Show sass file on the top of backtrace

lib/sassc/error.rb

@@ -1,9 +1,30 @@
+require 'pathname'
 require 'sass/error'
 
 module SassC
   class BaseError < StandardError; end
-  class SyntaxError < BaseError; end
   class NotRenderedError < BaseError; end
   class InvalidStyleError < BaseError; end
   class UnsupportedValue < BaseError; end
+
+  # When dealing with SyntaxErrors,
+  # it's important to provide filename and line number information.
+  # This will be used in various error reports to users, including backtraces;
+  class SyntaxError < BaseError
+    LINE_INFO_REGEX = /on line (\d+) of (.+)/
+
+    def backtrace
+      return nil if super.nil?
+      sass_backtrace + super
+    end
+
+    # The backtrace of the error within Sass files.
+    def sass_backtrace
+      line_info = message.split("\n").find { |line| line.match(LINE_INFO_REGEX) }
+      return [] unless line_info
+
+      _, line, filename = line_info.match(LINE_INFO_REGEX).to_a
+      ["#{Pathname.getwd.join(filename)}:#{line}"]
+    end
+  end
 end

test/error_test.rb

@@ -0,0 +1,37 @@
+require_relative "test_helper"
+
+module SassC
+  class ErrorTest < MiniTest::Test
+    def test_first_backtrace_is_sass
+      line     = 2
+      filename = "app/assets/stylesheets/application.scss"
+
+      begin
+        raise SassC::SyntaxError.new(<<-ERROR)
+Error: property "padding" must be followed by a ':'
+        on line #{line} of #{filename}
+>>   padding top: 10px;
+   --^
+        ERROR
+      rescue SassC::SyntaxError => err
+        expected = "#{Pathname.getwd.join(filename)}:#{line}"
+        assert_equal expected, err.backtrace.first
+      end
+
+      begin
+        raise SassC::SyntaxError.new(<<-ERROR)
+Error: no mixin named border-radius
+
+       Backtrace:
+       \t#{filename}:#{line}
+        on line #{line} of #{filename}
+>>     @include border-radius(5px);
+   -------------^
+        ERROR
+      rescue SassC::SyntaxError => err
+        expected = "#{Pathname.getwd.join(filename)}:#{line}"
+        assert_equal expected, err.backtrace.first
+      end
+    end
+  end
+end