Improve messages when test suites were not solvable

[ezyang]

I recently wrote a test suite, but typoed one of its dependencies, rendering it unbuildable. Despite there being a completely bogus test suite, the dependency solver happily passed the package, simply opting to disable test suites to make things work.

When I asked new-build to subsequently build the test, I got this message.

ezyang@sabre:~/Dev/backpack-str$ cabal new-build str-undefined:tests
cabal: Cannot build the test suites in the package str-undefined-0.1.0.0
because none of the components are available to build: the test suite
'implements' is not available because the solver did not find a plan that
included the test suites

This message is OK, but a better message would say how to find out what the solver error for test suites was. I think the correct way to do this is to rerun the command with --enable-tests.

[sebright]

The discussion in #3923 is related.

[refaelsh]

@ezyang Exact same thing happened to me too.

[chrissound]

It's also not clear at all what "plan" means here, and it is a very generic thing to search the web for.

For example if I search cabal docs for "plan":
https://cabal.readthedocs.io/en/stable/search.html?q=plan&check_keywords=yes&area=default

[geekosaur]

A build plan is the output of the solver. It's saved in cabal.plan under dist-newstyle, and the cabal-plan utility (installable from Hackage) formats it and displays it in a human-readable and at least somewhat comprehensible way. (I've also queried it with jq at times.)

[geekosaur]

We do have a PR open which improves solver output messages and discusses further improvements: #10524. #11062 is also related.

[sebright]

I think that we should solve this issue with the fix described at the end of #7883 (changing the default behavior to "solve for tests but don't build tests").