Add a new {% render %} tag

Example:

```
// the_count.liquid
{{ number }}! Ah ah ah.

// my_template.liquid
{% for number in range (1..3) %}
  {% render "the_count", number: number %}
{% endfor %}

Output:
1! Ah ah ah.
2! Ah ah ah.
3! Ah ah ah.
```

The `render` tag is a more strict version of the `include` tag. It is
designed to isolate itself from the parent rendering context both by
creating a new scope (which does not inherit the parent scope) and by
only inheriting "static" registers.

Static registers are those that do not hold mutable state which could
affect rendering. This again helps `render`ed templates remain entirely
separate from their calling context.

Unlike `include`, `render` does not permit specifying the target
template using a variable, only a string literal. For example, this
means that `{% render my_dynamic_template %}` is invalid syntax. This
will make it possible to statically analyze the dependencies between
templates without making Turing angry.

Note that the `static_environment` of a rendered template is inherited, unlike
the scope and regular environment. This environment is immutable from within the
template.

An alternate syntax, which mimics the `{% include ... for %}` tag is
currently in design discussion.

test/integration/tags/render_rag_test.rb

@@ -1,255 +0,0 @@
-require 'test_helper'
-
-class TestFileSystem
-  def read_template_file(template_path)
-    case template_path
-    when "product"
-      "Product: {{ product.title }} "
-
-    when "locale_variables"
-      "Locale: {{echo1}} {{echo2}}"
-
-    when "variant"
-      "Variant: {{ variant.title }}"
-
-    when "nested_template"
-      "{% include 'header' %} {% include 'body' %} {% include 'footer' %}"
-
-    when "body"
-      "body {% include 'body_detail' %}"
-
-    when "nested_product_template"
-      "Product: {{ nested_product_template.title }} {%include 'details'%} "
-
-    when "recursively_nested_template"
-      "-{% include 'recursively_nested_template' %}"
-
-    when "pick_a_source"
-      "from TestFileSystem"
-
-    when 'assignments'
-      "{% assign foo = 'bar' %}"
-
-    when 'break'
-      "{% break %}"
-
-    else
-      template_path
-    end
-  end
-end
-
-class OtherFileSystem
-  def read_template_file(template_path)
-    'from OtherFileSystem'
-  end
-end
-
-class CountingFileSystem
-  attr_reader :count
-  def read_template_file(template_path)
-    @count ||= 0
-    @count += 1
-    'from CountingFileSystem'
-  end
-end
-
-class CustomInclude < Liquid::Tag
-  Syntax = /(#{Liquid::QuotedFragment}+)(\s+(?:with|for)\s+(#{Liquid::QuotedFragment}+))?/o
-
-  def initialize(tag_name, markup, tokens)
-    markup =~ Syntax
-    @template_name = $1
-    super
-  end
-
-  def parse(tokens)
-  end
-
-  def render_to_output_buffer(context, output)
-    output << @template_name[1..-2]
-    output
-  end
-end
-
-class IncludeTagTest < Minitest::Test
-  include Liquid
-
-  def setup
-    Liquid::Template.file_system = TestFileSystem.new
-  end
-
-  def test_include_tag_looks_for_file_system_in_registers_first
-    assert_equal 'from OtherFileSystem',
-      Template.parse("{% include 'pick_a_source' %}").render!({}, registers: { file_system: OtherFileSystem.new })
-  end
-
-  def test_include_tag_with
-    assert_template_result "Product: Draft 151cm ",
-      "{% include 'product' with products[0] %}", "products" => [ { 'title' => 'Draft 151cm' }, { 'title' => 'Element 155cm' } ]
-  end
-
-  def test_include_tag_with_default_name
-    assert_template_result "Product: Draft 151cm ",
-      "{% include 'product' %}", "product" => { 'title' => 'Draft 151cm' }
-  end
-
-  def test_include_tag_for
-    assert_template_result "Product: Draft 151cm Product: Element 155cm ",
-      "{% include 'product' for products %}", "products" => [ { 'title' => 'Draft 151cm' }, { 'title' => 'Element 155cm' } ]
-  end
-
-  def test_include_tag_with_local_variables
-    assert_template_result "Locale: test123 ", "{% include 'locale_variables' echo1: 'test123' %}"
-  end
-
-  def test_include_tag_with_multiple_local_variables
-    assert_template_result "Locale: test123 test321",
-      "{% include 'locale_variables' echo1: 'test123', echo2: 'test321' %}"
-  end
-
-  def test_include_tag_with_multiple_local_variables_from_context
-    assert_template_result "Locale: test123 test321",
-      "{% include 'locale_variables' echo1: echo1, echo2: more_echos.echo2 %}",
-      'echo1' => 'test123', 'more_echos' => { "echo2" => 'test321' }
-  end
-
-  def test_included_templates_assigns_variables
-    assert_template_result "bar", "{% include 'assignments' %}{{ foo }}"
-  end
-
-  def test_nested_include_tag
-    assert_template_result "body body_detail", "{% include 'body' %}"
-
-    assert_template_result "header body body_detail footer", "{% include 'nested_template' %}"
-  end
-
-  def test_nested_include_with_variable
-    assert_template_result "Product: Draft 151cm details ",
-      "{% include 'nested_product_template' with product %}", "product" => { "title" => 'Draft 151cm' }
-
-    assert_template_result "Product: Draft 151cm details Product: Element 155cm details ",
