From c270a6f37828f42411b3c9bae73067f9813b44d0 Mon Sep 17 00:00:00 2001
From: "Charles-P. Clermont" <charles@cpclermont.com>
Date: Mon, 22 Mar 2021 14:19:10 -0400
Subject: [PATCH] Add ParseTreeVisitor to Echo tag

This fixes theme-check#218, wherein variables used in echo tags are not
considered used by the linter. It is because our visitor doesn't see the
:variable_lookup's in the echo tag since the children array is empty.
But this array is empty because it is swallowed by the @variable.
---
 lib/liquid/tags/echo.rb              | 8 ++++++++
 test/unit/parse_tree_visitor_test.rb | 7 +++++++
 2 files changed, 15 insertions(+)

diff --git a/lib/liquid/tags/echo.rb b/lib/liquid/tags/echo.rb
index 1f78937..19026a0 100644
--- a/lib/liquid/tags/echo.rb
+++ b/lib/liquid/tags/echo.rb
@@ -12,6 +12,8 @@ module Liquid
   #   {% echo user | link %}
   #
   class Echo < Tag
+    attr_reader :variable
+
     def initialize(tag_name, markup, parse_context)
       super
       @variable = Variable.new(markup, parse_context)
@@ -20,6 +22,12 @@ module Liquid
     def render(context)
       @variable.render_to_output_buffer(context, +'')
     end
+
+    class ParseTreeVisitor < Liquid::ParseTreeVisitor
+      def children
+        [@node.variable]
+      end
+    end
   end
 
   Template.register_tag('echo', Echo)
diff --git a/test/unit/parse_tree_visitor_test.rb b/test/unit/parse_tree_visitor_test.rb
index 32dbcf6..2c085b3 100644
--- a/test/unit/parse_tree_visitor_test.rb
+++ b/test/unit/parse_tree_visitor_test.rb
@@ -26,6 +26,13 @@ class ParseTreeVisitorTest < Minitest::Test
     )
   end
 
+  def test_echo
+    assert_equal(
+      ["test"],
+      visit(%({% echo test %}))
+    )
+  end
+
   def test_if_condition
     assert_equal(
       ["test"],