Use default install directory if not specified.

[m-renaud]

Cabal 3.0.0.0 added the --installdir option to specify the location that binaries should be installed in. Running a cabal user-config update would populate the config file with the default value, but the cabal install program would error if it wasn't set. This change uses the default value that would be written to the config if its unset, and outputs a warning.

Previous error:

cabal: installdir is not defined. Set it in your cabal config file or use
--installdir=<path>

New warning (install succeds):

Warning: installdir is not defined. Set it in your cabal config file or use
--installdir=<path>. Using default installdir: "/path/to/the/default/installdir/bin"

This resolves #5973

---

Please include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
• If the change is docs-only, [ci skip] is used to avoid triggering the build bots.

Please also shortly describe how you tested your change. Bonus points for added tests!

/cc @phadej @fgaz

[m-renaud]

The "Windows / test ghc-8.6.5" test has failed for every PR I've made in the last week or so, is this a known issue? Anyways, I think this is ready to merge.

[phadej]

@m-renaud yes it is, #6627

[m-renaud]

@phadej should we merge this in before the next cabal-install release? It appears that the Windows failure is blocking the merge, at least from the GitHub UI.

[phadej]

Please squash commits into one. (Don't use squash and merge button).

[m-renaud]

Done. CI re-running now.

OOC, why not use "Squash and Merge" button and re-write the commit message to be added to the master branch there?

[phadej]

Because perfectly, I'd not use GitHub merge button at all, but let CI push to master the merge commit it tested (given the PR is accepted). We could get quite close to that with current CI setup, but GitHub actions doesn't support restarting individual jobs inside the workflow, so we cannot have whole CI as a single workflow (and thus know that all checks passed).

[tomsmeding]

Sorry for coming back to this from #5973 — is there any reason why the warning message does not mention that cabal user-config update is something that the user might want to do?

[fgaz]

@tomsmeding not really, I just didn't think of adding it