-      "{% include 'nested_product_template' for products %}", "products" => [{ "title" => 'Draft 151cm' }, { "title" => 'Element 155cm' }]
-  end
-
-  def test_recursively_included_template_does_not_produce_endless_loop
-    infinite_file_system = Class.new do
-      def read_template_file(template_path)
-        "-{% include 'loop' %}"
-      end
-    end
-
-    Liquid::Template.file_system = infinite_file_system.new
-
-    assert_raises(Liquid::StackLevelError) do
-      Template.parse("{% include 'loop' %}").render!
-    end
-  end
-
-  def test_dynamically_choosen_template
-    assert_template_result "Test123", "{% include template %}", "template" => 'Test123'
-    assert_template_result "Test321", "{% include template %}", "template" => 'Test321'
-
-    assert_template_result "Product: Draft 151cm ", "{% include template for product %}",
-      "template" => 'product', 'product' => { 'title' => 'Draft 151cm' }
-  end
-
-  def test_include_tag_caches_second_read_of_same_partial
-    file_system = CountingFileSystem.new
-    assert_equal 'from CountingFileSystemfrom CountingFileSystem',
-      Template.parse("{% include 'pick_a_source' %}{% include 'pick_a_source' %}").render!({}, registers: { file_system: file_system })
-    assert_equal 1, file_system.count
-  end
-
-  def test_include_tag_doesnt_cache_partials_across_renders
-    file_system = CountingFileSystem.new
-    assert_equal 'from CountingFileSystem',
-      Template.parse("{% include 'pick_a_source' %}").render!({}, registers: { file_system: file_system })
-    assert_equal 1, file_system.count
-
-    assert_equal 'from CountingFileSystem',
-      Template.parse("{% include 'pick_a_source' %}").render!({}, registers: { file_system: file_system })
-    assert_equal 2, file_system.count
-  end
-
-  def test_include_tag_within_if_statement
-    assert_template_result "foo_if_true", "{% if true %}{% include 'foo_if_true' %}{% endif %}"
-  end
-
-  def test_custom_include_tag
-    original_tag = Liquid::Template.tags['include']
-    Liquid::Template.tags['include'] = CustomInclude
-    begin
-      assert_equal "custom_foo",
-        Template.parse("{% include 'custom_foo' %}").render!
-    ensure
-      Liquid::Template.tags['include'] = original_tag
-    end
-  end
-
-  def test_custom_include_tag_within_if_statement
-    original_tag = Liquid::Template.tags['include']
-    Liquid::Template.tags['include'] = CustomInclude
-    begin
-      assert_equal "custom_foo_if_true",
-        Template.parse("{% if true %}{% include 'custom_foo_if_true' %}{% endif %}").render!
-    ensure
-      Liquid::Template.tags['include'] = original_tag
-    end
-  end
-
-  def test_does_not_add_error_in_strict_mode_for_missing_variable
-    Liquid::Template.file_system = TestFileSystem.new
-
-    a = Liquid::Template.parse(' {% include "nested_template" %}')
-    a.render!
-    assert_empty a.errors
-  end
-
-  def test_passing_options_to_included_templates
-    assert_raises(Liquid::SyntaxError) do
-      Template.parse("{% include template %}", error_mode: :strict).render!("template" => '{{ "X" || downcase }}')
-    end
-    with_error_mode(:lax) do
-      assert_equal 'x', Template.parse("{% include template %}", error_mode: :strict, include_options_blacklist: true).render!("template" => '{{ "X" || downcase }}')
-    end
-    assert_raises(Liquid::SyntaxError) do
-      Template.parse("{% include template %}", error_mode: :strict, include_options_blacklist: [:locale]).render!("template" => '{{ "X" || downcase }}')
-    end
-    with_error_mode(:lax) do
-      assert_equal 'x', Template.parse("{% include template %}", error_mode: :strict, include_options_blacklist: [:error_mode]).render!("template" => '{{ "X" || downcase }}')
-    end
-  end
-
-  def test_render_raise_argument_error_when_template_is_undefined
-    assert_raises(Liquid::ArgumentError) do
-      template = Liquid::Template.parse('{% include undefined_variable %}')
-      template.render!
-    end
-    assert_raises(Liquid::ArgumentError) do
-      template = Liquid::Template.parse('{% include nil %}')
-      template.render!
-    end
-  end
-
-  def test_including_via_variable_value
-    assert_template_result "from TestFileSystem", "{% assign page = 'pick_a_source' %}{% include page %}"
-
-    assert_template_result "Product: Draft 151cm ", "{% assign page = 'product' %}{% include page %}", "product" => { 'title' => 'Draft 151cm' }
-
-    assert_template_result "Product: Draft 151cm ", "{% assign page = 'product' %}{% include page for foo %}", "foo" => { 'title' => 'Draft 151cm' }
-  end
-
-  def test_including_with_strict_variables
-    template = Liquid::Template.parse("{% include 'simple' %}", error_mode: :warn)
-    template.render(nil, strict_variables: true)
-
-    assert_equal [], template.errors
-  end
-
-  def test_break_through_include
-    assert_template_result "1", "{% for i in (1..3) %}{{ i }}{% break %}{{ i }}{% endfor %}"
-    assert_template_result "1", "{% for i in (1..3) %}{{ i }}{% include 'break' %}{{ i }}{% endfor %}"
-  end
-end # IncludeTagTest
-

