Implement chilled strings

[Feature #20205]

As a path toward enabling frozen string literals by default in the future,
this commit introduce "chilled strings". From a user perspective chilled
strings pretend to be frozen, but on the first attempt to mutate them,
they lose their frozen status and emit a warning rather than to raise a
`FrozenError`.

Implementation wise, `rb_compile_option_struct.frozen_string_literal` is
no longer a boolean but a tri-state of `enabled/disabled/unset`.

When code is compiled with frozen string literals neither explictly enabled
or disabled, string literals are compiled with a new `putchilledstring`
instruction. This instruction is identical to `putstring` except it marks
the String with the `STR_CHILLED (FL_USER3)` and `FL_FREEZE` flags.

Chilled strings have the `FL_FREEZE` flag as to minimize the need to check
for chilled strings across the codebase, and to improve compatibility with
C extensions.

Notes:
  - `String#freeze`: clears the chilled flag.
  - `String#-@`: acts as if the string was mutable.
  - `String#+@`: acts as if the string was mutable.
  - `String#clone`: copies the chilled flag.

Co-authored-by: Jean Boussier <[email protected]>

1 files changed, 33 insertions, 0 deletions

diff --git a/test/ruby/test_string.rb b/test/ruby/test_string.rb
index 58e1af2b61..3f1b91a6a9 100644
--- a/test/ruby/test_string.rb
+++ b/test/ruby/test_string.rb
@@ -3610,6 +3610,39 @@ CODE
     assert_bytesplice_raise(ArgumentError, S("hello"), 0..-1, "bye", 0, 3)
   end
 
+  def test_chilled_string
+    chilled_string = eval('"chilled"')
+
+    # Chilled strings pretend to be frozen
+    assert_predicate chilled_string, :frozen?
+
+    assert_not_predicate chilled_string.dup, :frozen?
+    assert_predicate chilled_string.clone, :frozen?
+
+    # @+ treat the original string as frozen
+    assert_not_predicate +chilled_string, :frozen?
+    assert_not_same chilled_string, +chilled_string
+
+    # @- the the original string as mutable
+    assert_predicate -chilled_string, :frozen?
+    assert_not_same chilled_string, -chilled_string
+  end
+
+  def test_chilled_string_setivar
+    String.class_eval <<~RUBY, __FILE__, __LINE__ + 1
+      def setivar!
+        @ivar = 42
+        @ivar
+      end
+    RUBY
+    chilled_string = eval('"chilled"')
+    begin
+      assert_equal 42, chilled_string.setivar!
+    ensure
+      String.undef_method(:setivar!)
+    end
+  end
+
   private
 
   def assert_bytesplice_result(expected, s, *args)