if I am ever taking in a

List

and expect the contents to be the same, I should create a copy, right? (since the

List

might also be Immutable) ... is there any good way to only copy if it is mutable or would that be too much of a micro-optimization?

If you are willing to pull in another dependency, Guava has immutable collections you can use: https://github.com/google/guava I also have a Kotlin library that makes them easier to use: https://github.com/Octogonapus/kt-guava

to be on the safe side you need to use either an immutable list or make a copy

There's also kotlinx.collections.immutable, which is probably better than using Guava when you already use Kotlin.

I need to test it but could do something like this:

fun <T> List<T>.ensureInmutable() = when(this) {
  is MutableList -> this.toList()
  else -> this
}

This is assuming that calls to Java code with a List would assume that is a MutableList (which I'm not 100% sure it does)