[Sass] Add a warning for selectors with no children.

lib/sass/engine.rb

@@ -268,6 +268,7 @@ END
           continued_rule, child = nil, continued_rule
         end
 
+        check_for_no_children(child)
         validate_and_append_child(parent, child, line, root)
       end
 
@@ -294,6 +295,22 @@ END
       end
     end
 
+    def check_for_no_children(node)
+      return unless node.is_a?(Tree::RuleNode) && node.children.empty?
+      warning = (node.rules.size == 1) ? <<SHORT : <<LONG
+WARNING:
+Selector #{node.rules.first.inspect} doesn't have any properties and will not be rendered.
+SHORT
+
+WARNING:
+Selector
+  #{node.rules.join("\n  ")}
+doesn't have any properties and will not be rendered.
+LONG
+
+      warn(warning.strip)
+    end
+
     def parse_line(parent, line, root)
       case line.text[0]
       when PROPERTY_CHAR

test/sass/engine_test.rb

@@ -130,7 +130,7 @@ class SassEngineTest < Test::Unit::TestCase
     define_method("test_exception (#{key.inspect})") do
       line = 10
       begin
-        Sass::Engine.new(key, :filename => __FILE__, :line => line).render
+        silence_warnings {Sass::Engine.new(key, :filename => __FILE__, :line => line).render}
       rescue Sass::SyntaxError => err
         value = [value] unless value.is_a?(Array)
 
@@ -704,6 +704,24 @@ a
 SASS
   end
 
+  def test_empty_selector_warning
+    assert_warning(<<END) {render("foo bar")}
+WARNING:
+Selector "foo bar" doesn't have any properties and will not be rendered.
+END
+
+    assert_warning(<<END) {render(<<SASS)}
+WARNING:
+Selector
+  foo, bar, baz,
+  bang, bip, bop
+doesn't have any properties and will not be rendered.
+END
+foo, bar, baz,
+bang, bip, bop
+SASS
+  end
+
   # Regression tests
 
   def test_parens_in_mixins

test/sass/script_test.rb

@@ -243,21 +243,6 @@ WARN
     Sass::Engine.new(sass, options).render
   end
 
-  def assert_warning(message)
-    the_real_stderr, $stderr = $stderr, StringIO.new
-    yield
-    assert_equal message.strip, $stderr.string.strip
-  ensure
-    $stderr = the_real_stderr
-  end
-
-  def silence_warnings
-    the_real_stderr, $stderr = $stderr, StringIO.new
-    yield
-  ensure
-    $stderr = the_real_stderr
-  end
-
   def env(hash = {})
     env = Sass::Environment.new
     hash.each {|k, v| env.set_var(k, v)}

test/sass/templates/complex.sass

@@ -99,8 +99,6 @@ body
     :clear both
     .column
       :float left
-      .left
-      .middle
       .right
         :float right
   a:link, a:visited

test/test_helper.rb

@@ -26,4 +26,19 @@ class Test::Unit::TestCase
     path = File.dirname(__FILE__) + "/../.sass-cache"
     FileUtils.rm_r(path) if File.exist?(path)
   end
+
+  def assert_warning(message)
+    the_real_stderr, $stderr = $stderr, StringIO.new
+    yield
+    assert_equal message.strip, $stderr.string.strip
+  ensure
+    $stderr = the_real_stderr
+  end
+
+  def silence_warnings
+    the_real_stderr, $stderr = $stderr, StringIO.new
+    yield
+  ensure
+    $stderr = the_real_stderr
+  end
 end