Add false positives warning

[peterjaap]

Is your feature request related to a problem? Please describe.
After fixing an error for which a baseline entry exists, and running phpcs, nothing happens. There's no difference in output.

Describe the solution you'd like
I would like to receive a warning or error from this tool saying that even though I fixed the phpcs error, the baseline entry is still there in phpcs.baseline.xml.

Because right now, when somebody re-introduces the original error, it would not trigger a phpcs error/warning because it is still in the baseline, causing a false positive (or an unmatched ignored error).

Describe alternatives you've considered
No alternative

Additional context
PHPStan supports this ootb, it's the reportUnmatchedIgnoredErrors parameter, see https://phpstan.org/user-guide/baseline

[peterjaap]

Hmm harder than I originally thought because of how this works - it looks for an entry in the baseline when it encounters an error/warning. So when the error/warning is fixed, there is nothing that triggers a search for it in the baseline.

So we should keep track of all "matched" baselines, and throw warnings on unmatched ones.

[peterjaap]

Hmm and that is exactly the part that gets injected into the phpcs code, no nice clean API for that unfortunately.

[peterjaap]

As far as I can see, we can't do this in just this package. We'd need more hacking in phpcs and that's not worth it.

We'll just have to wait for squizlabs/PHP_CodeSniffer#3387 to be merged and we can add it there (if it's not part of the eventual code that'll get merged).

Closing this.