You can get this happening by going to Preferences -> General -> Content types and adding *.drl as a sub-type of Java source.
This will get Java content assist working in .drl files, but it will unfortunately do a whole lot more. It will send your drl files to the Java compiler and it will show error markers for malformed Java code (and drl files contain lots of that).
You can turn off lots of this through various preferences:
- add *.drl files to the exclusion filters of your classpath or place them outside of the classpath altogether.
- Go to Preferences -> Java -> Editor and disable "Report problems as you type". No error markers in drl file. But, this will also affect your java files.
So, there is no complete solution here, but you can get some of what you want.