protected final RetryRule retryRule() {
        checkState(retryConfig != null, "No retryRule set. Are you using RetryConfigMapping?");
        final RetryRule retryRule = retryConfig.retryRule();
        checkState(retryRule != null, "retryRule is not set.");
        return retryRule;
    }
Returns the {@link RetryRule}.

@throws IllegalStateException if the {@link RetryRule} is not set
core/src/main/java/com/linecorp/armeria/client/retry/AbstractRetryingClient.java