@Suppress("NOTHING_TO_INLINE")
internal inline fun <K, V> Map<K, V>.unmodifiable(): Map<K, V> = Collections.unmodifiableMap(this)

/** Returns an immutable copy of this. */
@Suppress("UNCHECKED_CAST")
internal fun <T> List<T>.toImmutableList(): List<T> =
  when {
    this.isEmpty() -> emptyList()
    this.size == 1 -> Collections.singletonList(this[0])
    // Collection.toArray returns Object[] (covariant).