> thinking like the "borrow checker" it understands the scope of some objects and puts them on the stack and avoids garbage collection and allocation overhead.

On the other side however if you don't write code that the borrow checker would accept you likely won't get these optimizations. And even if it was accepted there's a chance the analysis needs to be too deep or complex for the escape analysis to work. Ultimately this is a nice speed up in practice but not something I would rely on.

The counter to that is that is that performance really matters in inner loops and in those cases the hot area is not that big, it doesn’t matter if your setup and tear down give up a lot of possible performance.

Early this year my son was playing chess which motivated me to write a chess program. I wrote one in Python really quickly that could challenge him. I was thinking of writing one I could bring to the chess club which would have had to have respected time control and with threads in Java this would have been a lot easier. I was able to get the inner loop to generate very little garbage in terms of move generation and search and evaluation with code that was only slightly stilted. To get decent play though I would have needed transposition table that were horribly slow using normal Java data structures but it could have been done off heap with something that would have looked nice on the outside but done it the way C would have done it in the inside.

I gave up because my son gave up on chess and started building and playing guitars in all his spare time.

Chess is a nice case of specialized programming where speed matters and it is branchy and not numeric.