Oh yes, immutability is crucial. Generational GC already makes you pay for mutation with a write barrier, and in our parallel GC we omit the locking when copyin

Oh yes, immutability is crucial. Generational GC already makes you pay for mutation with a write barrier, and in our parallel GC we omit the locking when copying immutable objects, accepting that a few might get copied twice. In the new GC mutable objects become even more expensive. I don't think local GC is viable at all in a language with ubiquitous mutation.