test/integration/tags/render_tag_test.rb

@@ -1,75 +1,69 @@
 require 'test_helper'
 
-class StubFileSystem
-  def initialize(values)
-    @values = values
-  end
-
-  def read_template_file(template_path)
-    @values.fetch(template_path)
-  end
-end
-
 class RenderTagTest < Minitest::Test
   include Liquid
 
   def test_render_with_no_arguments
     Liquid::Template.file_system = StubFileSystem.new('source' => 'rendered content')
-    assert_template_result 'rendered content', "{% render 'source' %}"
+    assert_template_result 'rendered content', '{% render "source" %}'
   end
 
   def test_render_tag_looks_for_file_system_in_registers_first
     file_system = StubFileSystem.new('pick_a_source' => 'from register file system')
     assert_equal 'from register file system',
-      Template.parse("{% render 'pick_a_source' %}").render!({}, registers: { file_system: file_system })
+      Template.parse('{% render "pick_a_source" %}').render!({}, registers: { file_system: file_system })
   end
 
   def test_render_passes_named_arguments_into_inner_scope
     Liquid::Template.file_system = StubFileSystem.new('product' => '{{ inner_product.title }}')
-    assert_template_result 'My Product', "{% render 'product', inner_product: outer_product %}",
+    assert_template_result 'My Product', '{% render "product", inner_product: outer_product %}',
       'outer_product' => { 'title' => 'My Product' }
   end
 
   def test_render_accepts_literals_as_arguments
     Liquid::Template.file_system = StubFileSystem.new('snippet' => '{{ price }}')
-    assert_template_result '123', "{% render 'snippet', price: 123 %}"
+    assert_template_result '123', '{% render "snippet", price: 123 %}'
   end
 
   def test_render_accepts_multiple_named_arguments
     Liquid::Template.file_system = StubFileSystem.new('snippet' => '{{ one }} {{ two }}')
-    assert_template_result '1 2', "{% render 'snippet', one: 1, two: 2 %}"
+    assert_template_result '1 2', '{% render "snippet", one: 1, two: 2 %}'
   end
 
