WP Template and Rewrite Rule Debug – just drop in `mu-plugins/`

debug-rewrite-rules.php

<?php

/**
 * Adapted from
 *
 * @param string $where
 *
 * @return bool
 */
function car_debug_rewrite_rules()
{
    // only do this on public side
    if (is_admin()) {
        return false;
    }

    add_filter(
        'wp_before_admin_bar_render',
        function () {
            global $wp_rewrite, $wp, $template;

            if (!empty($wp_rewrite->rules)) { ?>

                <style>h5 {
                        background: #000 !important;
                        color: #fff !important;
                        padding: 1em !important;
                        margin: 1em !important
                    }

                    table {
                        margin: 1em !important
                    }

                    table td {
                        border: 1px solid silver;
                        padding: 5px
                    }

                    tr.matchedrule td {
                        border-color: transparent
                    }

                    tr.matchedrule > td {
                        background: HSLA(52, 96%, 67%, 1)
                    }

                    tr.matchedrule + tr.matchedrule > td {
                        background: HSLA(52, 96%, 67%, .5)
                    }

                    tr.matchedrule table td + td {
                        font-weight: 700
                    }</style>

                <h5>Rewrite Rules</h5>
                <table>
                    <thead>
                    <tr>
                        <td></td>
                        <td>Rule</td>
                        <td>Rewrite</td>
                    </tr>
                    </thead>
                    <tbody>
                    <?php
                    $i = 1;
                    foreach ($wp_rewrite->rules as $name => $value) {
                        if ($name == $wp->matched_rule) {
                            ?>
                            <tr class="matchedrule">
                                <td><?php echo $i; ?></td>
                                <td><?php echo $name; ?></td>
                                <td><?php echo $value; ?></td>
                            </tr>
                            <tr class="matchedrule">
                                <td colspan="3">
                                    <table>
                                        <tr>
                                            <td>Request</td>
                                            <td><?php echo $wp->request; ?></td>
                                        <tr>
                                            <td>Matched Rewrite Query</td>
                                            <td title="<?php echo esc_attr(urldecode($wp->matched_query)); ?>">
                                                <?php echo $wp->matched_query; ?>
                                            </td>
                                        <tr>
                                            <td>Loaded template</td>
                                            <td><?php echo basename($template); ?></td>
                                    </table>
                                </td>
                            </tr>
                        <?php } else { ?>
                            <tr>
                                <td><?php echo $i; ?></td>
                                <td><?php echo $name; ?></td>
                                <td><?php echo $value; ?></td>
                            </tr>
                        <?php }
                        $i++;
                    } ?>
                    </tbody>
                </table>
                <?php
            }
        }
    );

    return true;
}

// If indicated, register our debug hook that puts a table in the footer.
if (defined('DEBUG_REWRITE_RULES') && DEBUG_REWRITE_RULES) {
    car_debug_rewrite_rules();
}

// If indicated, fresh rewrite rules on page load
if (defined('FORCE_FLUSH_REWRITE_RULES') && FORCE_FLUSH_REWRITE_RULES) {
    add_action('init', function () {
        flush_rewrite_rules(true);
    });
}

Based off of this and cleaned up a bit to use valid HTML and to be controlled via constants in wp-config.php and also to have a constant that turns on page load rewrite rule flushing.