vim-patch:8.2.4907: some users do not want a line comment always inserted (#18464)

Problem:    Some users do not want a line comment always inserted.
Solution:   Add the '/' flag to 'formatoptions' to not repeat the comment
            leader after a statement when using "o".
2bf875f881
(cherry picked from commit d4b9aa4f8e)

Co-authored-by: Christian Clason <c.clason@uni-graz.at>

runtime/doc/change.txt

@@ -1586,11 +1586,10 @@ You can use the 'formatoptions' option  to influence how Vim formats text.
 can separate the option letters with commas for readability.
 
 letter	 meaning when present in 'formatoptions'    ~
-
 							*fo-t*
-t	Auto-wrap text using textwidth
+t	Auto-wrap text using 'textwidth'
 							*fo-c*
-c	Auto-wrap comments using textwidth, inserting the current comment
+c	Auto-wrap comments using 'textwidth', inserting the current comment
 	leader automatically.
 							*fo-r*
 r	Automatically insert the current comment leader after hitting
@@ -1599,6 +1598,9 @@ r	Automatically insert the current comment leader after hitting
 o	Automatically insert the current comment leader after hitting 'o' or
 	'O' in Normal mode.  In case comment is unwanted in a specific place
 	use CTRL-U to quickly delete it. |i_CTRL-U|
+							*fo-/*
+/	When 'o' is included: do not insert the comment leader for a //
+	comment after a statement, only when // is at the start of the line.
 							*fo-q*
 q	Allow formatting of comments with "gq".
 	Note that formatting will not change blank lines or lines containing
@@ -1661,8 +1663,8 @@ B	When joining lines, don't insert a space between two multibyte
 1	Don't break a line after a one-letter word.  It's broken before it
 	instead (if possible).
 							*fo-]*
-]	Respect textwidth rigorously. With this flag set, no line can be
-	longer than textwidth, unless line-break-prohibition rules make this
+]	Respect 'textwidth' rigorously. With this flag set, no line can be
+	longer than 'textwidth', unless line-break-prohibition rules make this
 	impossible.  Mainly for CJK scripts and works only if 'encoding' is
 	"utf-8".
 							*fo-j*

src/nvim/change.c

@@ -1186,7 +1186,8 @@ int open_line(int dir, int flags, int second_line_indent, bool *did_do_comment)
   end_comment_pending = NUL;
   if (flags & OPENLINE_DO_COM) {
     lead_len = get_leader_len(saved_line, &lead_flags, dir == BACKWARD, true);
-    if (lead_len == 0 && curbuf->b_p_cin && do_cindent && dir == FORWARD) {
+    if (lead_len == 0 && curbuf->b_p_cin && do_cindent && dir == FORWARD
+        && !has_format_option(FO_NO_OPEN_COMS)) {
       // Check for a line comment after code.
       comment_start = check_linecomment(saved_line);
       if (comment_start != MAXCOL) {

src/nvim/option_defs.h

@@ -67,6 +67,7 @@
 #define FO_WRAP_COMS    'c'
 #define FO_RET_COMS     'r'
 #define FO_OPEN_COMS    'o'
+#define FO_NO_OPEN_COMS '/'
 #define FO_Q_COMS       'q'
 #define FO_Q_NUMBER     'n'
 #define FO_Q_SECOND     '2'
@@ -85,7 +86,7 @@
 
 #define DFLT_FO_VI      "vt"
 #define DFLT_FO_VIM     "tcqj"
-#define FO_ALL          "tcroq2vlb1mMBn,aw]jp"   // for do_set()
+#define FO_ALL          "tcro/q2vlb1mMBn,aw]jp"   // for do_set()
 
 // characters for the p_cpo option:
 #define CPO_ALTREAD     'a'     // ":read" sets alternate file name
@@ -118,7 +119,7 @@
 #define CPO_REMMARK     'R'     // remove marks when filtering
 #define CPO_BUFOPT      's'
 #define CPO_BUFOPTGLOB  'S'
-#define CPO_TAGPAT      't'
+#define CPO_TAGPAT      't'     // tag pattern is used for "n"
 #define CPO_UNDO        'u'     // "u" undoes itself
 #define CPO_BACKSPACE   'v'     // "v" keep deleted text
 #define CPO_FWRITE      'W'     // "w!" doesn't overwrite readonly files

src/nvim/testdir/test_textformat.vim

@@ -278,6 +278,18 @@ func Test_format_c_comment()
                       //
   END
   call assert_equal(expected, getline(1, '$'))
+  3delete
+
+  " No comment repeated with a slash in 'formatoptions'
+  set fo+=/
+  normal 2Gox
+  let expected =<< trim END
+      nop;
+      val = val;      // This is a comment
+      x
+  END
+  call assert_equal(expected, getline(1, '$'))
+  set fo-=/
 
   " using 'indentexpr' instead of 'cindent' does not repeat a comment
   setl nocindent indentexpr=2