use libsass provided error filename and line no

lib/sassc/engine.rb

@@ -40,7 +40,10 @@ def render
 
       if status != 0
         message = Native.context_get_error_message(context)
-        raise SyntaxError.new(message)
+        filename = Native.context_get_error_file(context)
+        line = Native.context_get_error_line(context)
+
+        raise SyntaxError.new(message, filename: filename, line: line)
       end
 
       css = Native.context_get_output_string(context)

lib/sassc/error.rb

@@ -11,7 +11,11 @@ class UnsupportedValue < BaseError; end
   # 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 initialize(message, filename: nil, line: nil)
+      @filename = filename
+      @line = line
+      super(message)
+    end
 
     def backtrace
       return nil if super.nil?
@@ -20,11 +24,8 @@ def backtrace
 
     # 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}"]
+      return [] unless @filename && @line
+      ["#{@filename}:#{@line}"]
     end
   end
 end

test/error_test.rb

@@ -2,34 +2,24 @@
 
 module SassC
   class ErrorTest < MiniTest::Test
+    def render(data, opts={})
+      Engine.new(data, opts).render
+    end
+
     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
+        template = <<-SCSS
+.foo {
+  baz: bang;
+  padding top: 10px;
+}
+      SCSS
 
-       Backtrace:
-       \t#{filename}:#{line}
-        on line #{line} of #{filename}
->>     @include border-radius(5px);
-   -------------^
-        ERROR
+        render(template, filename: filename)
       rescue SassC::SyntaxError => err
-        expected = "#{Pathname.getwd.join(filename)}:#{line}"
+        expected = "#{filename}:3"
         assert_equal expected, err.backtrace.first
       end
     end