[dr2chase]

The way to view it is "unless there is syntactic sharing, it is a for loop, same as before". The compiler uses a syntactic test (with little knowledge of control flow or value use) to exclude loops from the change. This excludes most loops.

After the change, escape analysis figures out if the changed iteration variable actually needs heap allocation; in an internal sample of code that was actually buggy (i.e., biased, guaranteed to have at least one loop like this) for 5/6 of the loops escape analysis decided that heap allocation wasn't needed.

The reason this optimization isn't part of the language change proposal is that escape analysis is "behind the curtain"; ignoring performance, a program should behave the same with or without it, and it is removing heap allocations all over the place already. Escape analysis is also extremely difficult to explain exactly, so you would not want it in the spec, and "make escape analysis better" (that is, change it) is one of the prominent items in the bag of things to do for Go.