[WIP] Add support for migrating liquid code away from several lax parse quirks

It transforms liquid code to equivalently parsed code, that doesn't depend
on lax parsing quirks.

Note that this transformation can hide the intent of the original code, so
it is best to review the changes, since removed syntax often would indicate
a bug in the original liquid code.

test/unit/migrate_unit_test.rb

@@ -0,0 +1,121 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class MigrateUnitTest < Minitest::Test
+  def test_migrate_preserves_valid_markup
+    [
+      "{{a}}",
+      " {{- \ta\n -}} ",
+      "{{a.b['c'].d[5]|default:6,allow_false:true|truncate:7,'..'}}",
+      "{{ a . b [ 'c' ] . d [ 5 ] | default : 6 , allow_false : true | truncate :  4 , '..'  }}",
+      "{%assign x=a.b['c'].d[5]|default:6,allow_false:true|truncate:7,'..'%}",
+      "{% assign x =\na . b [ 'c' ] . d [ 5 ] | default : 6 , allow_false : true | truncate :  4 , '..'  %}",
+      "{% if a and b > c %}A{% elsif d or f contains g %}B{% else %}C{% endif %}",
+      <<~LIQUID,
+        {% liquid
+          if x > 0
+            assign x = x | plus: 1
+
+          endif
+        %}
+      LIQUID
+    ].each do |source|
+      assert_no_migration(source)
+    end
+  end
+
+  def test_migrate_variable
+    with_error_mode(:lax) do
+      assert_migration({
+        %({{ ,|"' }}) => "{{  }}", # no MarkupWithQuotedFragment match, skipping characters
+        %({{ ,|"' 123 }}) => "{{ 123 }}", # MarkupWithQuotedFragment skipped characters
+        "{{  12 34  }}" => "{{  12  }}", # no FilterMarkupRegex match, skipping characters
+        "{{ -12 34 | abs }}" => "{{ -12 | abs }}", # FilterMarkupRegex skipped characters
+        %({{ -12 | '" abs }}) => "{{ -12 |  abs }}", # FilterParser skipped characters
+        "{{ -1 | abs ' plus: 1 }}" => "{{ -1 | abs | plus: 1 }}", # FilterParser unexpected separator
+        "{{ -1 | ! abs }}" => "{{ -1 | abs }}", # ignored non-word characters preceding filter name
+        "{{ 'a' | append WAT: 'b' }}" => "{{ 'a' | append : 'b' }}", # FilterArgsRegex skipped characters
+        "{{ '!' | replace, '!': '?' }}" => "{{ '!' | replace: '!', '?' }}", # FilterArgsRegex unexpected separators
+        "{{!-a-!}}" => "{{ -a- }}", # preserve separators when removing ignored characters
+      })
+    end
+  end
+
+  def test_migrate_expression
+    with_error_mode(:lax) do
+      assert_migration({
+        "{{ (1.9...2.8) }}" => "{{ (1..2) }}", # apply constant range coercion
+        "{{ 1.2.3.4 }}" => "{{ 1.2 }}", # multiple periods allowed by FLOATS_REGEX, truncated by to_f
+        "{{ 1. }}" => "{{ 1.0 }}", # FLOATS_REGEX didn't require digits after the period
+        "{{ .empty }}" => "{{ ['empty'] }}", # skipped character prevents exact literal lookup
+      })
+    end
+  end
+
+  def test_migrate_variable_lookup
+    with_error_mode(:lax) do
+      assert_migration({
+        "{{@-a[b].c-@}}" => "{{ -a[b].c- }}", # VariableParser skipped characters
+        "{{ a!b$c }}" => "{{ a.b.c }}", # VariableParser unexpected separators
+      })
+    end
+  end
+
+  def test_migrate_assign
+    with_error_mode(:lax) do
+      assert_migration({
+        "{% assign!a = b-!%}" => "{% assign a = b- %}", # Syntax skipped characters
+        "{% assign a = @b ! %}" => "{% assign a =  b %}", # Variable skipped characters
+        "{% assign|x=1 %}" => "{% assign x=1 %}", # ensure tag name separated from markup
+      })
+    end
+  end
+
+  def test_lax_migrate_if
+    with_error_mode(:lax) do
+      assert_migration({
+        "{% if@a-@%}Y{% endif %}" => "{% if a- %}Y{% endif %}", # Syntax skipped character
+        "{% if &a contains^b and *c %}A{% elsif %d or$e %}B{% endif %}" =>
+          "{% if  a contains b and  c %}A{% elsif  d or e %}B{% endif %}", # test more expressions
+        "{% if b 1 %}Y{% endif %}" => "{% if b  %}Y{% endif %}", # missing operator with right operand
+        "{% if c == %}Y{% endif %}" => "{% if c == nil %}Y{% endif %}", # operator with missing right operand
+        "{% if!a-!%}T{% endif %}" => "{% if a- %}T{% endif %}", # VariableParser skipped characters
+        "{% if!%}T{% endif %}" => "{% if nil %}T{% endif %}", # VariableParser skipping all characters
+      })
+    end
+  end
+
+  def test_migrate_liquid_tag
+    with_error_mode(:lax) do
+      source = <<~LIQUID
+        {% liquid
+          assign ! a = 1
+          assign a = @b !
+        %}
+      LIQUID
+      expect = <<~LIQUID
+        {% liquid
+          assign   a = 1
+          assign a =  b
+        %}
+      LIQUID
+      assert_migration({ source => expect })
+    end
+  end
+
+  private
+
+  def assert_no_migration(source)
+    assert_equal(source, Liquid::Template.migrate(source))
+  end
+
+  def assert_migration(source_to_expected_output_hash)
+    source_to_expected_output_hash.each do |source, expect|
+      message = "source: #{source.inspect}"
+      assert_equal(expect, Liquid::Template.migrate(source), message)
+      assert_no_migration(expect)
+      assert_equal(Liquid::Template.parse(expect).render!, Liquid::Template.parse(source).render!, message)
+    end
+  end
+end