This second thought seems to be well supported, since there is a built-in method that facilitates it: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/to-map.html

MutableMap implements the read-only Map interface, so the usual mechanic for this is along the lines of:

private val _myMap = mutableMapOf<Int, Int>()
val myMap : Map<Int, Int> = _myMap

so no need to copy everything unless you want to provide a snapshot of the current values instead

But remember this way all clients will still be able to cast

foo.myMap as MutableMap<Int,Int>

and modify its content.

Neat stuff. Thanks everyone.