Basically a re-request from #289.
Currently I'm relying on a hack onto core Jekyll to achieve this:
# _plugins/set_posts.rb
Jekyll::Hooks.register :site, :post_read do |site|
c = site.collections
c['posts'].docs.concat c['news'].docs
c['posts'].docs.concat c['planet'].docs
end
This way my feed.xml contains posts from all three collections. However as of Jekyll 4.3, jekyll doctor complains about multiple source files pointing to the same output file. It's absolutely right that I basically just duplicated all posts from desired collections into the _posts collection.
Now I can't find another good way to work around this, so I have to bring #289 up again. It's useful and hopefully easy to implement.