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>

8 files changed, 140 insertions, 12 deletions

diff --git a/spec/ruby/command_line/fixtures/string_literal_frozen_comment.rb b/spec/ruby/command_line/fixtures/string_literal_frozen_comment.rb
new file mode 100644
index 0000000000..fb84b546c0
--- /dev/null
+++ b/spec/ruby/command_line/fixtures/string_literal_frozen_comment.rb
@@ -0,0 +1,4 @@
+# frozen_string_literal: true
+frozen = "test".frozen?
+interned = "test".equal?("test")
+puts "frozen:#{frozen} interned:#{interned}"
diff --git a/spec/ruby/command_line/fixtures/string_literal_mutable_comment.rb b/spec/ruby/command_line/fixtures/string_literal_mutable_comment.rb
new file mode 100644
index 0000000000..381a742001
--- /dev/null
+++ b/spec/ruby/command_line/fixtures/string_literal_mutable_comment.rb
@@ -0,0 +1,4 @@
+# frozen_string_literal: false
+frozen = "test".frozen?
+interned = "test".equal?("test")
+puts "frozen:#{frozen} interned:#{interned}"
diff --git a/spec/ruby/command_line/fixtures/string_literal_raw.rb b/spec/ruby/command_line/fixtures/string_literal_raw.rb
new file mode 100644
index 0000000000..56b1841296
--- /dev/null
+++ b/spec/ruby/command_line/fixtures/string_literal_raw.rb
@@ -0,0 +1,3 @@
+frozen = "test".frozen?
+interned = "test".equal?("test")
+puts "frozen:#{frozen} interned:#{interned}"
diff --git a/spec/ruby/command_line/frozen_strings_spec.rb b/spec/ruby/command_line/frozen_strings_spec.rb
index 647b69daed..24b979b68b 100644
--- a/spec/ruby/command_line/frozen_strings_spec.rb
+++ b/spec/ruby/command_line/frozen_strings_spec.rb
@@ -19,6 +19,41 @@ describe "The --enable-frozen-string-literal flag causes string literals to" do
   end
 end
 
+describe "The --disable-frozen-string-literal flag causes string literals to" do
+
+  it "produce a different object each time" do
+    ruby_exe(fixture(__FILE__, "freeze_flag_one_literal.rb"), options: "--disable-frozen-string-literal").chomp.should == "false"
+  end
+
+end
+
+describe "With neither --enable-frozen-string-literal nor --disable-frozen-string-literal flag set" do
+
+  it "produce a different object each time" do
+    ruby_exe(fixture(__FILE__, "freeze_flag_one_literal.rb")).chomp.should == "false"
+  end
+
+  ruby_version_is "3.4" do
+    it "if file has no frozen_string_literal comment produce different frozen strings each time" do
+      ruby_exe(fixture(__FILE__, "string_literal_raw.rb")).chomp.should == "frozen:true interned:false"
+    end
+  end
+
+  ruby_version_is ""..."3.4" do
+    it "if file has no frozen_string_literal comment produce different mutable strings each time" do
+      ruby_exe(fixture(__FILE__, "string_literal_raw.rb")).chomp.should == "frozen:false interned:false"
+    end
+  end
+
+  it "if file has frozen_string_literal:true comment produce same frozen strings each time" do
+    ruby_exe(fixture(__FILE__, "string_literal_frozen_comment.rb")).chomp.should == "frozen:true interned:true"
+  end
+
+  it "if file has frozen_string_literal:false comment produce different mutable strings each time" do
+    ruby_exe(fixture(__FILE__, "string_literal_mutable_comment.rb")).chomp.should == "frozen:false interned:false"
+  end
+end
+
 describe "The --debug flag produces" do
   it "debugging info on attempted frozen string modification" do
     error_str = ruby_exe(fixture(__FILE__, 'debug_info.rb'), options: '--debug',  args: "2>&1")
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
diff --git a/spec/ruby/core/string/chilled_string_spec.rb b/spec/ruby/core/string/chilled_string_spec.rb
new file mode 100644
index 0000000000..8de4fc421b
--- /dev/null
+++ b/spec/ruby/core/string/chilled_string_spec.rb
@@ -0,0 +1,69 @@
+require_relative '../../spec_helper'
+
+describe "chilled String" do
+  guard -> { ruby_version_is "3.4" and !"test".equal?("test") } do
+    describe "#frozen?" do
+      it "returns true" do
+        "chilled".frozen?.should == true
+      end
+    end
+
+    describe "#-@" do
+      it "returns a different instance" do
+        input = "chilled"
+        interned = (-input)
+        interned.frozen?.should == true
+        interned.object_id.should_not == input.object_id
+      end
+    end
+
+    describe "#+@" do
+      it "returns a different instance" do
+        input = "chilled"
+        duped = (+input)
+        duped.frozen?.should == false
+        duped.object_id.should_not == input.object_id
+      end
+    end
+
+    describe "#clone" do
+      it "preserves chilled status" do
+        input = "chilled".clone
+        -> {
+          input << "-mutated"
+        }.should complain(/literal string will be frozen in the future/)
+        input.should == "chilled-mutated"
+      end
+    end
+
+    describe "mutation" do
+      it "emits a warning" do
+        input = "chilled"
+        -> {
+          input << "-mutated"
+        }.should complain(/literal string will be frozen in the future/)
+        input.should == "chilled-mutated"
+      end
+
+      it "emits a warning on singleton_class creaation" do
+        -> {
+          "chilled".singleton_class
+        }.should complain(/literal string will be frozen in the future/)
+      end
+
+      it "emits a warning on instance variable assignment" do
+        -> {
+          "chilled".instance_variable_set(:@ivar, 42)
+        }.should complain(/literal string will be frozen in the future/)
+      end
+
+      it "raises FrozenError after the string was explictly frozen" do
+        input = "chilled"
+        input.freeze
+        -> {
+          input << "mutated"
+        }.should raise_error(FrozenError)
+      end
+    end
+  end
+end
diff --git a/spec/ruby/language/string_spec.rb b/spec/ruby/language/string_spec.rb
index f2764eada0..1a1cd35850 100644
--- a/spec/ruby/language/string_spec.rb
+++ b/spec/ruby/language/string_spec.rb
@@ -232,8 +232,8 @@ describe "Ruby String literals" do
     end
 
     it "produce different objects for literals with the same content in different files if the other file doesn't have the comment" do
-      frozen_literals_by_default = eval("'test'").frozen?
-      ruby_exe(fixture(__FILE__, "freeze_magic_comment_across_files_no_comment.rb")).chomp.should == (!frozen_literals_by_default).to_s
+      frozen_string_literal = "test".frozen? && "test".equal?("test")
+      ruby_exe(fixture(__FILE__, "freeze_magic_comment_across_files_no_comment.rb")).chomp.should == (!frozen_string_literal).to_s
     end
 
     it "produce different objects for literals with the same content in different files if they have different encodings" do
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"