Link custom Rule class message() dynamic output to Validator Facade extensions.

CustomValidatorRuleProvider.php

<?php

class AppServiceProvider
{
    ...

    /**
     * @param string[] $rules
     */
    protected function loadCustomValidationRules(array $rules)
    {
        foreach ($rules as $name => $ruleClass) {
            //Logically, a string by-reference variable should work for this, but it's disallowed in this context
            $message = new class {
                /**
                 * @var Validator|null
                 */
                public static $validator;

                /**
                 * @var Rule|null
                 */
                public static $rule;

                /**
                 * @var string[]
                 */
                public static $parameters = [];

                /**
                 * @var mixed
                 */
                public static $attribute = null;

                /**
                 * @var string
                 */
                public static $ruleName = '';

                /**
                 * @return string
                 */
                public function __toString()
                {
                    return static::$validator->makeReplacements(
                        static::$rule->message(),
                        static::$attribute,
                        static::$ruleName,
                        static::$parameters
                    );
                }
            };

            $message::$rule = $custom = app($ruleClass);
            $instance = null;

            //Intercept the arguments passed to the Rule::passes() method
            $extension = function ($attribute, $value, $parameters, Validator $validator) use ($custom, $message) {
                $message::$validator = $validator;
                $message::$attribute = $attribute;
                $message::$parameters = $parameters;

                return $custom->passes($attribute, $value, $parameters, $validator);
            };

            //ValidatorFacade::replacer() would result in infinite recursion when using Validator::makeReplacements()
            ValidatorFacade::extend($message::$ruleName = $name, $extension, $message);
        }
    }
}