[ractor] shareable_constant_value: literal makes constants shareable between ractors

language/fixtures/shareable_constant_value_magic_comment.rb

@@ -0,0 +1,9 @@
+# shareable_constant_value: literal
+
+A = "rac"
+H = "tor"
+
+Ractor.new do
+  print A
+  print H
+end.take

language/ractor_spec.rb

@@ -0,0 +1,42 @@
+require_relative "../spec_helper"
+
+describe "magic comments in ractors" do
+  ruby_version_is "3.0" do
+    it "makes constants shareable between ractors" do
+      out = ruby_exe(fixture(__FILE__, "shareable_constant_value_magic_comment.rb"), options: "-W0")
+      out.should == "ractor"
+    end
+
+    it "makes constants shareable between ractors" do
+      a, b, c = Class.new.class_eval(<<~RUBY, __FILE__, __LINE__ + 1)
+        # shareable_constant_value: experimental_everything
+        A = [[1]]
+        # shareable_constant_value: none
+        B = [[2]]
+        # shareable_constant_value: literal
+        C = [["shareable", "constant#{self.class.name}"]]
+        [A, B, C]
+      RUBY
+      Ractor.shareable?(a).should == true
+      Ractor.shareable?(b).should == false
+      Ractor.shareable?(c).should == true
+    end
+
+    context "when a shared constant is not a literal" do
+      it "raises an error when is computed" do
+        -> { Class.new.class_eval(<<~RUBY, __FILE__, __LINE__ + 1) }.should raise_error(Ractor::IsolationError, "cannot assign unshareable object to C")
+          # shareable_constant_value: literal
+          C = ["Not " + "shareable"]
+        RUBY
+      end
+
+      it "raises an error when referencing a non-frozen variable" do
+        -> { Class.new.class_eval(<<~RUBY, __FILE__, __LINE__ + 1) }.should raise_error(Ractor::IsolationError, "cannot assign unshareable object to C")
+          # shareable_constant_value: literal
+          value = "value"
+          C = value
+        RUBY
+      end
+    end
+  end
+end