Adding Properties to Map that is collected from a sequence

Language Design
1

Anyone have a good way to add properties to a map on the toMap() call?

Often I have to create a mutable map, that I then add the properties to and then make immutable.

Would it be sensible to add something like this to the language? (or does something like it exist?)

public fun <K, V> Iterable<Pair<K, V>>.toMap(vararg pairs: Pair<K, V>): Map<K, V> {
    if (this is Collection) {
        if (pairs.isEmpty()) {
            return this.toMap()
        }

        return when (size + pairs.size) {
            1 -> { pairs.toMap() }
            else -> {
                val map = toMap(LinkedHashMap(mapCapacity(size + pairs.size)))
                map.putAll(pairs)
                map.toMap()
            }
        }
    }

    val map = toMap(LinkedHashMap(mapCapacity(pairs.size)))
    map.putAll(pairs)
    return toMap(map).optimizeReadOnlyMap()
}

Example usage

val collection = mapOf("k1" to "v1")
val result = collection.map { it.key to it.value }.toMap("example" to "extra")
// "k1": "v1"
// "example": "extra"
2

Can you just add a pair to a read-only map, thus creating another map with that pair?

collection.toMap() + ("example" to "extra")

Or is it unacceptable in your case due to performance reasons?