debug.rb

### Inspect where a method is called
def foo(x, y)
  p caller # Kernel#caller
  # ...
end

### Inspect where a method is defined
# Where is obj.foo defined?
p obj.method(:foo).source_location # Method#source_location

### Output all methods will be called
trace = TracePoint.new(:call, :c_call) do |tp|
  cls = tp.defined_class
  m = if cls.singleton_class?
        "#{cls.to_s[/#<Class:(.+)>/, 1]}.#{tp.method_id}"
      else
        "#{cls}##{tp.method_id}"
      end
  puts "#{m} (#{tp.path}:#{tp.lineno})"
end

trace.enable

# codes....

trace.disable

### Count the number of objects on memory
GC.start
ObjectSpace.each_object(Hash).count

### Inspect where a global variable is overwrote
trace_var(:$FOO) do |newval|
  p caller.first
end