AFAICT, the difference between inf-ruby-first-prompt-pattern and inf-ruby-prompt-pattern is meant to handle the continuation prompt (the one we see after an unfinished expression or unclosed literal).
Moreover, we're already not passing this argument to at least one of the four implementations (jruby, not sure about rbx defaults).
You always see something after pressing RET. Seeing lots of whitespace instead isn't too helpful, especially if the repl has something running in another thread that sometimes prints to stdout.
Pickaxe says that in addition to modifying the prompt, --inf-ruby-mode suppresses Readline. I tried both --readline and --noreadline, and couldn't find any difference with the latest Emacs. Maybe we'd have to pass --noreadline for older Emacsen. I suggest make the change and see if anyone complains.
There's no difference in input history. You'd think that continued expressions would be single entries in history before this change, but no: each time you press RET - separate entry.
Remove "--inf-ruby-mode" argument, we already handle continuation prompt
And use inf-ruby-first-prompt-pattern value in inf-ruby-output-filter, because
we actually want to know that the prompt on the current line is not continuation.