-  def test_render_does_inherit_parent_scope_variables
+  def test_render_does_not_inherit_parent_scope_variables
     Liquid::Template.file_system = StubFileSystem.new('snippet' => '{{ outer_variable }}')
-    assert_template_result '', "{% render 'snippet' %}", 'outer_variable' => 'should not be visible'
+    assert_template_result '', '{% assign outer_variable = "should not be visible" %}{% render "snippet" %}'
   end
 
   def test_render_does_not_inherit_variable_with_same_name_as_snippet
     Liquid::Template.file_system = StubFileSystem.new('snippet' => '{{ snippet }}')
-    assert_template_result '', "{% render 'snippet' %}", 'snippet' => 'should not be visible'
+    assert_template_result '', "{% assign snippet = 'should not be visible' %}{% render 'snippet' %}"
   end
 
   def test_render_sets_the_correct_template_name_for_errors
     Liquid::Template.file_system = StubFileSystem.new('snippet' => '{{ unsafe }}')
-    Liquid::Template.taint_mode = :error
 
-    template = Liquid::Template.parse("{% render 'snippet', unsafe: unsafe %}")
-    template.render('unsafe' => String.new('unsafe').tap(&:taint))
-    refute_empty template.errors
+    with_taint_mode :error do
+      template = Liquid::Template.parse('{% render "snippet", unsafe: unsafe %}')
+      context = Context.new('unsafe' => (+'unsafe').tap(&:taint))
+      template.render(context)
 
-    assert_equal 'snippet', template.errors.first.template_name
+      assert_equal [Liquid::TaintedError], template.errors.map(&:class)
+      assert_equal 'snippet', template.errors.first.template_name
+    end
   end
 
   def test_render_sets_the_correct_template_name_for_warnings
     Liquid::Template.file_system = StubFileSystem.new('snippet' => '{{ unsafe }}')
-    Liquid::Template.taint_mode = :warn
 
-    template = Liquid::Template.parse("{% render 'snippet', unsafe: unsafe %}")
-    template.render('unsafe' => String.new('unsafe').tap(&:taint))
-    refute_empty template.warnings
+    with_taint_mode :warn do
+      template = Liquid::Template.parse('{% render "snippet", unsafe: unsafe %}')
+      context = Context.new('unsafe' => (+'unsafe').tap(&:taint))
+      template.render(context)
 
-    assert_equal 'snippet', template.errors.first.template_name
+      assert_equal [Liquid::TaintedError], context.warnings.map(&:class)
+      assert_equal 'snippet', context.warnings.first.template_name
+    end
   end
 
   def test_render_does_not_mutate_parent_scope
@@ -79,152 +73,77 @@ class RenderTagTest < Minitest::Test
 
   def test_nested_render_tag
     Liquid::Template.file_system = StubFileSystem.new(
-      'one' => "one {{ render 'two' }}",
+      'one' => "one {% render 'two' %}",
       'two' => 'two'
     )
-    assert_template_result 'one two', "{% include 'one' %}"
+    assert_template_result 'one two', "{% render 'one' %}"
   end
 
-  def test_nested_include_with_variable
-    skip 'To be implemented'
-    assert_template_result "Product: Draft 151cm details ",
-      "{% include 'nested_product_template' with product %}", "product" => { "title" => 'Draft 151cm' }
+  def test_recursively_rendered_template_does_not_produce_endless_loop
+    Liquid::Template.file_system = StubFileSystem.new('loop' => '{% render "loop" %}')
 
-    assert_template_result "Product: Draft 151cm details Product: Element 155cm details ",
-      "{% include 'nested_product_template' for products %}", "products" => [{ "title" => 'Draft 151cm' }, { "title" => 'Element 155cm' }]
-  end
-
-  def test_recursively_included_template_does_not_produce_endless_loop
-    skip 'To be implemented'
-    infinite_file_system = Class.new do
-      def read_template_file(template_path)
-        "-{% include 'loop' %}"
-      end
-    end
-
-    Liquid::Template.file_system = infinite_file_system.new
-
-    assert_raises(Liquid::StackLevelError) do
-      Template.parse("{% include 'loop' %}").render!
+    assert_raises Liquid::StackLevelError do
+      Template.parse('{% render "loop" %}').render!
     end
   end
 
