>From 142261030f7759428701afa6bc4e1e5ac72dc7b4 Mon Sep 17 00:00:00 2001
From: Sean Whitton <spwhitton@spwhitton.name>
Date: Sun, 4 Sep 2022 16:20:15 -0700
Subject: [PATCH] Font lock long Git commit summary lines

* lisp/vc/vc-git.el (vc-git-log-edit-summary-target)
(vc-git-log-edit-summary-max): New defcustoms.
(vc-git-log-edit-summary-warning, vc-git-log-edit-summary-error): New faces.
(vc-git--log-edit-summary-check): New function.
(vc-git-log-edit-mode): Add vc-git--log-edit-summary-check to
log-edit-font-lock-keywords to font lock long Git commit summary
lines.
* .dir-locals.el: Set vc-git-log-edit-summary-target.
---
 .dir-locals.el    |  3 ++-
 lisp/vc/vc-git.el | 62 ++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 63 insertions(+), 2 deletions(-)

diff --git a/.dir-locals.el b/.dir-locals.el
index 7812beb001..666d8019cc 100644
--- a/.dir-locals.el
+++ b/.dir-locals.el
@@ -5,7 +5,8 @@
          (sentence-end-double-space . t)
          (fill-column . 70)
 	 (emacs-lisp-docstring-fill-column . 65)
-         (bug-reference-url-format . "https://debbugs.gnu.org/%s")))
+         (bug-reference-url-format . "https://debbugs.gnu.org/%s")
+	 (vc-git-log-edit-summary-target . 50)))
  (c-mode . ((c-file-style . "GNU")
             (c-noise-macro-names . ("INLINE" "ATTRIBUTE_NO_SANITIZE_UNDEFINED" "UNINIT" "CALLBACK" "ALIGN_STACK"))
             (electric-quote-comment . nil)
diff --git a/lisp/vc/vc-git.el b/lisp/vc/vc-git.el
index 7395253745..e7c04393de 100644
--- a/lisp/vc/vc-git.el
+++ b/lisp/vc/vc-git.el
@@ -858,6 +858,43 @@ vc-git-branches
 
 ;;; STATE-CHANGING FUNCTIONS
 
+(defcustom vc-git-log-edit-summary-target nil
+  "Target length for Git commit summary lines.
+If non-nil, characters in Summary: lines beyond this length are
+displayed in the `vc-git-log-edit-summary-warning' face.
+
+By setting this to an integer around 50, you can improve the
+compatibility of your commit messages with Git commands that
+print the summary line in width-constrained contexts.  However,
+many commit summaries will need to exceed this length.
+
+See also `vc-git-log-edit-summary-max'."
+  :type '(choice (const :tag "No target" nil)
+                 (natnum :tag "Target length"))
+  :safe (lambda (x) (or (not x) (integerp x))))
+
+(defface vc-git-log-edit-summary-warning
+  '((t :inherit warning))
+  "Face for Git commit summary lines beyond the target length.")
+
+(defcustom vc-git-log-edit-summary-max 68
+  "Maximum length for Git commit summary lines.
+If non-nil, characters in summary lines beyond this length are
+displayed in the `vc-git-log-edit-summary-error' face.
+
+It is good practice to avoid writing summary lines longer than
+this because otherwise the summary line will be truncated in many
+contexts in which Git commands display summary lines.
+
+See also `vc-git-log-edit-summary-target'."
+  :type '(choice (const :tag "No target" nil)
+                 (natnum :tag "Target length"))
+  :safe (lambda (x) (or (not x) (integerp x))))
+
+(defface vc-git-log-edit-summary-error
+  '((t :inherit error))
+  "Face for Git commit summary lines beyond the maximum length.")
+
 (defun vc-git-create-repo ()
   "Create a new Git repository."
   (vc-git-command nil 0 nil "init"))
@@ -911,9 +948,32 @@ vc-git-log-edit-mode-map
   "C-c C-n" #'vc-git-log-edit-toggle-no-verify
   "C-c C-e" #'vc-git-log-edit-toggle-amend)
 
+(defun vc-git--log-edit-summary-check (limit)
+  (and (re-search-forward "^Summary: " limit t)
+       (when-let ((regex
+                   (cond ((and vc-git-log-edit-summary-max
+                               vc-git-log-edit-summary-target)
+                          (format ".\\{,%d\\}\\(.\\{,%d\\}\\)\\(.*\\)"
+                                  vc-git-log-edit-summary-target
+                                  (- vc-git-log-edit-summary-max
+                                     vc-git-log-edit-summary-target)))
+                         (vc-git-log-edit-summary-max
+                          (format ".\\{,%d\\}\\(?2:.*\\)"
+                                  vc-git-log-edit-summary-max))
+                         (vc-git-log-edit-summary-target
+                          (format ".\\{,%d\\}\\(.*\\)"
+                                  vc-git-log-edit-summary-target)))))
+         (re-search-forward regex limit t))))
+
 (define-derived-mode vc-git-log-edit-mode log-edit-mode "Log-Edit/git"
   "Major mode for editing Git log messages.
-It is based on `log-edit-mode', and has Git-specific extensions.")
+It is based on `log-edit-mode', and has Git-specific extensions."
+  (setq-local
+   log-edit-font-lock-keywords
+   (append log-edit-font-lock-keywords
+           '((vc-git--log-edit-summary-check
+	      (1 'vc-git-log-edit-summary-warning prepend t)
+              (2 'vc-git-log-edit-summary-error prepend t))))))
 
 (defvar vc-git-patch-string nil)
 
-- 
2.30.2