It is, for the example. But it only works because it is known at that point that the input is not just an IEnumerable<int>, but rather an int[]. Knowing that they can generate a for loop, which is quite a bit faster than the enumerator. For an unknown IEnumerable they can still emit the loop and static helper method and cut down on the generated closure, but they cannot remove the IEnumerator usage. They could probably do the same as for the array if it's known that the source IEnumerable is a List<T> (and not a subclass, because GetEnumerator may have been overridden with side-effects).

Effectively this transformation is the same as doing "Convert LINQ to code" and "Extract method" in R# (although those tend to sometimes be unable to codify even simple LINQ expressions).