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

[ideasman42]

Add an minor mode which can be optionally used to run clang-format on save.

Formatting before saving works well and is convenient to avoid having to remember to manually run clang format.

I've written this as it's own package but it's probably better if the functionality is supported by clang-format.el.
See: melpa/melpa#8762

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[llvmbot]

@llvm/pr-subscribers-clang-format

Author: Campbell Barton (ideasman42)

Changes

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

Formatting before saving works well and is convenient to avoid having to remember to manually run clang format.

I've written this as it's own package but it's probably better if the functionality is supported by clang-format.el.
See: melpa/melpa#8762

---

Full diff: https://github.com/llvm/llvm-project/pull/104533.diff

1 Files Affected:

• (modified) clang/tools/clang-format/clang-format.el (+59)

diff --git a/clang/tools/clang-format/clang-format.el b/clang/tools/clang-format/clang-format.el
index f43bf063c62970..25a5865efdebd2 100644
--- a/clang/tools/clang-format/clang-format.el
+++ b/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

[mydeveloperday]

I'm not an emacs user and I'm unsure if the other maintainers are. If you are happy to address any issues that might arise I'd be happy for this to land.

[ideasman42]

I'm not an emacs user and I'm unsure if the other maintainers are. If you are happy to address any issues that might arise I'd be happy for this to land.

Yes, I'm happy to resolve any issues - for some context, I've been using a version of this for some years.
Originally from this answer https://emacs.stackexchange.com/q/48500/2418

I also maintain an auto-formatting package for autopep8: https://codeberg.org/ideasman42/emacs-py-autopep8

The minor mode just makes it convenient to enable/disable - I find it especially when using C/C++ on some projects that don't use clang-format to check for .clang-format and only format-on-save in this case (that behavior can be configured too).

[ideasman42]

Is there any action needed on my side? (the PR was approved over a week ago).

[ideasman42]

@mydeveloperday bump, it'd be nice to get this merged if there are no problems with the PR.

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