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.

6 files changed, 126 insertions, 9 deletions

diff --git a/doc/misc/cc-mode.texi b/doc/misc/cc-mode.texi
index 217261090bf..a802ca9886f 100644
--- a/doc/misc/cc-mode.texi
+++ b/doc/misc/cc-mode.texi
@@ -669,7 +669,7 @@ expression, to some statements, or perhaps to whole functions, the
 syntactic recognition can be wrong.  @ccmode{} manages to figure it
 out correctly most of the time, though.
 
-Some macros, when invoked, ''have their own semicolon''.  To get the
+Some macros, when invoked, ``have their own semicolon''.  To get the
 next line indented correctly, rather than as a continuation line,
 @xref{Macros with ;}.
 
@@ -1149,7 +1149,9 @@ find useful while writing new code or editing old code:
 @table @asis
 @item comment style
 This specifies whether comment commands (such as @kbd{M-;}) insert
-block comments or line comments.
+block comments or line comments@footnote{You can emphasize
+non-default style comments in your code by giving their delimiters
+@code{font-lock-warning-face}.  @xref{Wrong Comment Style}.}.
 @item electric mode
 When this is enabled, certain visible characters cause reformatting as
 they are typed.  This is normally helpful, but can be a nuisance when
@@ -1866,6 +1868,7 @@ sections apply to the other languages.
 * Font Locking Preliminaries::
 * Faces::
 * Doc Comments::
+* Wrong Comment Style::
 * Misc Font Locking::
 * AWK Mode Font Locking::
 @end menu
@@ -2069,7 +2072,7 @@ since those aren't syntactic errors in themselves.
 
 
 @comment !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
-@node    Doc Comments, Misc Font Locking, Faces, Font Locking
+@node    Doc Comments, Wrong Comment Style, Faces, Font Locking
 @comment node-name, next, previous, up
 @section Documentation Comments
 @cindex documentation comments
@@ -2149,7 +2152,29 @@ If you add support for another doc comment style, please consider
 contributing it: send a note to @email{bug-cc-mode@@gnu.org}.
 
 @comment !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
-@node    Misc Font Locking, AWK Mode Font Locking, Doc Comments, Font Locking
+@node    Wrong Comment Style, Misc Font Locking, Doc Comments, Font Locking
+@comment  node-name,  next,  previous,  up
+@section Marking ``Wrong'' style comments
+@comment !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+
+Most languages supported by @ccmode{} have two styles of comments,
+namely block comments and line comments.  Your project may have such a
+strong preference for one of them, that you wish ``wrong'' style
+comments to be clearly marked.
+
+You can get @ccmode{} to do this by setting the default comment style,
+if necessary, (@pxref{Minor Modes}) and setting the customizable
+option @code{c-mark-wrong-style-of-comment} to non-@code{nil}.
+
+@defvar c-mark-wrong-style-of-comment
+@vindex mark-wrong-style-of-comment (c-)
+When this customizable option is non-@code{nil}, comment delimiters
+which aren't of the default style will be fontified with
+@code{font-lock-warning-face}.
+@end defvar
+
+@comment !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
+@node    Misc Font Locking, AWK Mode Font Locking, Wrong Comment Style, Font Locking
 @comment  node-name,  next,  previous,  up
 @section Miscellaneous Font Locking
 @comment !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
@@ -2885,11 +2910,11 @@ should not be changed directly; use @code{c-add-style} instead.
 
 Instead of specifying a style, you can get @ccmode{} to @dfn{guess}
 your style by examining an already formatted code buffer.  @ccmode{}
-then determines the ''most frequent'' offset (@pxref{c-offsets-alist})
+then determines the ``most frequent'' offset (@pxref{c-offsets-alist})
 for each of the syntactic symbols (@pxref{Indentation Engine Basics})
-encountered in the buffer, and the ''most frequent'' value of
+encountered in the buffer, and the ``most frequent'' value of
 c-basic-offset (@pxref{Customizing Indentation}), then merges the
-current style with these ''guesses'' to form a new style.  This
+current style with these ``guesses'' to form a new style.  This
 combined style is known as the @dfn{guessed style}.
 
 To do this, call @code{c-guess} (or one of the other 5 guessing
@@ -6927,7 +6952,7 @@ is @code{nil}, all lines inside macro definitions are analyzed as
 Because a macro can expand into anything at all, near where one is
 invoked @ccmode{} can only indent and fontify code heuristically.
 Sometimes it gets it wrong.  Usually you should try to design your
-macros so that they ''look like ordinary code'' when you invoke them.
+macros so that they ``look like ordinary code'' when you invoke them.
 However, two situations are so common that @ccmode{} handles them
 specially: that is when certain macros needn't (or mustn't) be
 followed by a @samp{;}, and when certain macros (or compiler
diff --git a/etc/NEWS b/etc/NEWS
index 95d6b9b5bd4..61b9f933f15 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -2264,6 +2264,14 @@ file-local variable, you may need to update the value.
 *** Declare 'define-overload' and 'define-child-mode' as obsolete
 *** Rename several internal functions to use a ''mode-local-' prefix
 
+** CC Mode
+
++++
+*** You can now flag "wrong style" comments with font-lock-warning-face.
+To do this, use c-toggle-comment-style, if needed, to set the desired
+default comment style (block or line); then set the option
+c-mark-wrong-style-of-comment to non-nil.
+
 
 * New Modes and Packages in Emacs 27.1
 
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