use Ruby Hash as default expression cache

Gemfile

@@ -13,6 +13,7 @@ group :benchmark, :test do
   gem 'benchmark-ips'
   gem 'memory_profiler'
   gem 'terminal-table'
+  gem "lru_redux"
 
   install_if -> { RUBY_PLATFORM !~ /mingw|mswin|java/ && RUBY_ENGINE != 'truffleruby' } do
     gem 'stackprof'

lib/liquid/parse_context.rb

@@ -16,8 +16,12 @@ module Liquid
       # This StringScanner will be shared by all of them
       @string_scanner = StringScanner.new("")
 
-      @expression_cache = if options[:expression_cache] != false
-        options[:expression_cache] || LruRedux::Cache.new(10_000)
+      @expression_cache = if options[:expression_cache].nil?
+        {}
+      elsif options[:expression_cache].respond_to?(:[]) && options[:expression_cache].respond_to?(:[]=)
+        options[:expression_cache]
+      elsif options[:expression_cache]
+        {}
       end
 
       self.depth   = 0

liquid.gemspec

@@ -30,7 +30,6 @@ Gem::Specification.new do |s|
 
   s.add_dependency("strscan", ">= 3.1.1")
   s.add_dependency("bigdecimal")
-  s.add_dependency("lru_redux")
 
   s.add_development_dependency('rake', '~> 13.0')
   s.add_development_dependency('minitest')

test/integration/expression_test.rb

@@ -58,6 +58,51 @@ class ExpressionTest < Minitest::Test
   def test_expression_cache
     skip("Liquid-C does not support Expression caching") if defined?(Liquid::C) && Liquid::C.enabled
 
+    cache = {}
+    template = <<~LIQUID
+      {% assign x = 1 %}
+      {{ x }}
+      {% assign x = 2 %}
+      {{ x }}
+      {% assign y = 1 %}
+      {{ y }}
+    LIQUID
+
+    Liquid::Template.parse(template, expression_cache: cache).render
+
+    assert_equal(
+      ["1", "2", "x", "y"],
+      cache.to_a.map { _1[0] }.sort,
+    )
+  end
+
+  def test_expression_cache_with_true_boolean
+    skip("Liquid-C does not support Expression caching") if defined?(Liquid::C) && Liquid::C.enabled
+
+    template = <<~LIQUID
+      {% assign x = 1 %}
+      {{ x }}
+      {% assign x = 2 %}
+      {{ x }}
+      {% assign y = 1 %}
+      {{ y }}
+    LIQUID
+
+    parse_context = ParseContext.new(expression_cache: true)
+
+    Liquid::Template.parse(template, parse_context).render
+
+    cache = parse_context.instance_variable_get(:@expression_cache)
+
+    assert_equal(
+      ["1", "2", "x", "y"],
+      cache.to_a.map { _1[0] }.sort,
+    )
+  end
+
+  def test_expression_cache_with_lru_redux
+    skip("Liquid-C does not support Expression caching") if defined?(Liquid::C) && Liquid::C.enabled
+
     cache = LruRedux::Cache.new(10)
     template = <<~LIQUID
       {% assign x = 1 %}