Add :trace option.

Closes #702.

CHANGELOG.md

@@ -14,6 +14,8 @@
 * Make escape_once respect hexadecimal references. (Matt Wildig)
 * General performance and memory usage improvements. (Akira Matsuda)
 * Don't treat the 'data' attribute specially when merging attribute hashes. (Matt Wildig and Norman Clarke)
+* Add a tracing option. When enabled, Haml will output a data-trace attribute on each tag showing the path
+  to the source Haml file from which it was generated. Thanks [Alex Babkin](https://github.com/ababkin).
 
 ## 4.0.5
 

lib/haml/compiler.rb

@@ -500,7 +500,9 @@ END
     end
 
     def prerender_tag(name, self_close, attributes)
-      # TODO: consider just passing in the damn options here
+      if @options[:trace]
+        attributes.merge!({"data-trace" => @options.filename.split('/views').last + ":" + @node.line.to_s})
+      end
       attributes_string = Compiler.build_attributes(
         @options.html?, @options.attr_wrapper, @options.escape_attrs, @options.hyphenate_data_attrs, attributes)
       "<#{name}#{attributes_string}#{self_close && @options.xhtml? ? ' /' : ''}>"

lib/haml/options.rb

@@ -23,7 +23,8 @@ module Haml
       :ugly                 => false,
       :cdata                => false,
       :parser_class         => ::Haml::Parser,
-      :compiler_class       => ::Haml::Compiler
+      :compiler_class       => ::Haml::Compiler,
+      :trace                => false
     }
 
     @valid_formats = [:html4, :html5, :xhtml]
@@ -168,6 +169,14 @@ module Haml
     # The compiler class to use. Defaults to Haml::Compiler.
     attr_accessor :compiler_class
 
+    # Enable template tracing. If true, it will add a 'data-trace' attribute to
+    # each tag generated by Haml. The value of the attribute will be the
+    # source template name and the line number from which the tag was generated,
+    # separated by a colon. On Rails applications, the path given will be a
+    # relative path as from the views directory. On non-Rails applications,
+    # the path will be the full path.
+    attr_accessor :trace
+
     def initialize(values = {}, &block)
       defaults.each {|k, v| instance_variable_set :"@#{k}", v}
       values.each {|k, v| send("#{k}=", v) if defaults.has_key?(k) && !v.nil?}

test/engine_test.rb

@@ -1986,6 +1986,11 @@ HAML
     end
   end
 
+  def test_tracing
+    result = render('%p', :trace => true, :filename => 'foo').strip
+    assert_equal "<p data-trace='foo:1'></p>", result
+  end
+
   private
 
   def assert_valid_encoding_comment(comment)