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, 0 insertions, 1 deletions

diff --git a/include/ruby/internal/intern/error.h b/include/ruby/internal/intern/error.h
index bf8daadd3e..11e147a121 100644
--- a/include/ruby/internal/intern/error.h
+++ b/include/ruby/internal/intern/error.h
@@ -190,7 +190,6 @@ RBIMPL_ATTR_NONNULL(())
  */
 void rb_error_frozen(const char *what);
 
-RBIMPL_ATTR_NORETURN()
 /**
  * Identical  to  rb_error_frozen(),  except  it takes  arbitrary  Ruby  object
  * instead of C's string.