Fix rb_interned_str_* functions to not assume static strings

Fixes [Feature #13381]

When passed a `fake_str`, `register_fstring` would create new strings
with `str_new_static`. That's not what was expected, and answer
almost no use cases.

1 files changed, 12 insertions, 0 deletions

diff --git a/test/-ext-/string/test_interned_str.rb b/test/-ext-/string/test_interned_str.rb
new file mode 100644
index 0000000000..340dba41e8
--- /dev/null
+++ b/test/-ext-/string/test_interned_str.rb
@@ -0,0 +1,12 @@
+require 'test/unit'
+require '-test-/string'
+
+class Test_RbInternedStr < Test::Unit::TestCase
+  def test_interned_str
+    src = "a" * 20
+    interned_str = Bug::String.rb_interned_str_dup(src)
+    src.clear
+    src << "b" * 20
+    assert_equal "a" * 20, interned_str
+  end
+end