-  def test_dynamically_choosen_template
-    skip 'To be implemented'
-    assert_template_result "Test123", "{% include template %}", "template" => 'Test123'
-    assert_template_result "Test321", "{% include template %}", "template" => 'Test321'
+  def test_includes_and_renders_count_towards_the_same_recursion_limit
+    Liquid::Template.file_system = StubFileSystem.new(
+      'loop_render' => '{% render "loop_include" %}',
+      'loop_include' => '{% include "loop_render" %}'
+    )
 
-    assert_template_result "Product: Draft 151cm ", "{% include template for product %}",
-      "template" => 'product', 'product' => { 'title' => 'Draft 151cm' }
+    assert_raises Liquid::StackLevelError  do
+      Template.parse('{% render "loop_include" %}').render!
+    end
+  end
+
+  def test_dynamically_choosen_templates_are_not_allowed
+    Liquid::Template.file_system = StubFileSystem.new('snippet' => 'should not be rendered')
+
+    assert_raises Liquid::SyntaxError do
+      Liquid::Template.parse("{% assign name = 'snippet' %}{% render name %}")
+    end
   end
 
   def test_include_tag_caches_second_read_of_same_partial
-    skip 'To be implemented'
-    file_system = CountingFileSystem.new
-    assert_equal 'from CountingFileSystemfrom CountingFileSystem',
-      Template.parse("{% include 'pick_a_source' %}{% include 'pick_a_source' %}").render!({}, registers: { file_system: file_system })
-    assert_equal 1, file_system.count
+    file_system = StubFileSystem.new('snippet' => 'echo')
+    assert_equal 'echoecho',
+      Template.parse('{% render "snippet" %}{% render "snippet" %}')
+              .render!({}, registers: { file_system: file_system })
+    assert_equal 1, file_system.file_read_count
   end
 
-  def test_include_tag_doesnt_cache_partials_across_renders
-    skip 'To be implemented'
-    file_system = CountingFileSystem.new
-    assert_equal 'from CountingFileSystem',
-      Template.parse("{% include 'pick_a_source' %}").render!({}, registers: { file_system: file_system })
-    assert_equal 1, file_system.count
+  def test_render_tag_doesnt_cache_partials_across_renders
+    file_system = StubFileSystem.new('snippet' => 'my message')
 
-    assert_equal 'from CountingFileSystem',
-      Template.parse("{% include 'pick_a_source' %}").render!({}, registers: { file_system: file_system })
-    assert_equal 2, file_system.count
+    assert_equal 'my message',
+      Template.parse('{% include "snippet" %}').render!({}, registers: { file_system: file_system })
+    assert_equal 1, file_system.file_read_count
+
+    assert_equal 'my message',
+      Template.parse('{% include "snippet" %}').render!({}, registers: { file_system: file_system })
+    assert_equal 2, file_system.file_read_count
   end
 
-  def test_include_tag_within_if_statement
-    skip 'To be implemented'
-    assert_template_result "foo_if_true", "{% if true %}{% include 'foo_if_true' %}{% endif %}"
+  def test_render_tag_within_if_statement
+    Liquid::Template.file_system = StubFileSystem.new('snippet' => 'my message')
+    assert_template_result 'my message', '{% if true %}{% render "snippet" %}{% endif %}'
   end
 
-  def test_custom_include_tag
-    skip 'To be implemented'
-    original_tag = Liquid::Template.tags['include']
-    Liquid::Template.tags['include'] = CustomInclude
-    begin
-      assert_equal "custom_foo",
-        Template.parse("{% include 'custom_foo' %}").render!
-    ensure
-      Liquid::Template.tags['include'] = original_tag
-    end
+  def test_break_through_render
+    Liquid::Template.file_system = StubFileSystem.new('break' => '{% break %}')
+    assert_template_result '1', '{% for i in (1..3) %}{{ i }}{% break %}{{ i }}{% endfor %}'
+    assert_template_result '112233', '{% for i in (1..3) %}{{ i }}{% render "break" %}{{ i }}{% endfor %}'
   end
 
