[nixos] Add nixos-format-on-save (#14982)

* [nixos] Add nixos-format-on-save

layers/+os/nixos/README.org

@@ -4,11 +4,12 @@
 
 [[file:img/nixos.png]]
 
-* Table of Contents                     :TOC_5_gh:noexport:
+* Table of Contents                                       :TOC_5_gh:noexport:
 - [[#description][Description]]
   - [[#features][Features:]]
 - [[#install][Install]]
   - [[#layer][Layer]]
+  - [[#format-on-save][Format on save]]
 - [[#configuration][Configuration]]
 - [[#key-bindings][Key bindings]]
   - [[#nixos-options][NixOS Options]]
@@ -18,6 +19,7 @@ This layer adds tools for better integration of Emacs in NixOS.
 
 ** Features:
 - Nix-mode using [[https://github.com/NixOS/nix-mode][nix-mode]]
+- Automatic formatting via [[https://github.com/serokell/nixfmt][nixfmt]]
 - Auto-completion of NixOS Options using [[https://github.com/travisbhartwell/nix-emacs/blob/master/company-nixos-options.el][company-nixos-options]]
 - Helm Lookup for NixOS Options [[https://github.com/travisbhartwell/nix-emacs/blob/master/helm-nixos-options.el][helm-nixos-options]]
 
@@ -27,6 +29,14 @@ To use this configuration layer, add it to your =~/.spacemacs=. You will need to
 add =nixos= to the existing =dotspacemacs-configuration-layers= list in this
 file.
 
+** Format on save
+To enable automatic formatting on save, set the layer variable
+~nixos-format-on-save~ to ~t~:
+
+#+begin_src elisp
+  (nixos :variables nixos-format-on-save t)
+#+end_src
+
 * Configuration
 On some systems, =company-nixos-options= may be very slow. If this is the case,
 see the section on disabling the [[https://github.com/syl20bnr/spacemacs/blob/develop/doc/DOCUMENTATION.org#disabling-layer-services-in-other-layers][disabling auto-complete]] for the =nixos= layer.
@@ -34,6 +44,7 @@ see the section on disabling the [[https://github.com/syl20bnr/spacemacs/blob/de
 * Key bindings
 ** NixOS Options
 
-| Key binding | Description             |
-|-------------+-------------------------|
-| ~SPC h >~   | Call helm-nixos-options |
+| Key binding | Description                |
+|-------------+----------------------------|
+| ~SPC h >~   | Call helm-nixos-options    |
+| ~SPC m = =~ | Format buffer using nixfmt |

layers/+os/nixos/config.el

@@ -0,0 +1,27 @@
+;;; config.el --- nixos Layer Configuration File for Spacemacs
+;;
+;; Copyright (c) 2012-2021 Sylvain Benner & Contributors
+;;
+;; Author: Sylvain Benner <sylvain.benner@gmail.com>
+;; URL: https://github.com/syl20bnr/spacemacs
+;;
+;; This file is not part of GNU Emacs.
+;;
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+;;
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+;;
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+
+;; variables
+
+(defvar nixos-format-on-save nil
+  "If non-nil, nixfmt before saving.")

layers/+os/nixos/packages.el

@@ -56,7 +56,9 @@
     (progn
       (add-to-list 'spacemacs-indent-sensitive-modes 'nix-mode)
       (spacemacs/set-leader-keys-for-major-mode 'nix-mode
-        "==" 'nix-format-buffer))
+        "==" 'nix-format-buffer)
+      (when nixos-format-on-save
+        (add-hook 'before-save-hook 'nix-format-before-save)))
     :config
     (electric-indent-mode -1)))