Make Module#prepend affect the iclasses of the module

3556a834a2 added support for
Module#include to affect the iclasses of the module.  It didn't add
support for Module#prepend because there were bugs in the object model
and GC at the time that prevented it.  Those problems have been
addressed in ad729a1d11 and
98286e9850, and now adding support for
it is straightforward and does not break any tests or specs.

Fixes [Bug #9573]

class.c

@@ -1141,6 +1141,13 @@ rb_prepend_module(VALUE klass, VALUE module)
     if (changed) {
 	rb_vm_check_redefinition_by_prepend(klass);
     }
+    if (RB_TYPE_P(klass, T_MODULE)) {
+        rb_subclass_entry_t *iclass = RCLASS_EXT(klass)->subclasses;
+        while (iclass) {
+            include_modules_at(iclass->klass, iclass->klass, module, FALSE);
+            iclass = iclass->next;
+        }
+    }
 }
 
 /*

test/ruby/test_module.rb

@@ -587,6 +587,28 @@ class TestModule < Test::Unit::TestCase
     m1.include m2
     m1.include m3
     assert_equal([:m1, :sc, :m2, :m3, :c], o.foo)
+
+    m1, m2, m3, sc, o = modules.call
+    assert_equal([:sc, :c], o.foo)
+    sc.prepend m1
+    assert_equal([:m1, :sc, :c], o.foo)
+    m1.prepend m2
+    assert_equal([:m2, :m1, :sc, :c], o.foo)
+    m2.prepend m3
+    assert_equal([:m3, :m2, :m1, :sc, :c], o.foo)
+    m1, m2, m3, sc, o = modules.call
+    sc.include m1
+    assert_equal([:sc, :m1, :c], o.foo)
+    sc.prepend m2
+    assert_equal([:m2, :sc, :m1, :c], o.foo)
+    sc.prepend m3
+    assert_equal([:m3, :m2, :sc, :m1, :c], o.foo)
+    m1, m2, m3, sc, o = modules.call
+    sc.include m1
+    assert_equal([:sc, :m1, :c], o.foo)
+    m2.prepend m3
+    m1.include m2
+    assert_equal([:sc, :m1, :m3, :m2, :c], o.foo)
   end
 
   def test_included_modules