[eregon]

When did you try? Fixnum/Bignum are gone since many years (before 1.0RC1 which was April 2018, it's all Integer now). So I guess many years ago, and back then TruffleRuby was basically implementing Ruby features, had fairly limited compatibility and likely could not run Rails, very different from today.

There were compatibility issues with BigDecimal, but TruffleRuby now uses the C extension, hence it should be exactly the same behavior as CRuby.

> Numerical to string formatting is a bit different too.

AFAIK that was fixed years ago if you mean float formatting.

> Regexes behave differently if they worked at all.

TruffleRuby always used Joni, which is literally a translation of CRuby's Regexp engine to Java (by the JRuby team), so that is very surprising and I have a really hard time to believe it. At least "Regexes behave differently if they worked at all" seems harsh and highly inaccurate to me. There likely were a couple Regexp issues but the generalization seems wildly exaggerated.