guava/src/com/google/common/collect/RegularImmutableMap.java
static <K, V> ImmutableMapEntry<K, V> makeImmutable(Entry<K, V> entry, K key, V value) {
    boolean reusable =
        entry instanceof ImmutableMapEntry && ((ImmutableMapEntry<K, V>) entry).isReusable();
    return reusable ? (ImmutableMapEntry<K, V>) entry : new ImmutableMapEntry<K, V>(key, value);
  }
static <K, V> ImmutableMapEntry<K, V> makeImmutable(Entry<K, V> entry, K key, V value) {
    boolean reusable =
        entry instanceof ImmutableMapEntry && ((ImmutableMapEntry<K, V>) entry).isReusable();
    return reusable ? (ImmutableMapEntry<K, V>) entry : new ImmutableMapEntry<K, V>(key, value);
  }
Makes an entry usable internally by a new ImmutableMap without rereading its contents.