* object to facilitate testing.
   */
  private final SleepingStopwatch stopwatch;

  // Can't be initialized in the constructor because mocks don't call the constructor.
  private volatile @Nullable Object mutexDoNotUseDirectly;

  private Object mutex() {
    Object mutex = mutexDoNotUseDirectly;
    if (mutex == null) {
      synchronized (this) {

   *
   * <ul>
   *   <li>The non-private constructor or non-private static factory method with the most parameters
   *       is used to construct the sample instances. In case of tie, the candidate constructors or
   *       factories are tried one after another until one can be used to construct sample
   *       instances.
   *   <li>For the constructor or static factory method used to construct instances, it's checked

    Builder(int expectedKeys) {
      if (expectedKeys > 0) {
        builderMap = Platform.preservesInsertionOrderOnPutsMapWithExpectedSize(expectedKeys);
      }
      // otherwise, leave it null to be constructed lazily
    }

    Map<K, ImmutableCollection.Builder<V>> ensureBuilderMapNonNull() {
      Map<K, ImmutableCollection.Builder<V>> result = builderMap;
      if (result == null) {

    Builder(int expectedKeys) {
      if (expectedKeys > 0) {
        builderMap = Platform.preservesInsertionOrderOnPutsMapWithExpectedSize(expectedKeys);
      }
      // otherwise, leave it null to be constructed lazily
    }

    Map<K, ImmutableCollection.Builder<V>> ensureBuilderMapNonNull() {
      Map<K, ImmutableCollection.Builder<V>> result = builderMap;
      if (result == null) {

  /**
   * Returns {@code true} if this range is of the form {@code [v..v)} or {@code (v..v]}. (This does
   * not encompass ranges of the form {@code (v..v)}, because such ranges are <i>invalid</i> and
   * can't be constructed at all.)
   *
   * <p>Note that certain discrete ranges such as the integer range {@code (3..4)} are <b>not</b>
   * considered empty, even though they contain no actual values. In these cases, it may be helpful

// on the client side, meaning the request is recycled/pooled when a request is sent.
// CAREFUL: This should only be used when there are no pointers, slices that aren't freshly constructed.
func (h *SingleHandler[Req, Resp]) AllowCallRequestPool(b bool) *SingleHandler[Req, Resp] {
	h.callReuseReq = b
	return h
}

// IgnoreNilConn will ignore nil connections when calling.

  /**
   * Returns {@code true} if this range is of the form {@code [v..v)} or {@code (v..v]}. (This does
   * not encompass ranges of the form {@code (v..v)}, because such ranges are <i>invalid</i> and
   * can't be constructed at all.)
   *
   * <p>Note that certain discrete ranges such as the integer range {@code (3..4)} are <b>not</b>
   * considered empty, even though they contain no actual values. In these cases, it may be helpful

 *
 * <p>This class supports an optional fairness policy for ordering waiting producer and consumer
 * threads. By default, this ordering is not guaranteed. However, a queue constructed with fairness
 * set to {@code true} grants threads access in FIFO order. Fairness generally decreases throughput
 * but reduces variability and avoids starvation.
 *

      }
      return result;
    }

    /**
     * Creates a new copy of this SetBuilderImpl. Modifications to that SetBuilderImpl will not
     * affect this SetBuilderImpl or sets constructed from this SetBuilderImpl via build().
     */
    abstract SetBuilderImpl<E> copy();

    /**
     * Call this before build(). Does a final check on the internal data structures, e.g. shrinking

and function types. A pointer type may hold a Go pointer or a C pointer.
Array, slice, string, and struct types may or may not include Go pointers,
depending on their type and how they are constructed. All the discussion
below about Go pointers applies not just to pointer types,
but also to other types that include Go pointers.

All Go pointers passed to C must point to pinned Go memory. Go pointers