Evaluate compiled Ruby code safely on in debug option

See #762

CHANGELOG.md

@@ -18,6 +18,8 @@
   to the source Haml file from which it was generated. Thanks [Alex Babkin](https://github.com/ababkin).
 * Fix #@foo and #$foo style interpolation that was not working in html_safe mode. (Akira Matsuda)
 * Add `haml_tag_if` to render a block, conditionally wrapped in another element (Matt Wildig)
+* The `haml` command's debug option (`-d`) no longer executes the Haml code, but
+  rather checks the generated Ruby syntax for errors.
 
 ## 4.0.6
 

lib/haml/exec.rb

@@ -309,7 +309,12 @@ END
 
           if @options[:debug]
             puts engine.precompiled
-            puts '=' * 100
+            error = validate_ruby(engine.precompiled)
+            if error
+              puts '=' * 100
+              puts error.message.split("\n")[0]
+            end
+            return
           end
 
           result = engine.to_html
@@ -326,6 +331,15 @@ END
         output.write(result)
         output.close() if output.is_a? File
       end
+
+      def validate_ruby(code)
+        begin
+          eval("BEGIN {return nil}; #{code}")
+        # Not sure why, but rescuing "SyntaxError" does not work here.
+        rescue Exception
+          $!
+        end
+      end
     end
   end
 end