Let python/ruby modules handle version strings

Also, set RBENV_VERSION and PYENV_VERSION envvars.

modules/lang/ruby/config.el

@@ -1,5 +1,16 @@
 ;;; lang/ruby/config.el -*- lexical-binding: t; -*-
 
+(defvar +ruby-rbenv-versions nil
+  "Available versions of ruby in rbenv.")
+
+(defvar-local +ruby-current-version nil
+  "The currently active ruby version.")
+
+
+;;
+;; Plugins
+;;
+
 (def-package! ruby-mode
   :mode ("\\.rb$" "\\.rake$" "\\.gemspec$" "\\.?pryrc$"
          "/\\(Gem\\|Cap\\|Vagrant\\|Rake\\|Pod\\|Puppet\\|Berks\\)file$")
@@ -13,6 +24,29 @@
   ;; Don't interfere with my custom RET behavior
   (define-key ruby-mode-map [?\n] nil)
 
+  ;; Version management with rbenv
+  (defun +ruby|add-version-to-modeline ()
+    "Add version string to the major mode in the modeline."
+    (setq mode-name
+          (if +python-current-version
+              (format "Ruby %s" +ruby-current-version)
+            "Ruby")))
+  (add-hook 'ruby-mode-hook #'+ruby|add-version-to-modeline)
+
+  (if (not (executable-find "rbenv"))
+      (setq +ruby-current-version (string-trim (shell-command-to-string "ruby --version 2>&1 | cut -d' ' -f2")))
+    (setq +ruby-rbenv-versions (split-string (shell-command-to-string "rbenv versions --bare") "\n" t))
+
+    (defun +ruby|detect-rbenv-version ()
+      "Detect the rbenv version for the current project and set the relevant
+environment variables."
+      (when-let (version-str (shell-command-to-string "ruby --version 2>&1 | cut -d' ' -f2"))
+        (setq version-str (string-trim version-str)
+              +ruby-current-version version-str)
+        (when (member version-str +ruby-rbenv-versions)
+          (setenv "RBENV_VERSION" version-str))))
+    (add-hook 'ruby-mode-hook #'+ruby|detect-rbenv-version))
+
   (map! :map ruby-mode-map
         :localleader
         :prefix "r"