Source

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

public function wp_rewrite_rules() {
		$this->rules = get_option( 'rewrite_rules' );
		if ( empty( $this->rules ) ) {
			$this->matches = 'matches';
			$this->rewrite_rules();
			if ( ! did_action( 'wp_loaded' ) ) {
				add_action( 'wp_loaded', array( $this, 'flush_rules' ) );
				return $this->rules;
			}
			update_option( 'rewrite_rules', $this->rules );
		}

		return $this->rules;
	}