bug#56679: 28.1; [PATCH] whitespace-style cannot be configured for diff-mode via hook

[YE via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

[-- Attachment #1: Type: text/plain, Size: 650 bytes --]

>> Or maybe adding a defcustom 'diff-whitespace-style' would be a proper
>> solution here?
>
> Since `diff-setup-whitespace' unconditionally sets whitespace-style from
> a hardcoded value, this is the natural fix.
>
>> I started working on such a patch but stuck disliking the probable
>> need in the 'whitespace-style' large ':type' definition duplication.
>
> I think you don't need a duplication.  You can reuse an existing type
> definition by looking up the `custom-type` symbol property of an already
> defined option.  See e.g. "lisp/eshell/em-cmpl.el" for a few example
> definitions.

Thanks for the advice.

The proposed patch is attached.

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #2: Add user option diff-whitespace-style --]
[-- Type: text/x-patch, Size: 2545 bytes --]

From 86a90344673a8ca9645f11ec790c16eda6a4120e Mon Sep 17 00:00:00 2001
From: YugaEgo <yet@ego.team>
Date: Sat, 23 Jul 2022 22:25:10 +0300
Subject: [PATCH] Add new user option 'diff-whitespace-style'

* lisp/vc/diff-mode.el (diff-whitespace-style): New user option.
  (diff-setup-whitespace): Use it (Bug#56679).
  (top level): require 'whitespace.
  (whitespace-style, whitespace-trailing-regexp): Remove defvars.
---
 etc/NEWS             |  7 +++++++
 lisp/vc/diff-mode.el | 11 +++++++----
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/etc/NEWS b/etc/NEWS
index 412a93bbf9..dcece83767 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1190,6 +1190,13 @@ the run/continue command.
 This is bound to 'H' and toggles whether to hide or show the widget
 contents.
 
+** Diff mode
+
+---
+*** New user option 'diff-whitespace-style'.
+This option determines buffer-local 'whitespace-style' value.
+
+
 ** Ispell
 
 ---
diff --git a/lisp/vc/diff-mode.el b/lisp/vc/diff-mode.el
index 30ba4153a9..8d9caf35a3 100644
--- a/lisp/vc/diff-mode.el
+++ b/lisp/vc/diff-mode.el
@@ -56,6 +56,7 @@
 (eval-when-compile (require 'cl-lib))
 (eval-when-compile (require 'subr-x))
 (require 'easy-mmode)
+(require 'whitespace)
 
 (autoload 'vc-find-revision "vc")
 (autoload 'vc-find-revision-no-save "vc")
@@ -147,6 +148,11 @@ diff-font-lock-syntax
                  (const :tag "Highlight syntax" t)
                  (const :tag "Allow hunk-based fallback" hunk-also)))
 
+(defcustom diff-whitespace-style '(face trailing)
+  "Specify `whitespace-style' variable for the current Diff mode buffer."
+  :type (get 'whitespace-style 'custom-type)
+  :version "29.1")
+
 (defvar diff-vc-backend nil
   "The VC backend that created the current Diff buffer, if any.")
 
@@ -1476,9 +1482,6 @@ diff--font-lock-cleanup
     ;; Added when diff--font-lock-prettify is non-nil!
     (cl-pushnew 'display font-lock-extra-managed-props)))
 
-(defvar whitespace-style)
-(defvar whitespace-trailing-regexp)
-
 (defvar-local diff-mode-read-only nil
   "Non-nil when read-only diff buffer uses short keys.")
 
@@ -1572,7 +1575,7 @@ diff-setup-whitespace
 This sets `whitespace-style' and `whitespace-trailing-regexp' so
 that Whitespace mode shows trailing whitespace problems on the
 modified lines of the diff."
-  (setq-local whitespace-style '(face trailing))
+  (setq-local whitespace-style diff-whitespace-style)
   (let ((style (save-excursion
 		 (goto-char (point-min))
                  ;; FIXME: For buffers filled from async processes, this search
-- 
2.34.1