Add # inline comment tag.

This commit adds a new tag named `#` that behaves like a comment.

Therefore it behaves as you'd expect any tag would work. The difference
with the comment tag is that the comment is in the tag markup and that
there is no block delimiter.

What it looks like in practice:

```liquid
{%- # this is an inline comment -%}
{% # this too is an inline comment %}

{% liquid
  # required args:
  assign product = product

  # optional args:
  assign should_show_border = should_show_border | default: true
  assign should_show_cursor = should_show_cursor | default: true
%}

{% liquid
  # This is a very long comment that spans multiple lines.
  # It looks very similar to what it would look like if you wrote
  # ruby code instead of liquid. But it doesn't have all the clunk
  # of having an open tag and a close tag with so many characters.
%}
```

Co-authored-by: Dylan Thacker-Smith <Dylan.Smith@shopify.com>

Gemfile

@@ -23,6 +23,6 @@ group :test do
   gem 'rubocop-performance', require: false
 
   platform :mri, :truffleruby do
-    gem 'liquid-c', github: 'Shopify/liquid-c', ref: 'master'
+    gem 'liquid-c', github: 'Shopify/liquid-c', ref: 'inline-comment-fresh'
   end
 end

lib/liquid.rb

@@ -29,6 +29,7 @@ module Liquid
   WhitespaceControl           = '-'
   TagStart                    = /\{\%/
   TagEnd                      = /\%\}/
+  TagName                     = /#|\w+/
   VariableSignature           = /\(?[\w\-\.\[\]]\)?/
   VariableSegment             = /[\w\-]/
   VariableStart               = /\{\{/

lib/liquid/block_body.rb

@@ -4,8 +4,8 @@
 
 module Liquid
   class BlockBody
-    LiquidTagToken      = /\A\s*(\w+)\s*(.*?)\z/o
-    FullToken           = /\A#{TagStart}#{WhitespaceControl}?(\s*)(\w+)(\s*)(.*?)#{WhitespaceControl}?#{TagEnd}\z/om
+    LiquidTagToken      = /\A\s*(#{TagName})\s*(.*?)\z/o
+    FullToken           = /\A#{TagStart}#{WhitespaceControl}?(\s*)(#{TagName})(\s*)(.*?)#{WhitespaceControl}?#{TagEnd}\z/om
     ContentOfVariable   = /\A#{VariableStart}#{WhitespaceControl}?(.*?)#{WhitespaceControl}?#{VariableEnd}\z/om
     WhitespaceOrNothing = /\A\s*\z/
     TAGSTART            = "{%"

lib/liquid/locales/en.yml

@@ -13,15 +13,16 @@
       for_invalid_attribute: "Invalid attribute in for loop. Valid attributes are limit and offset"
       if: "Syntax Error in tag 'if' - Valid syntax: if [expression]"
       include: "Error in tag 'include' - Valid syntax: include '[template]' (with|for) [object|collection]"
-      unknown_tag: "Unknown tag '%{tag}'"
+      inline_comment_invalid: "Syntax error in tag '#' - Each line of comments must be prefixed by the '#' character"
       invalid_delimiter: "'%{tag}' is not a valid delimiter for %{block_name} tags. use %{block_delimiter}"
+      render: "Syntax error in tag 'render' - Template name must be a quoted string"
+      table_row: "Syntax Error in 'table_row loop' - Valid syntax: table_row [item] in [collection] cols=3"
+      tag_never_closed: "'%{block_name}' tag was never closed"
+      tag_termination: "Tag '%{token}' was not properly terminated with regexp: %{tag_end}"
       unexpected_else: "%{block_name} tag does not expect 'else' tag"
       unexpected_outer_tag: "Unexpected outer '%{tag}' tag"
-      tag_termination: "Tag '%{token}' was not properly terminated with regexp: %{tag_end}"
+      unknown_tag: "Unknown tag '%{tag}'"
       variable_termination: "Variable '%{token}' was not properly terminated with regexp: %{tag_end}"
-      tag_never_closed: "'%{block_name}' tag was never closed"
-      table_row: "Syntax Error in 'table_row loop' - Valid syntax: table_row [item] in [collection] cols=3"
-      render: "Syntax error in tag 'render' - Template name must be a quoted string"
     argument:
       include: "Argument error in tag 'include' - Illegal template name"
     disabled:

lib/liquid/tags/inline_comment.rb

@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+module Liquid
+  class InlineComment < Tag
+    def initialize(tag_name, markup, options)
+      super
+
+      # Semantically, a comment should only ignore everything after it on the line.
+      # Currently, this implementation doesn't support mixing a comment with another tag
+      # but we need to reserve future support for this and prevent the introduction
+      # of inline comments to be a backward incompatible changes.
+      #
+      # As such, we're forcing users to put a # symbol on every line otherwise this
+      # tag will throw an error.
+      if markup.match?(/\n\s*[^#\s]/)
+        raise SyntaxError, options[:locale].t("errors.syntax.inline_comment_invalid")
+      end
+    end
+
+    def render_to_output_buffer(_context, output)
+      output
+    end
+
+    def blank?
+      true
+    end
+  end
+
+  Template.register_tag('#', InlineComment)
+end

test/integration/tags/inline_comment_test.rb

@@ -0,0 +1,68 @@
+# frozen_string_literal: true
+
+require 'test_helper'
+
+class InlineCommentTest < Minitest::Test
+  include Liquid
+
+  def test_inline_comment_returns_nothing
+    assert_template_result('', '{%- # this is an inline comment -%}')
+    assert_template_result('', '{%-# this is an inline comment -%}')
+    assert_template_result('', '{% # this is an inline comment %}')
+    assert_template_result('', '{%# this is an inline comment %}')
+  end
+
+  def test_inline_comment_does_not_require_a_space_after_the_pound_sign
+    assert_template_result('', '{%#this is an inline comment%}')
+  end
+
+  def test_liquid_inline_comment_returns_nothing
+    assert_template_result('Hey there, how are you doing today?', <<~LIQUID)
+      {%- liquid
+        # This is how you'd write a block comment in a liquid tag.
+        # It looks a lot like what you'd have in ruby.
+
+        # You can use it as inline documentation in your
+        # liquid blocks to explain why you're doing something.
+        echo "Hey there, "
+
+        # It won't affect the output.
+        echo "how are you doing today?"
+      -%}
+    LIQUID
+  end
+
+  def test_inline_comment_can_be_written_on_multiple_lines
+    assert_template_result('', <<~LIQUID)
+      {%-
+        # That kind of block comment is also allowed.
+        # It would only be a stylistic difference.
+        # Much like JavaScript's /* */ comments and their
+        # leading * on new lines.
+      -%}
+    LIQUID
+  end
+
+  def test_inline_comment_multiple_pound_signs
+    assert_template_result('', <<~LIQUID)
+      {%- liquid
+        ######################################
+        # We support comments like this too. #
+        ######################################
+      -%}
+    LIQUID
+  end
+
+  def test_inline_comments_require_the_pound_sign_on_every_new_line
+    assert_match_syntax_error("Each line of comments must be prefixed by the '#' character", <<~LIQUID)
+      {%-
+        # some comment
+        echo 'hello world'
+      -%}
+    LIQUID
+  end
+
+  def test_inline_comment_does_not_support_nested_tags
+    assert_template_result(' -%}', "{%- # {% echo 'hello world' %} -%}")
+  end
+end