The alternative can be that whenever address of iteration variable is used inside the loop the variable is per iteration and otherwise it is per loop. This way it is not breaking the old code and have new semantics.
That wouldn't fix the closure problem where the address isn't taken.
Would it be possible to add closure use as another case?
It's one per module though. In large enough modules you'll have tens of x:=x though. I assume this also opens the option of doing more such changes through the same system in the future.
The “static analysis” section says that it is impossible to catch all cases where address is used which is true. However if the analysis checks for whether address is TAKEN, then it is trivial. I would like to propose that as an alternative- whenever address is taken the variable is per iteration. Otherwise per loop.
This seems the simplest and logical modification.