Mixin indexing does not depend on Project Builder - it is performed automatically in the background
diff --git a/plugins/org.eclipse.dltk.ruby.launching/src/org/eclipse/dltk/ruby/internal/launching/RubyGenericInstall.java b/plugins/org.eclipse.dltk.ruby.launching/src/org/eclipse/dltk/ruby/internal/launching/RubyGenericInstall.java
index a67a7e5..50c36e0 100644
--- a/plugins/org.eclipse.dltk.ruby.launching/src/org/eclipse/dltk/ruby/internal/launching/RubyGenericInstall.java
+++ b/plugins/org.eclipse.dltk.ruby.launching/src/org/eclipse/dltk/ruby/internal/launching/RubyGenericInstall.java
@@ -140,10 +140,13 @@
 				} catch (CoreException e) {
 					e.printStackTrace();
 				}
+				lastModified = System.currentTimeMillis();
 			}
 
 			return sources;
 		}
+		
+		long lastModified;
 	}
 
 	private BuiltinsHelper helper = new BuiltinsHelper();
@@ -175,6 +178,11 @@
 		final Map sources = helper.getSources();
 		return (String) sources.get(name);
 	}
+	
+	public long lastModified() {
+		helper.getSources();
+		return helper.lastModified;
+	}
 
 	public String[] getBuiltinModules() {
 		final Map sources = helper.getSources();