It gets even worse. In the general case, you need to use a separate object for each captured mutable variable for space safety reasons (avoiding unwanted object

It gets even worse. In the general case, you need to use a separate object for each captured mutable variable for space safety reasons (avoiding unwanted object retention not present in the source code). The easiest way to achieve space safety involves a separate object for each captured variable, but of course that is unnecessarily wasteful in many cases. But in order to coalesce captured locals into fewer reference objects, you need to do some sort of lifetime analysis to find cases where locals become unused at the same time.