deprecate gem build -C flag

Author: gustavothecoder

lib/rubygems/commands/build_command.rb

@@ -26,6 +26,9 @@ def initialize
     add_option "-C PATH", "Run as if gem build was started in <PATH> instead of the current working directory." do |value, options|
       options[:build_path] = value
     end
+    deprecate_option "-C",
+                     version: "4.0",
+                     extra_msg: "-C is a global flag now. Use `gem -C PATH build GEMSPEC_FILE [options]` instead"
   end
 
   def arguments # :nodoc:

test/rubygems/test_gem_commands_build_command.rb

@@ -41,6 +41,16 @@ def test_handle_options
     assert_includes Gem.platforms, Gem::Platform.local
   end
 
+  def test_handle_deprecated_options
+    use_ui @ui do
+      @cmd.handle_options %w[-C ./test/dir]
+    end
+
+    assert_equal "WARNING:  The \"-C\" option has been deprecated and will be removed in Rubygems 4.0. " \
+                 "-C is a global flag now. Use `gem -C PATH build GEMSPEC_FILE [options]` instead\n",
+                 @ui.error
+  end
+
   def test_options_filename
     gemspec_file = File.join(@tempdir, @gem.spec_name)