The bundle-config manpage says:
This command allows you to interact with the system switch configuration. The Bundler retrieves the configuration from the local application (app / .bundle / config), environment variables, and user home (~ / .bundle / config) in this priority order. Therefore, make sure that you do not have configuration files that take precedence over the one you want to use.
You can configure this file yourself or set parameters using bundle config (option) , running bundle config without any parameters will print the current configuration. For example, you can set compile time options for them: mysql gem:
bundle config build.mysql --with-mysql-config=/usr/local/mysql/bin/mysql_config
So yes, app/.bundle/config used to set package options for the current project.
Edit: This change was added in commit efa85055 to the rails github repository. You can view this version here and commit here .
The commit message comes from José Valim and mentions the line you have a question for:
Make bin / rails a call to rails / commands / application, fix the use of generators and update .gitignores.
Edit again: This is a quote from bundler about why you should not check the .bundle directory in any VCS.
Do not check the .bundle directory or any of the files inside it. These files are specific to each machine and are used to save installation options between starts the installation of the package command.