Fix GC compaction for compare-by-identity sets

[Bug #22064]

Compare-by-identity sets use the address for hashing, so we must pin it
so the object does not move in GC compaction. Objects in a compare-by-identity
set is not currently pinned, causing the set to be broken if the object
is moved.

For example:

    set = Set.new.compare_by_identity

    o = Object.new
    set.add(o)

    puts set.include?(o)

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

    puts set.include?(o)

It should output true twice, but it outputs true and false.

1 files changed, 19 insertions, 0 deletions

diff --git a/test/ruby/test_set.rb b/test/ruby/test_set.rb
index 46d649ee73..427dd4b6b0 100644
--- a/test/ruby/test_set.rb
+++ b/test/ruby/test_set.rb
@@ -902,6 +902,25 @@ class TC_Set < Test::Unit::TestCase
     assert_equal(array.uniq.sort, set.sort)
   end
 
+  def test_compare_by_identity_compact
+    omit "compaction is not supported on this platform" unless GC.respond_to?(:compact)
+
+    # [Bug #22064]
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      set = Set.new.compare_by_identity
+
+      o = Object.new
+      set.add(o)
+
+      assert_include(set, o)
+
+      GC.verify_compaction_references(expand_heap: true, toward: :empty)
+
+      assert_include(set, o)
+    end;
+  end
+
   def test_reset
     [Set, Class.new(Set)].each { |klass|
       a = [1, 2]