一段很有意思的代码 没见过的符号 ??=

/**
 * Creates a new fiber asynchronously using the given closure, returning a Future that is completed with the
 * eventual return value of the passed function or will fail if the closure throws an exception.
 *
 * @template T
 *
 * @param \Closure(...):T $closure
 * @param mixed ...$args Arguments forwarded to the closure when starting the fiber.
 *
 * @return Future<T>
 */
function async(\Closure $closure, mixed ...$args): Future
{
    static $run = null;
    // 这里 ??=
    $run ??= static function (FutureState $state, \Closure $closure, array $args): void {
        $s = $state;
        $c = $closure;

        /* Null function arguments so an exception thrown from the closure does not contain the FutureState object
         * in the stack trace, which would create a circular reference, preventing immediate garbage collection */
        $state = $closure = null;

        try {
            // Clear $args to allow garbage collection of arguments during fiber execution
            $s->complete($c(...$args, ...($args = [])));
        } catch (\Throwable $exception) {
            $s->error($exception);
        }
    };

    $state = new Internal\FutureState;

    EventLoop::queue($run, $state, $closure, $args);

    return new Future($state);
}