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, 1 insertions, 0 deletions

diff --git a/prism_compile.h b/prism_compile.h
index 427fa54b51..0c1510d67f 100644
--- a/prism_compile.h
+++ b/prism_compile.h
@@ -47,6 +47,7 @@ typedef struct {
     bool parsed;
 } pm_parse_result_t;
 
+void pm_options_frozen_string_literal_init(pm_parse_result_t *result, int frozen_string_literal);
 VALUE pm_load_file(pm_parse_result_t *result, VALUE filepath);
 VALUE pm_parse_file(pm_parse_result_t *result, VALUE filepath);
 VALUE pm_load_parse_file(pm_parse_result_t *result, VALUE filepath);