Source

File: wp-includes/post.php

function wp_trash_post_comments( $post = null ) {
	global $wpdb;

	$post = get_post( $post );
	if ( empty( $post ) ) {
		return;
	}

	$post_id = $post->ID;

	/**
	 * Fires before comments are sent to the Trash.
	 *
	 * @since 2.9.0
	 *
	 * @param int $post_id Post ID.
	 */
	do_action( 'trash_post_comments', $post_id );

	$comments = $wpdb->get_results( $wpdb->prepare( "SELECT comment_ID, comment_approved FROM $wpdb->comments WHERE comment_post_ID = %d", $post_id ) );
	if ( empty( $comments ) ) {
		return;
	}

	// Cache current status for each comment.
	$statuses = array();
	foreach ( $comments as $comment ) {
		$statuses[ $comment->comment_ID ] = $comment->comment_approved;
	}
	add_post_meta( $post_id, '_wp_trash_meta_comments_status', $statuses );

	// Set status for all comments to post-trashed.
	$result = $wpdb->update( $wpdb->comments, array( 'comment_approved' => 'post-trashed' ), array( 'comment_post_ID' => $post_id ) );

	clean_comment_cache( array_keys( $statuses ) );

	/**
	 * Fires after comments are sent to the Trash.
	 *
	 * @since 2.9.0
	 *
	 * @param int   $post_id  Post ID.
	 * @param array $statuses Array of comment statuses.
	 */
	do_action( 'trashed_post_comments', $post_id, $statuses );

	return $result;
}