Add Emacs version check to noninteractive session

Changing Emacs versions can cause incompatibilities. Doom now warns the
user about these.

core/core-dispatcher.el

@@ -1,5 +1,32 @@
 ;;; -*- lexical-binding: t; no-byte-compile: t; -*-
 
+;; Do an Emacs version check and warn the user if it has changed.
+(defvar doom--last-emacs-file (concat doom-local-dir "emacs-version.el"))
+(defvar doom--last-emacs-version nil)
+
+(defun doom-refresh-emacs-version ()
+  (with-temp-file doom--last-emacs-file
+    (princ `(setq doom--last-emacs-version ,(prin1-to-string emacs-version))
+           (current-buffer))))
+
+(defun doom-same-emacs-version-p ()
+  (if (or doom--last-emacs-version
+          (load doom--last-emacs-file t t t))
+      (equal emacs-version doom--last-emacs-version)
+    (setq doom--last-emacs-version emacs-version)
+    (doom-refresh-emacs-version)
+    t))
+
+(unless (doom-same-emacs-version-p)
+  (unless (y-or-n-p
+           (format (concat "Your version of Emacs has changed from %s to %s, which may cause incompatibility\n"
+                           "issues. Please run `bin/doom compile :plugins` afterwards to resolve any problems.\n\n"
+                           "Continue?")
+                   doom--last-emacs-version
+                   emacs-version))
+    (error "Aborting"))
+  (doom-refresh-emacs-version))
+
 ;; Eagerly load these libraries because this module may be loaded in a session
 ;; that hasn't been fully initialized (where autoloads files haven't been
 ;; generated or `load-path' populated).