[Feature #20884] Reserve "Ruby" toplevel name

defs/id.def

@@ -101,6 +101,8 @@ firstline, predefined = __LINE__+1, %[\
   $_                                                    LASTLINE
   $~                                                    BACKREF
   $!                                                    ERROR_INFO
+
+  Ruby
 ]
 
 # VM ID         OP      Parser Token

test/ruby/test_rubyoptions.rb

@@ -1326,4 +1326,19 @@ def test_free_at_exit_env_var
     assert_ruby_status([env, "-e;"])
     assert_in_out_err([env, "-W"], "", [], /Free at exit is experimental and may be unstable/)
   end
+
+  def test_toplevel_ruby
+    reserved = ["", [], /::Ruby is reserved/]
+    env = {"RUBYOPT"=>""}
+    args = %w[-e Ruby=1]
+    assert_in_out_err([env, *args])
+    assert_in_out_err([env, "-w", *args], *reserved)
+    assert_in_out_err([env, "-W:deprecated", *args], *reserved)
+    assert_in_out_err([env, "-w", "-W:no-deprecated", *args])
+
+    args = ["-e", "class A; Ruby=1; end"]
+    assert_in_out_err([env, *args])
+    assert_in_out_err([env, "-w", *args])
+    assert_in_out_err([env, "-W:deprecated", *args])
+  end
 end

variable.c

@@ -3626,6 +3626,9 @@ const_set(VALUE klass, ID id, VALUE val)
             }
         }
     }
+    if (klass == rb_cObject && id == idRuby) {
+        rb_warn_reserved_name_at(3.5, "::Ruby");
+    }
 }
 
 void