[test] Remove duplicated Dummy language module

[oowekyala]

Is your feature request related to a problem? Please describe.
There's currently a "dummy" language module in the pmd-core test sources and one in the main pmd-test sources. They're a pain to keep in sync on the 7.0.x branch, and they're perfect duplicates. It also seems like the only usages of those DummyNodes in the pmd-test module are... in the tests of pmd-test

Describe the solution you'd like
Just make the test sources of pmd-test depend on the test sources of pmd-core, and remove the dummy module from pmd-test. https://stackoverflow.com/questions/174560/sharing-test-code-in-maven

This would remove the ability to use dummy nodes in external code that depends on pmd-test, but I think, they may use a real language module to fake nodes? I can't really think of a reason why anyone would want to build fake trees by hand outside of our test sources

Describe alternatives you've considered

1. Don't change anything, keep syncing the sources manually. This is a pain
2. Put the "dummy" language module in the main sources of PMD core. This would have some advantages, but then we'd have to make a commitment to binary compatibility over versions... and it would show up in LanguageRegistry

[adangel]

I didn't look into the details yet, but

It also seems like the only usages of those DummyNodes in the pmd-test module are... in the tests of pmd-test

would it be possible to move these tests then to pmd-core?

[oowekyala]

I looked a bit into it, and apparently there are only two unit tests in the module. But afaik they can't be moved to pmd-core, since they need the RuleTst class

[oowekyala]

I now think we should move PlainTextLanguage (pmd/pmd-designer#25) from the designer into the main sources of PMD core. The goal is not to write rules for plain text files, so we don't have to register this language in LanguageRegistry. We could just make it a singleton available with PlainTextLanguage.getInstance().

The advantages of this would be

• There is always a language to fall back on if you need one (which is why the designer has that PlainTextLanguage)
• TextDocuments/ TextFile ([core] Text documents epic #3784) always require a language to be specified. It is convenient to have a neutral language when the only thing you want to do with a TextDocument is to query line/columns and stuff, and don't plan to parse it or execute rules. Use cases for this have popped up in the designer (Compat with pmd/pmd#3893 pmd-designer#51)
• Tests of pmd-test and pmd-test-schema ([test] Extract xml schema module #3976) would use this instead of a DummyLanguageModule, they can just use plain text. DummyLanguageModule is useful to mock nodes and set properties on them, but only the tests of pmd-core currently use this flexibility.

[oowekyala]

Another possibility is to make the test classes (and resources) of pmd-core visible to the tests of other modules. This is what #4089 does for pmd-ant, to get access to dummy languages, rules and rulesets. This is maybe the easiest way.