Source

File: wp-includes/post.php

function wp_add_trashed_suffix_to_post_name_for_trashed_posts( $post_name, $post_ID = 0 ) {
	$trashed_posts_with_desired_slug = get_posts(
		array(
			'name'         => $post_name,
			'post_status'  => 'trash',
			'post_type'    => 'any',
			'nopaging'     => true,
			'post__not_in' => array( $post_ID ),
		)
	);

	if ( ! empty( $trashed_posts_with_desired_slug ) ) {
		foreach ( $trashed_posts_with_desired_slug as $_post ) {
			wp_add_trashed_suffix_to_post_name_for_post( $_post );
		}
	}
}