-  def test_custom_include_tag_within_if_statement
-    skip 'To be implemented'
-    original_tag = Liquid::Template.tags['include']
-    Liquid::Template.tags['include'] = CustomInclude
-    begin
-      assert_equal "custom_foo_if_true",
-        Template.parse("{% if true %}{% include 'custom_foo_if_true' %}{% endif %}").render!
-    ensure
-      Liquid::Template.tags['include'] = original_tag
-    end
+  def test_increment_is_isolated_between_renders
+    Liquid::Template.file_system = StubFileSystem.new('incr' => '{% increment %}')
+    assert_template_result '010', '{% increment %}{% increment %}{% render "incr" %}'
   end
 
-  def test_does_not_add_error_in_strict_mode_for_missing_variable
-    skip 'To be implemented'
-    Liquid::Template.file_system = TestFileSystem.new
-
-    a = Liquid::Template.parse(' {% include "nested_template" %}')
-    a.render!
-    assert_empty a.errors
+  def test_decrement_is_isolated_between_renders
+    Liquid::Template.file_system = StubFileSystem.new('decr' => '{% decrement %}')
+    assert_template_result '-1-2-1', '{% decrement %}{% decrement %}{% render "decr" %}'
   end
-
-  def test_passing_options_to_included_templates
-    skip 'To be implemented'
-    assert_raises(Liquid::SyntaxError) do
-      Template.parse("{% include template %}", error_mode: :strict).render!("template" => '{{ "X" || downcase }}')
-    end
-    with_error_mode(:lax) do
-      assert_equal 'x', Template.parse("{% include template %}", error_mode: :strict, include_options_blacklist: true).render!("template" => '{{ "X" || downcase }}')
-    end
-    assert_raises(Liquid::SyntaxError) do
-      Template.parse("{% include template %}", error_mode: :strict, include_options_blacklist: [:locale]).render!("template" => '{{ "X" || downcase }}')
-    end
-    with_error_mode(:lax) do
-      assert_equal 'x', Template.parse("{% include template %}", error_mode: :strict, include_options_blacklist: [:error_mode]).render!("template" => '{{ "X" || downcase }}')
-    end
-  end
-
-  def test_render_raise_argument_error_when_template_is_undefined
-    skip 'To be implemented'
-    assert_raises(Liquid::ArgumentError) do
-      template = Liquid::Template.parse('{% include undefined_variable %}')
-      template.render!
-    end
-    assert_raises(Liquid::ArgumentError) do
-      template = Liquid::Template.parse('{% include nil %}')
-      template.render!
-    end
-  end
-
-  def test_including_via_variable_value
-    skip 'To be implemented'
-    assert_template_result "from TestFileSystem", "{% assign page = 'pick_a_source' %}{% include page %}"
-
-    assert_template_result "Product: Draft 151cm ", "{% assign page = 'product' %}{% include page %}", "product" => { 'title' => 'Draft 151cm' }
-
-    assert_template_result "Product: Draft 151cm ", "{% assign page = 'product' %}{% include page for foo %}", "foo" => { 'title' => 'Draft 151cm' }
-  end
-
-  def test_including_with_strict_variables
-    skip 'To be implemented'
-    template = Liquid::Template.parse("{% include 'simple' %}", error_mode: :warn)
-    template.render(nil, strict_variables: true)
-
-    assert_equal [], template.errors
-  end
-
-  def test_break_through_include
-    skip 'To be implemented'
-    assert_template_result "1", "{% for i in (1..3) %}{{ i }}{% break %}{{ i }}{% endfor %}"
-    assert_template_result "1", "{% for i in (1..3) %}{{ i }}{% include 'break' %}{{ i }}{% endfor %}"
-  end
-end # IncludeTagTest
-
+end

test/test_helper.rb

@@ -121,3 +121,17 @@ class ErrorDrop < Liquid::Drop
     raise Exception, 'exception'
   end
 end
+
+class StubFileSystem
+  attr_reader :file_read_count
+
+  def initialize(values)
+    @file_read_count = 0
+    @values = values
+  end
+
+  def read_template_file(template_path)
+    @file_read_count += 1
+    @values.fetch(template_path)
+  end
+end

test/unit/context_unit_test.rb

@@ -468,6 +468,16 @@ class ContextUnitTest < Minitest::Test
     assert_equal 'hi filtered', context.apply_global_filter('hi')
   end
 
