update variable's context before invoking its to_liquid

[ggmichaelgo]

What are you trying to solve?

Liquid-C PR: Shopify/liquid-c#212 (Liquid-C PR needs to be merged first)

On variable lookups, Context is invoking the drop's to_liquid function, and update the values context to itself.

This becomes problematic if the drop's to_liquid has some contextual checks:

require 'liquid'

class ProductsDrop < Liquid::Drop
  def initialize(products)
    @products = products
  end

  def size
    @products.size
  end

  def to_liquid
    if @context["forloop"]
      @products.first(@context["forloop"].length)
    else
      @products
    end
  end
end

context = {
  "products" => ProductsDrop.new(["A", "B", "C", "D", "E"]),
}

template = Liquid::Template.parse(<<~LIQUID)
  {% for i in (1..3) %}
    for-loop: {{ products | size }} # returns 3
  {% endfor %}

  {{ products }} # it still returns 3 instead of 5
LIQUID

puts template.render(context)

We could solve issue by updating the Context#find_variable to update the drop's context before invoking its to_liquid method.

[ggmichaelgo]

ping @Shopify/guardians-of-the-liquid