Also, is there any performance impact of using

for (a in withIndex()) ...

rather than

var i = 0; for (a in iterable) { ...; a++ }

?

Yes, because withIndex creates IndexedValue for each item

I was wondering that, because it could happen to generate lazily rather than eagerly, which could make the impact negligible depending on the application.

IndexedValue generated lazily on each element, but if you traverse all elements will be generated + 1 object for each element