+  def test_static_environments_are_read_with_lower_priority_than_environments
+    context = Context.build(
+      static_environments: { 'shadowed' => 'static', 'unshadowed' => 'static' },
+      environments: { 'shadowed' => 'dynamic' }
+    )
+
+    assert_equal 'dynamic', context['shadowed']
+    assert_equal 'static', context['unshadowed']
+  end
+
   def test_apply_global_filter_when_no_global_filter_exist
     context = Context.new
     assert_equal 'hi', context.apply_global_filter('hi')
@@ -481,11 +491,11 @@ class ContextUnitTest < Minitest::Test
     assert_nil subcontext['my_variable']
   end
 
-  def test_new_isolated_subcontext_inherits_environment
-    super_context = Context.new('my_environment_value' => 'my value')
+  def test_new_isolated_subcontext_inherits_static_environment
+    super_context = Context.build(static_environments: { 'my_environment_value' => 'my value' })
     subcontext = super_context.new_isolated_subcontext
 
-    assert_equal 'my value',subcontext['my_environment_value']
+    assert_equal 'my value', subcontext['my_environment_value']
   end
 
   def test_new_isolated_subcontext_inherits_resource_limits
@@ -497,7 +507,7 @@ class ContextUnitTest < Minitest::Test
 
   def test_new_isolated_subcontext_inherits_exception_renderer
     super_context = Context.new
-    super_context.exception_renderer = -> (_e) { 'my exception message' }
+    super_context.exception_renderer = ->(_e) { 'my exception message' }
     subcontext = super_context.new_isolated_subcontext
     assert_equal 'my exception message', subcontext.handle_error(Liquid::Error.new)
   end

test/unit/partial_cache_unit_test.rb

@@ -0,0 +1,91 @@
+require 'test_helper'
+
+class PartialCacheUnitTest < Minitest::Test
+  def test_uses_the_file_system_register_if_present
+    context = Liquid::Context.build(
+      registers: {
+        file_system: StubFileSystem.new('my_partial' => 'my partial body')
+      }
+    )
+
+    partial = Liquid::PartialCache.load(
+      'my_partial',
+      context: context,
+      parse_context: Liquid::ParseContext.new
+    )
+
+    assert_equal 'my partial body', partial.render
+  end
+
+  def test_reads_from_the_file_system_only_once_per_file
+    file_system = StubFileSystem.new('my_partial' => 'some partial body')
+    context = Liquid::Context.build(
+      registers: { file_system: file_system }
+    )
+
+    2.times do
+      Liquid::PartialCache.load(
+        'my_partial',
+        context: context,
+        parse_context: Liquid::ParseContext.new
+      )
+    end
+
+    assert_equal 1, file_system.file_read_count
+  end
+
+  def test_cache_state_is_stored_per_context
+    parse_context = Liquid::ParseContext.new
+    shared_file_system = StubFileSystem.new(
+      'my_partial' => 'my shared value'
+    )
+    context_one = Liquid::Context.build(
+      registers: {
+        file_system: shared_file_system
+      }
+    )
+    context_two = Liquid::Context.build(
+      registers: {
+        file_system: shared_file_system
+      }
+    )
+
+    2.times do
+      Liquid::PartialCache.load(
+        'my_partial',
+        context: context_one,
+        parse_context: parse_context
+      )
+    end
+
+    Liquid::PartialCache.load(
+      'my_partial',
+      context: context_two,
+      parse_context: parse_context
+    )
+
+    assert_equal 2, shared_file_system.file_read_count
+  end
+
+  def test_cache_is_not_broken_when_a_different_parse_context_is_used
+    file_system = StubFileSystem.new('my_partial' => 'some partial body')
+    context = Liquid::Context.build(
+      registers: { file_system: file_system }
+    )
+
+    Liquid::PartialCache.load(
+      'my_partial',
+      context: context,
+      parse_context: Liquid::ParseContext.new(my_key: 'value one')
+    )
+    Liquid::PartialCache.load(
+      'my_partial',
+      context: context,
+      parse_context: Liquid::ParseContext.new(my_key: 'value two')
+    )
+
+    # Technically what we care about is that the file was parsed twice,
+    # but measuring file reads is an OK proxy for this.
+    assert_equal 1, file_system.file_read_count
+  end
+end