CC Mode. Allow fontification of "wrong" style comments with warning face.

This fixes bug #4192.

* etc/NEWS: Add a new entry.

* lisp/progmodes/cc-defs.el (c-font-lock-flush): New macro.

* lisp/progmodes/cc-cmds.el (c-toggle-comment-style): On toggling the comment
style, invoke c-font-lock-flush when c-mark-wrong-style-of-comment is non-nil,
to cause that marking to be done instead on the other style of comment.

* lisp/progmodes/cc-fonts.el (c-maybe-font-lock-wrong-style-comments): New
function.
(c-cpp-matchers): Call c-maybe-font-lock-wrong-style-comments when
appropriate.

* lisp/progmodes/cc-vars.el (c-mark-wrong-style-of-comment): New customizable
option.

* doc/misc/cc-mode.texi (top level, Indentation Commands, Guessing the Style,
Custom Macros): For some opening quote marks, correct '' to ``.
(Minor Modes): Add an xref to the new page "Wrong Comment Style" in a
footnote.
(Wrong Comment Style): New page.

4 files changed, 85 insertions, 1 deletions

diff --git a/lisp/progmodes/cc-cmds.el b/lisp/progmodes/cc-cmds.el
index 9aa2019ae60..af2efb63fc3 100644
--- a/lisp/progmodes/cc-cmds.el
+++ b/lisp/progmodes/cc-cmds.el
@@ -391,6 +391,16 @@ This action does nothing when the mode only has one comment style."
         (if c-block-comment-flag
             (concat " " c-block-comment-ender)
           ""))
+  ;; If necessary, invert the sense of fontification of wrong style comments.
+  (when (and c-mark-wrong-style-of-comment
+             font-lock-mode
+             c-block-comment-starter
+             c-block-comment-ender)
+    (save-excursion
+      (save-restriction
+        (widen)
+        (goto-char (point-min))
+        (c-font-lock-flush (point-min) (point-max)))))
   (c-update-modeline)
   (c-keep-region-active))
 
diff --git a/lisp/progmodes/cc-defs.el b/lisp/progmodes/cc-defs.el
index e9a3e16db36..6a9371e6f19 100644
--- a/lisp/progmodes/cc-defs.el
+++ b/lisp/progmodes/cc-defs.el
@@ -219,6 +219,13 @@ This variant works around bugs in `eval-when-compile' in various
         `(cl-delete-duplicates ,cl-seq ,@cl-keys)
       `(delete-duplicates ,cl-seq ,@cl-keys))))
 
+(defmacro c-font-lock-flush (beg end)
+  "Declare the region BEG...END's fontification as out-of-date.
+On XEmacs and older Emacsen, this refontifies that region immediately."
+  (if (fboundp 'font-lock-flush)
+      `(font-lock-flush ,beg ,end)
+    `(font-lock-fontify-region ,beg ,end)))
+
 (defmacro c-point (position &optional point)
   "Return the value of certain commonly referenced POSITIONs relative to POINT.
 The current point is used if POINT isn't specified.  POSITION can be
diff --git a/lisp/progmodes/cc-fonts.el b/lisp/progmodes/cc-fonts.el
index c27b70603ed..0daea8c84c9 100644
--- a/lisp/progmodes/cc-fonts.el
+++ b/lisp/progmodes/cc-fonts.el
@@ -95,6 +95,7 @@
 ;; during compilation.
 (cc-bytecomp-defvar c-preprocessor-face-name)
 (cc-bytecomp-defvar c-reference-face-name)
+(cc-bytecomp-defvar c-block-comment-flag)
 (cc-bytecomp-defun c-fontify-recorded-types-and-refs)
 (cc-bytecomp-defun c-font-lock-declarators)
 (cc-bytecomp-defun c-font-lock-objc-method)
@@ -532,7 +533,12 @@ stuff.  Used on level 1 and higher."
                  (sws-depth (c-lang-const c-syntactic-ws-depth))
                  (nsws-depth (c-lang-const c-nonempty-syntactic-ws-depth)))
 
