1 files changed, 72 insertions, 0 deletions
diff --git a/test/-ext-/string/test_fstring.rb b/test/-ext-/string/test_fstring.rb
new file mode 100644
index 0000000000..fcec6be543
--- /dev/null
+++ b/test/-ext-/string/test_fstring.rb
@@ -0,0 +1,72 @@
+# frozen_string_literal: false
+require 'test/unit'
+require '-test-/string'
+require_relative '../symbol/noninterned_name'
+
+class Test_String_Fstring < Test::Unit::TestCase
+  include Test_Symbol::NonInterned
+
+  def assert_fstring(str)
+    fstr = Bug::String.fstring(str)
+    yield str
+    yield fstr
+  end
+
+  def test_rb_enc_interned_str_autoloaded_encoding
+    assert_separately([], <<~RUBY)
+      require '-test-/string'
+      assert_include(Encoding::CESU_8.inspect, 'autoload')
+      Bug::String.rb_enc_interned_str(Encoding::CESU_8)
+    RUBY
+  end
+
+  def test_rb_enc_interned_str_null_encoding
+    assert_equal Encoding::ASCII_8BIT, Bug::String.rb_enc_interned_str(nil).encoding
+  end
+
+  def test_rb_enc_str_new_autoloaded_encoding
+    assert_separately([], <<~RUBY)
+      require '-test-/string'
+      assert_include(Encoding::CESU_8.inspect, 'autoload')
+      Bug::String.rb_enc_str_new(Encoding::CESU_8)
+    RUBY
+  end
+
+  def test_rb_enc_str_new_null_encoding
+    assert_equal Encoding::ASCII_8BIT, Bug::String.rb_enc_str_new(nil).encoding
+  end
+
+  def test_instance_variable
+    str = __method__.to_s * 3
+    str.instance_variable_set(:@test, 42)
+    str.freeze
+    assert_fstring(str) {|s| assert_send([s, :instance_variable_defined?, :@test])}
+  end
+
+  def test_singleton_method
+    str = __method__.to_s * 3
+    def str.foo
+    end
+    str.freeze
+    assert_fstring(str) {|s| assert_send([s, :respond_to?, :foo])}
+  end
+
+  def test_singleton_class
+    str = noninterned_name
+    fstr = Bug::String.fstring(str)
+    assert_raise(TypeError) {fstr.singleton_class}
+  end
+
+  def test_fake_str
+    assert_equal([*"a".."z"].join(""), Bug::String.fstring_fake_str)
+  end
+
+  class S < String
+  end
+
+  def test_subclass
+    str = S.new(__method__.to_s * 3)
+    str.freeze
+    assert_fstring(str) {|s| assert_instance_of(S, s)}
+  end
+end