Using Spacemacs with several Emacs versions

Since Spacemacs officially supports at least two major versions of Emacs, as a collaborator I have to switch between Emacs versions from time to time. Since installed packages don't usually work between different major versions of Emacs, I came up with a hack that allows me to seamlessly switch Emacs versions.

#2Change Log

• [2016-10-11]: Thanks to bmag, Spacemacs now has an option to set the packages directory based on Emacs version. Simply set dotspacemacs-elpa-subdirectory to emacs-version and enjoy seamless version switching. This option is available starting with Spacemacs v0.200.

Usually when I need to switch Emacs versions, it means I have to completely nuke the ~/.emacs.d/elpa directory and wait several minutes to install 300+ packages. This isn't critical, but there's room for improvement. Thanks to bmag, it's now possible to easily change the directory where all packages are installed (without any dirty hacks). I use separate directories for each Emacs version, so packages are only downloaded the first time I use a specific version. On subsequent switches, everything is ready to use.

;; setup package-user-dir to allow seamless switching between Emacs versions
(setq package-user-dir (file-name-as-directory
                        (concat user-emacs-directory "elpa/" emacs-version)))

Add this expression to the dotspacemacs/user-init function in your .spacemacs to make switching between versions easier.