Source

File: wp-includes/link-template.php

function get_post_comments_feed_link( $post_id = 0, $feed = '' ) {
	$post_id = absint( $post_id );

	if ( ! $post_id ) {
		$post_id = get_the_ID();
	}

	if ( empty( $feed ) ) {
		$feed = get_default_feed();
	}

	$post       = get_post( $post_id );
	$unattached = 'attachment' === $post->post_type && 0 === (int) $post->post_parent;

	if ( get_option( 'permalink_structure' ) ) {
		if ( 'page' === get_option( 'show_on_front' ) && get_option( 'page_on_front' ) == $post_id ) {
			$url = _get_page_link( $post_id );
		} else {
			$url = get_permalink( $post_id );
		}

		if ( $unattached ) {
			$url = home_url( '/feed/' );
			if ( get_default_feed() !== $feed ) {
				$url .= "$feed/";
			}
			$url = add_query_arg( 'attachment_id', $post_id, $url );
		} else {
			$url = trailingslashit( $url ) . 'feed';
			if ( get_default_feed() != $feed ) {
				$url .= "/$feed";
			}
			$url = user_trailingslashit( $url, 'single_feed' );
		}
	} else {
		if ( $unattached ) {
			$url = add_query_arg(
				array(
					'feed'          => $feed,
					'attachment_id' => $post_id,
				),
				home_url( '/' )
			);
		} elseif ( 'page' === $post->post_type ) {
			$url = add_query_arg(
				array(
					'feed'    => $feed,
					'page_id' => $post_id,
				),
				home_url( '/' )
			);
		} else {
			$url = add_query_arg(
				array(
					'feed' => $feed,
					'p'    => $post_id,
				),
				home_url( '/' )
			);
		}
	}

	/**
	 * Filters the post comments feed permalink.
	 *
	 * @since 1.5.1
	 *
	 * @param string $url Post comments feed permalink.
	 */
	return apply_filters( 'post_comments_feed_link', $url );
}