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, 11 insertions, 8 deletions

diff --git a/spec/ruby/core/kernel/eval_spec.rb b/spec/ruby/core/kernel/eval_spec.rb
index 15c9d511fc..5d82f57e44 100644
--- a/spec/ruby/core/kernel/eval_spec.rb
+++ b/spec/ruby/core/kernel/eval_spec.rb
@@ -350,9 +350,11 @@ CODE
     end
 
     it "allows a magic encoding comment and a subsequent frozen_string_literal magic comment" do
+      frozen_string_default = "test".frozen?
+
       code = <<CODE.b
 # encoding: UTF-8
-# frozen_string_literal: true
+# frozen_string_literal: #{!frozen_string_default}
 class EvalSpecs
   Vπstring = "frozen"
 end
@@ -362,7 +364,7 @@ CODE
       EvalSpecs.constants(false).should include(:"Vπstring")
       EvalSpecs::Vπstring.should == "frozen"
       EvalSpecs::Vπstring.encoding.should == Encoding::UTF_8
-      EvalSpecs::Vπstring.frozen?.should be_true
+      EvalSpecs::Vπstring.frozen?.should == !frozen_string_default
     end
 
     it "allows a magic encoding comment and a frozen_string_literal magic comment on the same line in emacs style" do
@@ -381,8 +383,9 @@ CODE
     end
 
     it "ignores the magic encoding comment if it is after a frozen_string_literal magic comment" do
+      frozen_string_default = "test".frozen?
       code = <<CODE.b
-# frozen_string_literal: true
+# frozen_string_literal: #{!frozen_string_default}
 # encoding: UTF-8
 class EvalSpecs
   Vπfrozen_first = "frozen"
@@ -396,24 +399,24 @@ CODE
       value = EvalSpecs.const_get(binary_constant)
       value.should == "frozen"
       value.encoding.should == Encoding::BINARY
-      value.frozen?.should be_true
+      value.frozen?.should == !frozen_string_default
     end
 
     it "ignores the frozen_string_literal magic comment if it appears after a token and warns if $VERBOSE is true" do
-      default_frozen_string_literal = "test".frozen?
+      frozen_string_default = "test".frozen?
       code = <<CODE
 some_token_before_magic_comment = :anything
-# frozen_string_literal: true
+# frozen_string_literal: #{!frozen_string_default}
 class EvalSpecs
   Vπstring_not_frozen = "not frozen"
 end
 CODE
       -> { eval(code) }.should complain(/warning: [`']frozen_string_literal' is ignored after any tokens/, verbose: true)
-      EvalSpecs::Vπstring_not_frozen.frozen?.should == default_frozen_string_literal
+      EvalSpecs::Vπstring_not_frozen.frozen?.should == frozen_string_default
       EvalSpecs.send :remove_const, :Vπstring_not_frozen
 
       -> { eval(code) }.should_not complain(verbose: false)
-      EvalSpecs::Vπstring_not_frozen.frozen?.should == default_frozen_string_literal
+      EvalSpecs::Vπstring_not_frozen.frozen?.should == frozen_string_default
       EvalSpecs.send :remove_const, :Vπstring_not_frozen
     end
   end