This is a overview issue about improvements to reduce warnings during the build. Since we moved trunk to Java 8 and branch_5x to Java 7, there were introduced some additional warnings, mostly in generated code.
ANTLR4 does automatically add the needed @SuppressWarnings, but jflex does not. There are also some rawtypes warnings, which changed in Java 7 (before Java 7 there were only "unsafe" warnings, now you need both.
I enabled the Warnings tracker in Policeman Jenkins, we can therefore very easily check the warnings: e.g. on trunk: http://jenkins.thetaphi.de/job/Lucene-Solr-trunk-Linux/ (see diagram on the right), you can click on it, e.g. http://jenkins.thetaphi.de/job/Lucene-Solr-trunk-Linux/warnings4
LUCENE-6741 Ant regenerate fails to be no-op on 5.x