Eiffel-like export features syntax for fine-grained method visibility

export_features.rb

class A; def bar; D.new.foo end end
class B; def bar; D.new.foo end end
class C; def bar; D.new.foo end end

class D
  def foo; "HELLO WORLD!" end
  
  # Export `foo` to A and B, but not C
  export :foo, A, B 
end

puts A.new.bar
puts B.new.bar
puts C.new.bar

# Output:
# 
# HELLO WORLD!
# HELLO WORLD!
# export_features.rb:3:in `bar': `foo' is not accessible to C:Class (NoMethodError)
#   from export_features.rb:14:in `<main>'

secret_sauce.rb

require_relative 'ext/callsite'

module Export
  def export_list
    @__exported__ ||= {}
  end
  
  def export(meth, *classes)
    meth = meth.to_s
    (export_list[meth] ||= []).push(*classes)
    alias_method "__exported__#{meth}", meth
    define_method(meth) do |*args, &block|
      cclass, classes = caller_class, self.class.export_list[meth]
      if classes && !classes.any? {|k| cclass >= k }
        raise NoMethodError, 
          "`#{meth}' is not accessible to #{cclass.inspect}:#{cclass.class.inspect}",
          caller
      end
      send("__exported__#{meth}", *args, &block)
    end
  end
end

BasicObject.send(:extend, Export)

secret_sauce_callsite.c

#include "callsite.h"

extern rb_thread_t *ruby_current_thread;

VALUE
callsite_caller_class(VALUE self)
{
	rb_control_frame_t *cfp = ruby_current_thread->cfp;
	/* pop 2 stack frames */
	cfp = RUBY_VM_PREVIOUS_CONTROL_FRAME(cfp);
	cfp = RUBY_VM_PREVIOUS_CONTROL_FRAME(cfp);
	if (cfp->iseq != 0 && cfp->pc != 0) {
		return cfp->iseq->klass;
	}
	else if (cfp->block_iseq) {
		while (!cfp->iseq) cfp = RUBY_VM_PREVIOUS_CONTROL_FRAME(cfp);
		return cfp->iseq->klass;
	}
	return Qnil;
}

void
Init_callsite()
{
	rb_define_method(rb_mKernel, "caller_class", callsite_caller_class, 0);
}