Moved parsing of partials to parse time of include tags if it's possible (eg. context not required to know template name)

lib/liquid/tags/include.rb

@@ -30,6 +30,11 @@ module Liquid
           @attributes[key] = value
         end
 
+        if partial_parsable_at_parse_time?
+          source = read_template_from_file_system_at_parse
+          @partial = Liquid::Template.parse(source, pass_options)
+        end
+
       else
         raise SyntaxError.new(options[:locale].t("errors.syntax.include".freeze))
       end
@@ -66,17 +71,21 @@ module Liquid
         template_name = context[@template_name]
 
         if cached = cached_partials[template_name]
-          return cached
+          cached
+        else
+          if @partial && context.registers[:file_system].nil?
+            partial = @partial
+          else
+            partial = Liquid::Template.parse(read_template_from_file_system(context), pass_options)
+          end
+          cached_partials[template_name] = partial
+          context.registers[:cached_partials] = cached_partials
+          partial
         end
-        source = read_template_from_file_system(context)
-        partial = Liquid::Template.parse(source, pass_options)
-        cached_partials[template_name] = partial
-        context.registers[:cached_partials] = cached_partials
-        partial
       end
 
       def read_template_from_file_system(context)
-        file_system = context.registers[:file_system] || Liquid::Template.file_system
+        file_system = context.registers[:file_system] || parsed_file_system || Liquid::Template.file_system
 
         # make read_template_file call backwards-compatible.
         case file_system.method(:read_template_file).arity
@@ -89,6 +98,23 @@ module Liquid
         end
       end
 
+      def read_template_from_file_system_at_parse
+        parsed_file_system.read_template_file(parsed_template_name)
+      end
+
+      def parsed_file_system
+        options[:file_system]
+      end
+
+      def partial_parsable_at_parse_time?
+        template_name_is_string_constant = parsed_template_name.is_a?(String)
+        options[:file_system] && template_name_is_string_constant
+      end
+
+      def parsed_template_name
+        Expression.parse(@template_name)
+      end
+
       def pass_options
         dont_pass = @options[:include_options_blacklist]
         return {locale: @options[:locale]} if dont_pass == true

test/integration/tags/include_tag_test.rb

@@ -33,6 +33,13 @@ class TestFileSystem
   end
 end
 
+# FileSystems at parse time don't have a context
+class ParseFileSystem
+  def read_template_file(template_path)
+    'from ParseFileSystem'
+  end
+end
+
 class OtherFileSystem
   def read_template_file(template_path, context)
     'from OtherFileSystem'
@@ -77,6 +84,20 @@ class IncludeTagTest < Minitest::Test
       Template.parse("{% include 'pick_a_source' %}").render!({}, :registers => {:file_system => OtherFileSystem.new})
   end
 
+  def test_include_tag_can_use_file_system_at_parse_so_it_can_be_parsed_before_rendered
+    assert_equal 'from ParseFileSystem',
+      Template.parse("{% include 'pick_a_source' %}",file_system: ParseFileSystem.new).render!({})
+  end
+
+  def test_include_tag_looks_at_file_system_passed_in_registers_over_parse_options
+    assert_equal 'from OtherFileSystem',
+      Template.parse("{% include 'pick_a_source' %}",file_system: ParseFileSystem.new).render!({}, :registers => {:file_system => OtherFileSystem.new})
+  end
+
+  def test_include_tag_use_parse_option_file_system_even_if_partial_can_be_preparsed
+    assert_equal 'from ParseFileSystem',
+      Template.parse("{% include template %}",file_system: ParseFileSystem.new).render!({})
+  end 
 
   def test_include_tag_with
     assert_template_result "Product: Draft 151cm ",