That only works if you are incrementing by 1.
++current
is the same as
{ current = current + 1; return current; }
In fact, ++i is a perfect example of an assignment as an expression and it is immensly useful. ++i both assigns to i and as an expression it returns the value that was assigned to i. Increment and decrement operators are just examples of situations where assignments as expressions are very useful, I would just like to see them extended to more than the trivial increment-by-one and decrement-by-one operations.
For the slightly more complex case consider:
i += 2
To take it a little further:
current = FetchNext(current)
where FetchNext is some operation that gets the next item in a series from a remote source.
Currently, in Kotlin these would have to be,
{ i += 2; return i}
{ current = FetchNext(current); return current }
respectively.