Don't pin named structs defined in Ruby

[Bug #20311]

`rb_define_class_under` assumes it's called from C and that the
reference might be held in a C global variable, so it adds the
class to the VM root.

In the case of `Struct.new('Name')` it's wasteful and make
the struct immortal.

1 files changed, 14 insertions, 0 deletions

diff --git a/test/ruby/test_struct.rb b/test/ruby/test_struct.rb
index ed750b91f7..a451e400cb 100644
--- a/test/ruby/test_struct.rb
+++ b/test/ruby/test_struct.rb
@@ -534,6 +534,20 @@ module TestStruct
     assert_equal [[:req, :_]], klass.instance_method(:c=).parameters
   end
 
+  def test_named_structs_are_not_rooted
+    # [Bug #20311]
+    assert_no_memory_leak([], <<~PREP, <<~CODE, rss: true)
+      code = proc do
+        Struct.new("A")
+        Struct.send(:remove_const, :A)
+      end
+
+      1_000.times(&code)
+    PREP
+      300_000.times(&code)
+    CODE
+  end
+
   class TopStruct < Test::Unit::TestCase
     include TestStruct