Show source of a block from a rails ERB file

erb_source.rb

class ERBSource
  ERB = ::ActionView::Template::Handlers::ERB

  def self.for(block)
    new(block).source
  end

  attr_reader :block, :file, :line_number
  def initialize(block)
    @block = block
    @file, @line_number = *block.source_location
    @line_number += 1
  end

  def source
    lines = File.readlines(file)

    relevant_lines = lines[(line_number - 1)..-1] || []

    extract_first_expression(relevant_lines)
  end

  private

  def extract_first_expression(lines)
    code = ""

    lines.each do |v|
      code << v
      return code if correct_syntax?(compile_erb(code))
    end
    raise SyntaxError, "unexpected $end"
  end

  def correct_syntax?(code)
    stderr = $stderr
    $stderr.reopen(IO::NULL)
    RubyVM::InstructionSequence.compile(code)
    $stderr.reopen(stderr)
    true
  rescue Exception
    $stderr.reopen(stderr)
    false
  end

  def compile_erb(code)
    ERB.erb_implementation.new(
      code,
      :escape => false,
      :trim => (ERB.erb_trim_mode == "-")
    ).src
  end
end

style_guide.html.erb

<%= render_example "article" do %>
  <% article title: "Settings", text: "description of settings" do %>
    <%= text_field_tag :name %>
    <%= text_field_tag :description %>
  <%- end %>
<% end %>

style_guide_helper.rb

module StyleGuideHelper
  def render_example(name, &block)
    code = ERBSource.for(block)
    content_tag(:h2, name) +
      content_tag(:div, &block) +
      content_tag(:pre, content_tag(:code, code))
  end
end