Rails 6 should in general work on TruffleRuby, including rails new.
Please report to Issues · oracle/truffleruby · GitHub if not.
fork() is not currently supported.
Puma works well on TruffleRuby and TruffleRuby is tested in Puma’s CI.
That said, TruffleRuby doesn’t run Rails tests in CI yet. That would be something valuable to improve on.
Application startup and VM startup are quite different things. Right now, TruffleRuby is good at VM startup but not so much at application startup. Currently we’re focusing on peak performance (when the server is running for a while and JIT’ed the relevant methods/blocks) and on warmup time (how long to find those relevant methods/blocks and to JIT-compile them).