We get a lot of questions in #gentoo about why SomeFeature's use flag is masked. I usually show them a grep for the profiles dir. Maybe, given we have a standard format for masks, we should show the reason for the USE flag mask, like we do for regular package masks, to prevent users needing to search the profiles manually.
I found it quite surprising that a USE flag I enabled in /etc/portage/package.use was completely ignored due to being masked in the profile One would think that an explicit user config choice would override the profile. May I also suggest that the user's config choice either override the profile, or better yet, have a contradiction between /etc/portage and the profile be flagged as a fatal usage error? So yes I agree that it should explain that a flag is masked, but I think it should go further and treat it as a fatal error in the same way as if you try to install a masked package. Blunt version: Treat enabling a masked flag as a fatal error the same way as trying to install a masked package (i.e., hard masked, keywording, license, blocked by another package, etc)
I agree this would be useful. I tried toying with this a bit (where does a mask come from) in q, but I think it needs something clearer. At least the grep is sort of eliminated, e.g. `q -mv ruby` will tell a ruby:2.4 mask lives in profiles/package.mask, but it doesn't allow to show mask overrides, e.g. something globally masked, and unmasked in a profile, and therefore explain why a package is NOT masked.
*** This bug has been marked as a duplicate of bug 489304 ***
