Add "clang-format-on-save-mode" minor mode to clang-format.el

Add a minor mode to run clang-format on save.

Author: ideasman42

clang/tools/clang-format/clang-format.el

@@ -70,6 +70,20 @@ in such buffers."
   :safe #'stringp)
 (make-variable-buffer-local 'clang-format-fallback-style)
 
+(defcustom clang-format-on-save-p 'clang-format-on-save-check-config-exists
+  "Only reformat on save if this function returns non-nil.
+
+You may wish to choose one of the following options:
+- `always': To always format on save.
+- `clang-format-on-save-check-config-exists':
+  Only reformat when \".clang-format\" exists.
+
+Otherwise you can set this to a user defined function."
+  :group 'clang-format
+  :type 'function
+  :risky t)
+(make-variable-buffer-local 'clang-format-on-save-p)
+
 (defun clang-format--extract (xml-node)
   "Extract replacements and cursor information from XML-NODE."
   (unless (and (listp xml-node) (eq (xml-node-name xml-node) 'replacements))
@@ -217,5 +231,50 @@ the function `buffer-file-name'."
 ;;;###autoload
 (defalias 'clang-format 'clang-format-region)
 
+;; Format on save minor mode.
+;;
+;; Optional minor mode for formatting on save.
+
+(defun clang-format--on-save-buffer-hook ()
+  "The hook to run on buffer saving to format the buffer."
+  ;; Demote errors as this is user configurable, we can't be sure it wont error.
+  (when (with-demoted-errors "clang-format-on-save: Error %S"
+          (funcall clang-format-on-save-p))
+    (clang-format-buffer))
+  ;; Continue to save.
+  nil)
+
+(defun clang-format--on-save-enable ()
+  "Disable the minor mode."
+  (add-hook 'before-save-hook #'clang-format--on-save-buffer-hook nil t))
+
+(defun clang-format--on-save-disable ()
+  "Enable the minor mode."
+  (remove-hook 'before-save-hook #'clang-format--on-save-buffer-hook t))
+
+;; Default value for `clang-format-on-save-p'.
+(defun clang-format-on-save-check-config-exists ()
+  "Return non-nil when `.clang-format' is found in a parent directory."
+  ;; Unlikely but possible this is nil.
+  (let ((filepath buffer-file-name))
+    (cond
+     (filepath
+      (null (null (locate-dominating-file (file-name-directory filepath) ".clang-format"))))
+     (t
+      nil))))
+
+;;;###autoload
+(define-minor-mode clang-format-on-save-mode
+  "Clang-format on save minor mode."
+  :global nil
+  :lighter ""
+  :keymap nil
+
+  (cond
+   (clang-format-on-save-mode
+    (clang-format--on-save-enable))
+   (t
+    (clang-format--on-save-disable))))
+
 (provide 'clang-format)
 ;;; clang-format.el ends here