could be optional via if impl(ghc == 7.10.*)? Is there a ticket for this? Would be cool!

I suspect that might depend on my/duncan's parsec stuff...

[hasufell]

bump!