Wait a moment. Is that a YAML file with C-style preprocessor macros?

https://hg.mozilla.org/mozilla-central/file/3e4e0fef13e01bf3376f61590d5d5744d4b0b22f/modules/libpref/init/StaticPrefList.yaml 

 @eedb8cb1 well how do you conditionally compile your yaml? ;)