Implemented shareable_constant_value

It does shallow freeze only for now.

parse.y

@@ -10925,19 +10925,33 @@ mark_lvar_used(struct parser_params *p, NODE *rhs)
     }
 }
 
+static NODE *
+shareable_constant_value(struct parser_params *p, NODE *value, const YYLTYPE *loc)
+{
+    if (p->ctxt.shareable_constant_value) {
+	NODE *ractor = NEW_COLON3(rb_intern("Ractor"), loc);
+	value = NEW_CALL(ractor, rb_intern("make_shareable"),
+			NEW_LIST(value, loc), loc);
+    }
+    return value;
+}
+
 static NODE *
 node_assign(struct parser_params *p, NODE *lhs, NODE *rhs, const YYLTYPE *loc)
 {
     if (!lhs) return 0;
 
     switch (nd_type(lhs)) {
+      case NODE_CDECL:
+	rhs = shareable_constant_value(p, rhs, loc);
+	/* fallthru */
+
       case NODE_GASGN:
       case NODE_IASGN:
       case NODE_LASGN:
       case NODE_DASGN:
       case NODE_DASGN_CURR:
       case NODE_MASGN:
-      case NODE_CDECL:
       case NODE_CVASGN:
 	lhs->nd_value = rhs;
 	nd_set_loc(lhs, loc);

ractor.rb

@@ -51,6 +51,10 @@ class Ractor
     }
   end
 
+  def self.make_shareable(obj)
+    obj.freeze
+  end
+
   # Multiplex multiple Ractor communications.
   #
   #   r, obj = Ractor.select(r1, r2)

test/ruby/test_parse.rb

@@ -1178,6 +1178,23 @@ x = __ENCODING__
     assert_warning(/invalid value/) do
       assert_valid_syntax("# shareable_constant_value: invalid-option", verbose: true)
     end
+    a = Class.new.class_eval("#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      # shareable_constant_value: true
+      A = []
+    end;
+    assert_predicate(a, :frozen?)
+    a, b = Class.new.class_eval("#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      # shareable_constant_value: false
+      class X # shareable_constant_value: true
+        A = []
+      end
+      B = []
+      [X::A, B]
+    end;
+    assert_predicate(a, :frozen?)
+    assert_not_predicate(b, :frozen?)
   end
 
 =begin