[Bug #20868] Fix Method#hash to not change after compaction

The hash value of a Method must remain constant after a compaction, otherwise
it may not work as the key in a hash table.

For example:

    def a; end

    # Need this method here because otherwise the iseq may be on the C stack
    # which would get pinned and not move during compaction
    def get_hash
      method(:a).hash
    end

    puts get_hash # => 2993401401091578131

    GC.verify_compaction_references(expand_heap: true, toward: :empty)

    puts get_hash # => -2162775864511574135

test/ruby/test_method.rb

@@ -209,6 +209,27 @@ def o.foo; end
     assert_kind_of(String, o.method(:foo).hash.to_s)
   end
 
+  def test_hash_does_not_change_after_compaction
+    omit "compaction is not supported on this platform" unless GC.respond_to?(:compact)
+
+    # iseq backed method
+    assert_separately([], <<~RUBY)
+      def a; end
+
+      # Need this method here because otherwise the iseq may be on the C stack
+      # which would get pinned and not move during compaction
+      def get_hash
+        method(:a).hash
+      end
+
+      hash = get_hash
+
+      GC.verify_compaction_references(expand_heap: true, toward: :empty)
+
+      assert_equal(hash, get_hash)
+    RUBY
+  end
+
   def test_owner
     c = Class.new do
       def foo; end

vm_method.c

@@ -2249,7 +2249,7 @@ rb_hash_method_definition(st_index_t hash, const rb_method_definition_t *def)
 
     switch (def->type) {
       case VM_METHOD_TYPE_ISEQ:
-        return rb_hash_uint(hash, (st_index_t)def->body.iseq.iseqptr);
+        return rb_hash_uint(hash, (st_index_t)def->body.iseq.iseqptr->body);
       case VM_METHOD_TYPE_CFUNC:
         hash = rb_hash_uint(hash, (st_index_t)def->body.cfunc.func);
         return rb_hash_uint(hash, def->body.cfunc.argc);