doomemacs/early-init.el

;;; early-init.el --- Doom's universal bootstrapper -*- lexical-binding: t -*-
;;; Commentary:
;;
;; early-init.el was introduced in Emacs 27.1 and is loaded before init.el, and
;; before Emacs initializes its UI or package.el, and before site files are
;; loaded. This is good place for startup optimizating, because only here can
;; you *prevent* things from loading, rather than turn them off after-the-fact.
;; As such, Doom does all its initializing here.
;;
;; This file is Doom's "universal bootstrapper" for both interactive and
;; non-interactive sessions. It's also the heart of its profile bootloader,
;; which allows you to switch between Emacs configs on demand using
;; `--init-directory DIR' (which was backported from Emacs 29) or `--profile
;; NAME` (more about profiles at `https://docs.doomemacs.org/-/developers' or
;; docs/developers.org).
;;
;; In summary, this file is responsible for:
;; - Setting up some universal startup optimizations.
;; - Determining where `user-emacs-directory' is from one of:
;;   - `--init-directory DIR' (backported from 29)
;;   - `--profile PROFILENAME'
;; - Do one of the following:
;;   - Load `doom' and one of `doom-start' or `doom-cli'.
;;   - Or (if the user is trying to load a non-Doom config) load
;;     `user-emacs-directory'/early-init.el.
;;
;;; Code:

;; PERF: Garbage collection is a big contributor to startup times. This fends it
;;   off, but will be reset later by `gcmh-mode'. Not resetting it later will
;;   cause stuttering/freezes.
(setq gc-cons-threshold most-positive-fixnum)

;; PERF: Don't use precious startup time checking mtime on elisp bytecode.
;;   Ensuring correctness is 'doom sync's job, not the interactive session's.
;;   Still, stale byte-code will cause *heavy* losses in startup efficiency.
(setq load-prefer-newer noninteractive)


;;
;;; Bootstrap

(or
 ;; PERF: `file-name-handler-alist' is consulted often. Unsetting it offers a
 ;;   notable saving in startup time.
 (let (file-name-handler-alist)
   ;; FEAT: First, we process --init-directory and --profile to detect what
   ;;   `user-emacs-directory' to load from. I avoid using
   ;;   `command-switch-alist' to process --profile and --init-directory because
   ;;   it is processed too late to change `user-emacs-directory' in time.

   ;; REVIEW: Backported from Emacs 29. Remove when 28 support is dropped.
   (let ((initdir (or (cadr (member "--init-directory" command-line-args))
                      (getenv-internal "EMACSDIR"))))
     (when initdir
       ;; FIX: Discard the switch to prevent "invalid option" errors later.
       (push (cons "--init-directory" (lambda (_) (pop argv))) command-switch-alist)
       (setq user-emacs-directory (expand-file-name initdir))))
   ;; Initialize a known profile, if requested.
   (let ((profile (or (cadr (member "--profile" command-line-args))
                      (getenv-internal "DOOMPROFILE"))))
     (when profile
       ;; FIX: Discard the switch to prevent "invalid option" errors later.
       (push (cons "--profile" (lambda (_) (pop argv))) command-switch-alist)
       ;; While processing the requested profile, Doom loosely expects
       ;; `user-emacs-directory' to be changed. If it doesn't, then you're using
       ;; profiles.el as a glorified, runtime dir-locals.el (which is fine, if
       ;; intended).
       (catch 'found
         (let ((profiles-file (expand-file-name "profiles.el" user-emacs-directory)))
           (when (file-exists-p profiles-file)
             (with-temp-buffer
               (let ((coding-system-for-read 'utf-8-auto))
                 (insert-file-contents profiles-file))
               (condition-case-unless-debug e
                   (let ((profile-data (cdr (assq (intern profile) (read (current-buffer))))))
                     (dolist (var profile-data (if profile-data (throw 'found t)))
                       (if (eq (car var) 'env)
                           (dolist (env (cdr var)) (setenv (car env) (cdr env)))
                         (set (car var) (cdr var)))))
                 (error (error "Failed to parse profiles.el: %s" (error-message-string e))))))
           ;; If the requested profile isn't in profiles.el, then see if
           ;; $EMACSDIR/profiles/$DOOMPROFILE exists. These are implicit
           ;; profiles, where `emacs --profile foo` will be equivalent to `emacs
           ;; --init-directory $EMACSDIR/profile/foo', if that directory exists.
           (let ((profile-dir
                  (expand-file-name
                   profile (or (getenv-internal "DOOMPROFILESDIR")
                               (expand-file-name "profiles/" user-emacs-directory)))))
             (when (file-directory-p profile-dir)
               (setq user-emacs-directory profile-dir)
               (throw 'found t)))
           (user-error "No %S profile found" profile)))
       (when init-file-debug
         (message "Selected profile: %s" profile))
       ;; Ensure the selected profile persists through the session
       (setenv "DOOMPROFILE" profile)))

   ;; PERF: When `load'ing or `require'ing files, each permutation of
   ;;   `load-suffixes' and `load-file-rep-suffixes' (then `load-suffixes' +
   ;;   `load-file-rep-suffixes') is used to locate the file.  Each permutation
   ;;   is a file op, which is normally very fast, but they can add up over the
   ;;   hundreds/thousands of files Emacs needs to load.
   ;;
   ;;   To reduce that burden -- and since Doom doesn't load any dynamic modules
   ;;   -- I remove `.so' from `load-suffixes' and pass the `must-suffix' arg to
   ;;   `load'. See the docs of `load' for details.
   (or (let ((load-suffixes '(".elc" ".el")))
         ;; Load the heart of Doom Emacs.
         (if (load (expand-file-name "lisp/doom" user-emacs-directory)
                   'noerror 'nomessage nil 'must-suffix)
             ;; ...and prepare for the rest of the session.
             (if noninteractive
                 (doom-require 'doom-cli)
               ;; HACK: This advice hijacks Emacs' initfile resolver to replace
               ;;   $EMACSDIR/init.el (and ~/.emacs or ~/_emacs) with a
               ;;   a Doom-provided init file. Later, this file will be
               ;;   generated by 'doom sync' for the active Doom profile;
               ;;   `doom-start' is its stand-in until that's implemented.
               ;;
               ;;   This effort spares Emacs the overhead of searching for
               ;;   initfiles we don't care about, enables savvier hackers to
               ;;   use $EMACSDIR as their $DOOMDIR, and gives us an opportunity
               ;;   to fall back to a "safe mode", so we can present a more
               ;;   user-friendly failure state.
               (define-advice startup--load-user-init-file (:filter-args (args) init-doom)
                 "Initialize Doom Emacs in an interactive session."
                 (list (lambda ()
                         (file-name-concat doom-core-dir "doom-start"))
                       (lambda ()
                         (file-name-concat doom-profiles-dir "safe-mode" "init.el"))
                       (caddr args))))))
       ;; Failing that, assume we're loading a non-Doom config and prepare.
       (ignore
        (setq early-init-file (expand-file-name "early-init" user-emacs-directory)
              ;; I make no assumptions about the config we're about to load, so
              ;; to limit side-effects, undo any leftover optimizations:
              load-prefer-newer t))))

 ;; Then continue on to the config/profile we want to load.
 (load early-init-file 'noerror 'nomessage nil 'must-suffix))

;;; early-init.el ends here