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 <byroot@ruby-lang.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/ext/objspace/objspace_dump.c b/ext/objspace/objspace_dump.c
index d05fe66037..bb479b91c5 100644
--- a/ext/objspace/objspace_dump.c
+++ b/ext/objspace/objspace_dump.c
@@ -476,6 +476,8 @@ dump_object(VALUE obj, struct dump_config *dc)
             dump_append(dc, ", \"embedded\":true");
         if (FL_TEST(obj, RSTRING_FSTR))
             dump_append(dc, ", \"fstring\":true");
+        if (CHILLED_STRING_P(obj))
+            dump_append(dc, ", \"chilled\":true");
         if (STR_SHARED_P(obj))
             dump_append(dc, ", \"shared\":true");
         else