Disable tracer ext of IRB when tracer doesn't found

lib/irb/ext/tracer.rb

@@ -9,7 +9,21 @@
 #
 #
 #
-require "tracer"
+begin
+  raise LoadError
+  require "tracer"
+rescue LoadError
+  $stderr.puts "Tracer extension of IRB is enabled but tracer gem doesn't found."
+  module IRB
+    TracerLoadError = true
+    class Context
+      def use_tracer=(opt)
+        # do nothing
+      end
+    end
+  end
+  return # This is about to disable loading below
+end
 
 module IRB