Instrument usage of bug with iteration of String with offset or 0 limit

lib/liquid/utils.rb

@@ -16,7 +16,11 @@ def self.slice_collection_using_each(collection, from, to)
 
       # Maintains Ruby 1.8.7 String#each behaviour on 1.9
       if collection.is_a?(String)
-        return collection.empty? ? [] : [collection]
+        return [] if collection.empty?
+        if from > 0 || to == 0
+          Usage.increment("string_slice_bug")
+        end
+        return [collection]
       end
       return [] unless collection.respond_to?(:each)
 

test/unit/tags/for_tag_unit_test.rb

@@ -12,4 +12,34 @@ def test_for_else_nodelist
     template = Liquid::Template.parse('{% for item in items %}FOR{% else %}ELSE{% endfor %}')
     assert_equal(['FOR', 'ELSE'], template.root.nodelist[0].nodelist.map(&:nodelist).flatten)
   end
+
+  def test_for_string_slice_bug_usage
+    template = Liquid::Template.parse("{% for x in str, offset: 1 %}{{ x }},{% endfor %}")
+    assert_usage("string_slice_bug") do
+      assert_equal("abc,", template.render({ "str" => "abc" }))
+    end
+  end
+
+  def test_for_string_0_limit_usage
+    template = Liquid::Template.parse("{% for x in str, limit: 0 %}{{ x }},{% endfor %}")
+    assert_usage("string_slice_bug") do
+      assert_equal("abc,", template.render({ "str" => "abc" }))
+    end
+  end
+
+  def test_for_string_no_slice_usage
+    template = Liquid::Template.parse("{% for x in str, offset: 0, limit: 1 %}{{ x }},{% endfor %}")
+    assert_usage("string_slice_bug", times: 0) do
+      assert_equal("abc,", template.render({ "str" => "abc" }))
+    end
+  end
+
+  private
+
+  def assert_usage(name, times: 1, &block)
+    count = 0
+    result = Liquid::Usage.stub(:increment, ->(n) { count += 1 if n == name }, &block)
+    assert_equal(times, count)
+    result
+  end
 end