Mark WordPress posts in query if they are in main or secondary.

in_main_query.php

<?php

add_action( 'loop_start', 'mark_posts_with_query' );

function mark_posts_with_query( $query ) {

	$in_main_query = $query->is_main_query();

	foreach ( $query->posts as $post ) {
	
		$post->in_main_query = $in_main_query;
	}
}