-            `(;; The stuff after #error and #warning is a message, so
+            `(;; Fontify "invalid" comment delimiters
+              ,@(when (and (c-lang-const c-block-comment-starter)
+                           (c-lang-const c-line-comment-starter))
+                  `(c-maybe-font-lock-wrong-style-comments))
+
+              ;; The stuff after #error and #warning is a message, so
               ;; fontify it as a string.
               ,@(when (c-lang-const c-cpp-message-directives)
                   (let* ((re (c-make-keywords-re 'appendable ; nil
@@ -715,6 +721,59 @@ stuff.  Used on level 1 and higher."
       (parse-partial-sexp end limit nil nil state 'syntax-table)))
     nil)
 
+(defun c-maybe-font-lock-wrong-style-comments (limit)
+  ;; This function will be called from font-lock-for a region bounded by POINT
+  ;; and LIMIT, as though it were to identify a keyword for
+  ;; font-lock-keyword-face.  It always returns NIL to inhibit this and
+  ;; prevent a repeat invocation.  See elisp/lispref page "Search-based
+  ;; Fontification".
+  ;;
+  ;; This function fontifies "invalid" comment delimiters with
+  ;; `font-lock-warning-face'.  A delimiter is "invalid" when
+  ;; `c-mark-wrong-style-of-comment' is non-nil, and the delimiter style is
+  ;; not the default specified by `c-block-comment-flag'.
+  (when c-mark-wrong-style-of-comment
+    (let* ((lit (c-semi-pp-to-literal (point)))
+           (s (car lit))                ; parse-partial-sexp state.
+           )
+      ;; First, deal with and move out of any literal we start in.
+      (cond
+       ((null (cadr lit)))              ; Not in a literal
+       ((eq (cadr lit) 'string)
+        (setq s (parse-partial-sexp (point) limit nil nil s 'syntax-table)))
+       ((and (not c-block-comment-flag) ; In an "invalid" block comment
+             (eq (cadr lit) 'c))
+        (setq s (parse-partial-sexp (point) limit nil nil s 'syntax-table))
+        ;; Font lock the block comment ender with warning face.
+        (when (not (nth 4 s))
+          (c-put-font-lock-face (- (point) (length c-block-comment-ender))
+                                (point) font-lock-warning-face)))
+       (t ; In a line comment, or a "valid" block comment
+        (setq s (parse-partial-sexp (point) limit nil nil s 'syntax-table))))
+
+      (while (< (point) limit)
+        (setq s (parse-partial-sexp (point) limit nil nil s 'syntax-table))
+        (cond
+         ((or (nth 3 s)                 ; In a string
+              (and (nth 4 s)            ; In a comment
+                   (eq (nth 7 s)        ; Comment style
+                       (if c-block-comment-flag
+                           nil          ; Block comment
+                         1))))  ; Line comment
+            ;; Move over a "valid" literal.
+          (setq s (parse-partial-sexp (point) limit nil nil s 'syntax-table)))
+         ((nth 4 s)                     ; In an invalid comment
+         ;; Fontify the invalid comment opener.
+          (c-put-font-lock-face (nth 8 s) (point) font-lock-warning-face)
+          ;; Move to end of comment or LIMIT.
+          (setq s (parse-partial-sexp (point) limit nil nil s 'syntax-table))
+          ;; Fontify an invalid block comment ender, if that's what we have.
+          (when (and (not c-block-comment-flag)
+                     (not (nth 4 s)))   ; We're outside the comment
+            (c-put-font-lock-face (- (point) (length c-block-comment-ender))
+                                  (point) font-lock-warning-face)))))))
+  nil)
+
 (c-lang-defconst c-basic-matchers-before
   "Font lock matchers for basic keywords, labels, references and various
 other easily recognizable things that should be fontified before generic
diff --git a/lisp/progmodes/cc-vars.el b/lisp/progmodes/cc-vars.el
index 60ad42f24f4..273174e0413 100644
--- a/lisp/progmodes/cc-vars.el
+++ b/lisp/progmodes/cc-vars.el
@@ -1751,6 +1751,14 @@ variables.")
 c-macro-names-with-semicolon: %s"
                     c-macro-names-with-semicolon))))))
 
+(defcustom c-mark-wrong-style-of-comment nil
+  "Fontify \"invalid\" comment delims with `font-lock-warning-face' if non-nil.
+\"Invalid\" means a line comment when the default comment style (set by
+`c-toggle-comment-style') is block, or a block comment otherwise."
+  :type 'boolean
+  :group 'c
+  :version 27.1)
+
 (defvar c-file-style nil
   "Variable interface for setting style via File Local Variables.
 In a file's Local Variable section, you can set this variable to a