liquid_block.rb

html = <<~HTML
{% assign message = "hello" %}
  <div>
    {% product 123 %}
      <div>{{ product.title }}</div>
    {% endproduct %}

    {% product 456 %}
      Liquid works inside these blocks
      <div>{{ message }}</div>
      <div>{{ product.title }}</div>
    {% endproduct %}

    Shouldn't render: 
    {{ product.title }}
  </div>
HTML

class ProductLiquid < Liquid::Block
  def initialize(tag_name, markup, tokens)
    super
    # We can look up the product here, which could be cached. 
    # Alterntively, we could use the Liquid::Document to find all
    # product tags and pull the ids ahead of time to do a single lookup. 
    @product_id = markup.to_i
    puts @product_id
  end

  def render(context)
    output = ""
    context.stack do 
      context["product"] = { "title" => @product_id }
      output = super(context)
    end

    output
  end 
end

Liquid::Template.register_tag("product", ProductLiquid)

Liquid::Template.parse(html).render