Freeze $/ and make it ractor safe

[Feature #21109]

By always freezing when setting the global rb_rs variable, we can ensure
it is not modified and can be accessed from a ractor.

We're also making sure it's an instance of String and does not have any
instance variables.

Of course, if $/ is changed at runtime, it may cause surprising behavior
but doing so is deprecated already anyway.

Co-authored-by: Jean Boussier <[email protected]>

Author: 2 people

command_line/dash_0_spec.rb

@@ -0,0 +1,13 @@
+require_relative '../spec_helper'
+
+describe "The -0 command line option" do
+  it "sets $/ and $-0" do
+    ruby_exe("puts $/, $-0", options: "-072").should == ":\n:\n"
+  end
+
+  ruby_version_is "3.5" do
+    it "sets $/ and $-0 as a frozen string" do
+      ruby_exe("puts $/.frozen?, $-0.frozen?", options: "-072").should == "true\ntrue\n"
+    end
+  end
+end

language/predefined_spec.rb

@@ -559,12 +559,39 @@ def foo
     $VERBOSE = @verbose
   end
 
-  it "can be assigned a String" do
-    str = "abc"
-    $/ = str
-    $/.should equal(str)
+  ruby_version_is ""..."3.5" do
+    it "can be assigned a String" do
+      str = +"abc"
+      $/ = str
+      $/.should equal(str)
+    end
   end
 
+  ruby_version_is "3.5" do
+    it "makes a new frozen String from the assigned String" do
+      string_subclass = Class.new(String)
+      str = string_subclass.new("abc")
+      str.instance_variable_set(:@ivar, 1)
+      $/ = str
+      $/.should.frozen?
+      $/.should be_an_instance_of(String)
+      $/.should_not.instance_variable_defined?(:@ivar)
+      $/.should == str
+    end
+
+    it "makes a new frozen String if it's not frozen" do
+      str = +"abc"
+      $/ = str
+      $/.should.frozen?
+      $/.should == str
+    end
+
+    it "assigns the given String if it's frozen and has no instance variables" do
+      str = "abc".freeze
+      $/ = str
+      $/.should equal(str)
+    end
+  end
   it "can be assigned nil" do
     $/ = nil
     $/.should be_nil
@@ -608,10 +635,38 @@ def foo
     $VERBOSE = @verbose
   end
 
-  it "can be assigned a String" do
-    str = "abc"
-    $-0 = str
-    $-0.should equal(str)
+  ruby_version_is ""..."3.5" do
+    it "can be assigned a String" do
+      str = +"abc"
+      $-0 = str
+      $-0.should equal(str)
+    end
+  end
+
+  ruby_version_is "3.5" do
+    it "makes a new frozen String from the assigned String" do
+      string_subclass = Class.new(String)
+      str = string_subclass.new("abc")
+      str.instance_variable_set(:@ivar, 1)
+      $-0 = str
+      $-0.should.frozen?
+      $-0.should be_an_instance_of(String)
+      $-0.should_not.instance_variable_defined?(:@ivar)
+      $-0.should == str
+    end
+
+    it "makes a new frozen String if it's not frozen" do
+      str = +"abc"
+      $-0 = str
+      $-0.should.frozen?
+      $-0.should == str
+    end
+
+    it "assigns the given String if it's frozen and has no instance variables" do
+      str = "abc".freeze
+      $-0 = str
+      $-0.should equal(str)
+    end
   end
 
   it "can be assigned nil" do