New option to specify additional configuration

This enables emacs configuration to refer nix store paths.

default.nix

@@ -9,6 +9,16 @@
        extraPackages = epkgs: [ pkgs.mu ];
   */
 , extraPackages ? epkgs: []
+  /* Extra configuration to source during initialization
+
+     Use this to refer other nix derivations.
+
+     Example:
+       extraConfig = ''
+         (setq mu4e-mu-binary = "${pkgs.mu}/bin/mu")
+       '';
+  */
+, extraConfig ? ""
   /* Package set to install emacs and dependent packages from
 
      Only used to get emacs package, if `bundledPackages` is set.
@@ -70,7 +80,7 @@ let
     phases = ["unpackPhase" "patchPhase" "installPhase"];
     patches = [
       (substituteAll {
-        src = ./fix-paths-pre.patch;
+        src = ./fix-paths.patch;
         private = builtins.toString doomPrivateDir;
       })
     ];
@@ -141,7 +151,7 @@ let
     src = doomSrc;
     patches = [
       (substituteAll {
-        src = ./fix-paths.patch;
+        src = ./nix-integration.patch;
         local = doomLocal;
       })
     ];
@@ -154,14 +164,23 @@ let
 
   # Stage 4: catch-all wrapper capable to run doom-emacs even
   # without installing ~/.emacs.d
-  emacs = (emacsPackages.emacsWithPackages (epkgs: [
-    (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}/init.el")
-      '')
+  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}/init.el")
+    '';
+
+    load-extra-config = writeTextDir "share/emacs/site-lisp/nix-integration.el" ''
+      ;;; -*- lexical-binding: t; -*-
+
+      ${extraConfig}
+    '';
+  in (emacsPackages.emacsWithPackages (epkgs: [
+    load-extra-config
+    load-config-from-site
   ])).overrideAttrs (super: {
     outputs = [ "out" "emacsd" ];
     buildInputs = [ doom-emacs ];

fix-paths-pre.patch

@@ -1,13 +0,0 @@
-diff --git a/core/autoload/config.el b/core/autoload/config.el
-index 6bb9ae300..81ab62860 100644
---- a/core/autoload/config.el
-+++ b/core/autoload/config.el
-@@ -20,7 +20,7 @@
- (defun doom/find-file-in-private-config ()
-   "Search for a file in `doom-private-dir' inside nixos-config."
-   (interactive)
--  (doom-project-find-file doom-private-dir))
-+  (doom-project-find-file "@private@"))
- 
- ;;;###autoload
- (defun doom/reload ()

fix-paths.patch

@@ -1,40 +1,13 @@
-diff --git a/core/core.el b/core/core.el
-index cb12f8d08..03f370691 100644
---- a/core/core.el
-+++ b/core/core.el
-@@ -63,22 +63,21 @@ decrease this. If you experience stuttering, increase this.")
-   "The root directory for Doom's modules. Must end with a slash.")
+diff --git a/core/autoload/config.el b/core/autoload/config.el
+index 6bb9ae300..81ab62860 100644
+--- a/core/autoload/config.el
++++ b/core/autoload/config.el
+@@ -20,7 +20,7 @@
+ (defun doom/find-file-in-private-config ()
+   "Search for a file in `doom-private-dir' inside nixos-config."
+   (interactive)
+-  (doom-project-find-file doom-private-dir))
++  (doom-project-find-file "@private@"))
  
- (defconst doom-local-dir
--  (if-let (localdir (getenv "DOOMLOCALDIR"))
--      (expand-file-name (file-name-as-directory localdir))
--    (concat doom-emacs-dir ".local/"))
-+  (or (getenv "DOOMLOCALDIR")
-+      "@local@/")
-   "Root directory for local storage.
- 
- Use this as a storage location for this system's installation of Doom Emacs.
- These files should not be shared across systems. By default, it is used by
- `doom-etc-dir' and `doom-cache-dir'. Must end with a slash.")
- 
--(defconst doom-etc-dir (concat doom-local-dir "etc/")
-+(defconst doom-etc-dir "~/.local/doom"
-   "Directory for non-volatile local storage.
- 
- Use this for files that don't change much, like server binaries, external
- dependencies or long-term shared data. Must end with a slash.")
- 
--(defconst doom-cache-dir (concat doom-local-dir "cache/")
-+(defconst doom-cache-dir "~/.cache/doom/"
-   "Directory for volatile local storage.
- 
- Use this for files that change often, like cache files. Must end with a slash.")
-@@ -196,7 +195,7 @@ users).")
- (setq abbrev-file-name             (concat doom-local-dir "abbrev.el")
-       async-byte-compile-log-file  (concat doom-etc-dir "async-bytecomp.log")
-       bookmark-default-file        (concat doom-etc-dir "bookmarks")
--      custom-file                  (concat doom-private-dir "init.el")
-+      custom-file                  (concat doom-etc-dir "init.el")
-       custom-theme-directory       (concat doom-private-dir "themes/")
-       desktop-dirname              (concat doom-etc-dir "desktop")
-       desktop-base-file-name       "autosave"
+ ;;;###autoload
+ (defun doom/reload ()

nix-integration.patch

@@ -0,0 +1,48 @@
+diff --git a/core/core.el b/core/core.el
+index cb12f8d08..03f370691 100644
+--- a/core/core.el
++++ b/core/core.el
+@@ -63,22 +63,21 @@ decrease this. If you experience stuttering, increase this.")
+   "The root directory for Doom's modules. Must end with a slash.")
+ 
+ (defconst doom-local-dir
+-  (if-let (localdir (getenv "DOOMLOCALDIR"))
+-      (expand-file-name (file-name-as-directory localdir))
+-    (concat doom-emacs-dir ".local/"))
++  (or (getenv "DOOMLOCALDIR")
++      "@local@/")
+   "Root directory for local storage.
+ 
+ Use this as a storage location for this system's installation of Doom Emacs.
+ These files should not be shared across systems. By default, it is used by
+ `doom-etc-dir' and `doom-cache-dir'. Must end with a slash.")
+ 
+-(defconst doom-etc-dir (concat doom-local-dir "etc/")
++(defconst doom-etc-dir "~/.local/doom"
+   "Directory for non-volatile local storage.
+ 
+ Use this for files that don't change much, like server binaries, external
+ dependencies or long-term shared data. Must end with a slash.")
+ 
+-(defconst doom-cache-dir (concat doom-local-dir "cache/")
++(defconst doom-cache-dir "~/.cache/doom/"
+   "Directory for volatile local storage.
+ 
+ Use this for files that change often, like cache files. Must end with a slash.")
+@@ -196,7 +195,7 @@ users).")
+ (setq abbrev-file-name             (concat doom-local-dir "abbrev.el")
+       async-byte-compile-log-file  (concat doom-etc-dir "async-bytecomp.log")
+       bookmark-default-file        (concat doom-etc-dir "bookmarks")
+-      custom-file                  (concat doom-private-dir "init.el")
++      custom-file                  (concat doom-etc-dir "init.el")
+       custom-theme-directory       (concat doom-private-dir "themes/")
+       desktop-dirname              (concat doom-etc-dir "desktop")
+       desktop-base-file-name       "autosave"
+@@ -489,6 +488,7 @@ to least)."
+                    (daemonp))
+                (file-exists-p doom-env-file))
+       (doom-load-envvars-file doom-env-file))
++    (load "nix-integration.el" t)
+ 
+     (require 'core-modules)
+     (let (;; `doom-autoload-file' tells Emacs where to load all its functions