Use --init-directory in Emacs 29+ (#191)

* Use `--init-directory` in Emacs 29+

This will allow a few advantages:
- We can load `early-init.el` file properly, fixing Flash Of Unstyled
Contents (FOUC) issues and improving the performance slightly
- Eventually we can drop `default.el` file loading in Home-Manager
module. Not done here since this would complicate the code without
necessity, however this can be done once Emacs 29+ is default

* Fix build in Emacs <29

* Add #checks.<arch>.init-example-el-emacsGit

* Add .#checks.x86_64-linux.init-example-el-emacsGit to CI

* Fix review issues

* Move config files to share/emacs.d

* Separate tests in two steps

* Add TODOS

* Simplify emacs-dir generation

default.nix

@@ -1,5 +1,5 @@
 { # The files would be going to ~/.config/doom (~/.doom.d)
-doomPrivateDir
+  doomPrivateDir
 /* Extra packages to install
 
    Useful for non-emacs packages containing emacs bindings (e.g.
@@ -8,8 +8,7 @@ doomPrivateDir
    Example:
      extraPackages = epkgs: [ pkgs.mu ];
 */
-, extraPackages ? epkgs:
-  [ ]
+, extraPackages ? epkgs: [ ]
   /* Extra configuration to source during initialization
 
      Use this to refer other nix derivations.
@@ -36,8 +35,7 @@ doomPrivateDir
        });
      };
 */
-, emacsPackagesOverlay ? self: super:
-  { }
+, emacsPackagesOverlay ? self: super: { }
   /* Use bundled revision of github.com/nix-community/emacs-overlay
      as `emacsPackages`.
   */
@@ -54,7 +52,8 @@ doomPrivateDir
          "emacs-overlay" = fetchFromGitHub { owner = /* ...*\/; };
        };
   */
-, dependencyOverrides ? { }, lib, pkgs, stdenv, buildEnv, makeWrapper
+, dependencyOverrides ? { }
+, lib, pkgs, stdenv, buildEnv, makeWrapper
 , runCommand, fetchFromGitHub, substituteAll, writeShellScript
 , writeShellScriptBin, writeTextDir }:
 
@@ -63,6 +62,7 @@ assert (lib.assertMsg ((builtins.isPath doomPrivateDir)
   "doomPrivateDir must be either a path, a derivation or a stringified store path");
 
 let
+  isEmacs29 = lib.versionAtLeast emacsPackages.emacs.version "29";
   flake =
     (import
       (let lock = with builtins; fromJSON (readFile ./flake.lock); in
@@ -193,7 +193,7 @@ let
   };
 
   # Stage 4: `extraConfig` is merged into private configuration
-  doomDir = pkgs.runCommand "doom-private" {
+  doomDir = runCommand "doom-private" {
     inherit extraConfig;
     passAsFile = [ "extraConfig" ];
   } ''
@@ -202,23 +202,38 @@ let
     chmod u+w $out/config.el
     cat $extraConfigPath > $out/config.extra.el
     cat > $out/config.el << EOF
-    (load "${builtins.toString doomPrivateDir}/config.el")
+    (load "${doomPrivateDir}/config.el")
     (load "$out/config.extra.el")
     EOF
   '';
 
   # Stage 5: catch-all wrapper capable to run doom-emacs even
   # without installing ~/.emacs.d
+  # TODO: remove once Emacs 29+ is released and commonly available
   emacs = let
     load-config-from-site = writeTextDir "share/emacs/site-lisp/default.el" ''
       (message "doom-emacs is not placed in `doom-private-dir',
       loading from `site-lisp'")
-      (when (> emacs-major-version 26)
-            (load "${doom-emacs}/early-init.el"))
-      (load "${doom-emacs}/core/core-start.el")
+      ${lib.optionalString (!isEmacs29) ''
+        (load "${doom-emacs}/early-init.el")
+        (load "${doom-emacs}/core/core-start.el")
+      ''}
     '';
   in (emacsPackages.emacsWithPackages (epkgs: [ load-config-from-site ]));
 
+  # create a `emacs.d` dir to be loaded using `--init-directory` flag from Emacs 29+
+  # this will allow proper usage of `early-init.el`, fixing FOUC issues and improving
+  # startup performance
+  emacs-dir = runCommand "emacs-dir" { } ''
+    mkdir -p $out
+    cat > $out/early-init.el << EOF
+    (load "${doom-emacs}/early-init.el")
+    EOF
+    cat > $out/init.el << EOF
+    (load "${doom-emacs}/core/core-start.el")
+    EOF
+  '';
+
   build-summary = writeShellScript "build-summary" ''
     printf "\n${fmt.green}Successfully built nix-doom-emacs!${fmt.reset}\n"
     printf "${fmt.bold}  ==> doom-emacs is installed to ${doom-emacs}${fmt.reset}\n"
@@ -227,13 +242,23 @@ let
   '';
 in emacs.overrideAttrs (esuper:
   let
+    # `--init-directory` is supported by Emacs 29+ only
+    initDirArgs = lib.optionalString isEmacs29 ''
+      if [[ $(basename $1) == emacs ]] || [[ $(basename $1) == emacs-* ]]; then
+        wrapArgs+=(--add-flags '--init-directory ${emacs-dir}')
+      fi
+    '';
     cmd = ''
       wrapEmacs() {
-          wrapProgram $1 \
-                    --set DOOMDIR ${doomDir} \
-                    --set NIX_DOOM_EMACS_BINARY $1 \
-                    --set __DEBUG_doom_emacs_DIR ${doom-emacs} \
-                    --set __DEBUG_doomLocal_DIR ${doomLocal}
+          local -a wrapArgs=(
+              --set DOOMDIR ${doomDir}
+              --set NIX_DOOM_EMACS_BINARY $1
+              --set __DEBUG_doom_emacs_DIR ${doom-emacs}
+              --set __DEBUG_doomLocal_DIR ${doomLocal}
+          )
+          ${initDirArgs}
+
+          wrapProgram $1 "''${wrapArgs[@]}"
       }
 
       for prog in $out/bin/*; do