bug#40658: Fforward-comment: Incorrect recognition of end of comment in C++

[Alan Mackenzie]

Hello, Emacs.

C++ comments can be continued onto the next line by an escaped NL.  This
is configured in Emacs with the variable comment-end-can-be-escaped.

However, Fforward_comment (in the backward direction) doesn't take this
into account.  A BOL following an escaped NL cannot be at the end of a
C++ line comment:

    // C++ comment first line \
    Second line of comment
    ^
    |
  point

Point here is in the middle of a comment.

It is therefore wrong in Fforward_comment not to check
comment-end-can-be-escaped when immediately after an Sendcomment.  This
can be fixed thusly:


diff --git a/src/syntax.c b/src/syntax.c
index e24b98da32..ff125b137c 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -2572,8 +2572,9 @@ between them, return t; otherwise return nil.  */)
            }
          else if (code == Sendcomment)
            {
-             found = back_comment (from, from_byte, stop, comnested, comstyle,
-                                   &out_charpos, &out_bytepos);
+              found = (!quoted || !Vcomment_end_can_be_escaped)
+                && back_comment (from, from_byte, stop, comnested, comstyle,
+                                 &out_charpos, &out_bytepos);
              if (!found)
                {
                  if (c == '\n')


This should clearly go into the master branch.  I think it should also go
into the emacs-27 release branch, since it is relevant for a nasty bug,
bug #40052.  There, scrolling in a C++ include file, which consists of a
#define macro ~2,400 lines long is deadly slow.  With the above fix in
place, scrolling is over 4 times as fast, although still too slow.

-- 
Alan Mackenzie (Nuremberg, Germany).