Source

File: wp-includes/class-wp-rewrite.php

public function get_feed_permastruct() {
		if ( isset( $this->feed_structure ) ) {
			return $this->feed_structure;
		}

		if ( empty( $this->permalink_structure ) ) {
			$this->feed_structure = '';
			return false;
		}

		$this->feed_structure = $this->root . $this->feed_base . '/%feed%';

		return $this->feed_structure;
	}