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, 1 insertions, 1 deletions

diff --git a/ruby.c b/ruby.c
index 3b9cce01f1..5c1d44ec6d 100644
--- a/ruby.c
+++ b/ruby.c
@@ -2116,7 +2116,7 @@ prism_script(ruby_cmdline_options_t *opt, pm_parse_result_t *result)
     pm_options_t *options = &result->options;
     pm_options_line_set(options, 1);
 
-    pm_options_frozen_string_literal_set(&result->options, rb_iseq_opt_frozen_string_literal());
+    pm_options_frozen_string_literal_init(result, rb_iseq_opt_frozen_string_literal());
 
     if (opt->ext.enc.name != 0) {
         pm_options_encoding_set(options, StringValueCStr(opt->ext.enc.name));