mapKeysTo
Populates the given destination map with entries having the keys obtained by applying the transform function to each entry in this Map and the values of this map.
@IgnorableReturnValueinline fun <K, V, R, M : MutableMap<in R, in V>> Map<out K, V>.mapKeysTo(destination: M, transform: (Map.Entry<K, V>) -> R): M(source)
val original = mapOf(1 to "one", 2 to "two", 3 to "three")
val destination = mutableMapOf<String, String>()
original.mapKeysTo(destination) { it.key.toString() }
println(destination) // Output: {1=one, 2=two, 3=three}