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, 12 insertions, 2 deletions

diff --git a/spec/ruby/shared/kernel/object_id.rb b/spec/ruby/shared/kernel/object_id.rb
index 3e032102f1..099df8ff94 100644
--- a/spec/ruby/shared/kernel/object_id.rb
+++ b/spec/ruby/shared/kernel/object_id.rb
@@ -52,7 +52,7 @@ describe :object_id, shared: true do
     o1.send(@method).should_not == o2.send(@method)
   end
 
-  guard -> { "test".frozen? } do # --enable-frozen-string-literal in $RUBYOPT
+  guard -> { "test".frozen? && "test".equal?("test") } do # --enable-frozen-string-literal in $RUBYOPT
     it "returns the same value for two identical String literals" do
       o1 = "hello"
       o2 = "hello"
@@ -60,7 +60,17 @@ describe :object_id, shared: true do
     end
   end
 
-  guard_not -> { "test".frozen? } do
+  guard -> { "test".frozen? && !"test".equal?("test") } do # chilled string literals
+    it "returns a different frozen value for two String literals" do
+      o1 = "hello"
+      o2 = "hello"
+      o1.send(@method).should_not == o2.send(@method)
+      o1.frozen?.should == true
+      o2.frozen?.should == true
+    end
+  end
+
+  guard -> { !"test".frozen? } do
     it "returns a different value for two String literals" do
       o1 = "hello"
       o2 = "hello"