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, 15 insertions, 1 deletions

diff --git a/ractor.c b/ractor.c
index 4eee3cc731..7b9c088ceb 100644
--- a/ractor.c
+++ b/ractor.c
@@ -2985,7 +2985,10 @@ rb_obj_traverse(VALUE obj,
 static int
 frozen_shareable_p(VALUE obj, bool *made_shareable)
 {
-    if (!RB_TYPE_P(obj, T_DATA)) {
+    if (CHILLED_STRING_P(obj)) {
+        return false;
+    }
+    else if (!RB_TYPE_P(obj, T_DATA)) {
         return true;
     }
     else if (RTYPEDDATA_P(obj)) {
@@ -3014,6 +3017,17 @@ make_shareable_check_shareable(VALUE obj)
     if (rb_ractor_shareable_p(obj)) {
         return traverse_skip;
     }
+    else if (CHILLED_STRING_P(obj)) {
+        rb_funcall(obj, idFreeze, 0);
+
+        if (UNLIKELY(!RB_OBJ_FROZEN_RAW(obj))) {
+            rb_raise(rb_eRactorError, "#freeze does not freeze object correctly");
+        }
+
+        if (RB_OBJ_SHAREABLE_P(obj)) {
+            return traverse_skip;
+        }
+    }
     else if (!frozen_shareable_p(obj, &made_shareable)) {
         if (made_shareable) {
             return traverse_skip;