bug#62951: 29.0.90; c-ts-mode: Incorrect fontification due to FOR_EACH_TAIL_SAFE

[Eli Zaretskii <eliz@gnu.org>]

> From: Yuan Fu <casouri@gmail.com>
> Date: Fri, 21 Apr 2023 13:37:08 -0700
> Cc: 62951@debbugs.gnu.org
> 
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > To reproduce:
> >
> >   emacs -Q
> >   C-x C-f src/fns.c RET
> >   C-u 3365 M-g g
> >
> > Observe that "if" and "STRINGP" in the body of FOR_EACH_TAIL_SAFE are
> > fontified in font-lock-function-name-face.  This is because the
> > FOR_EACH_TAIL_SAFE macro is misparsed by tree-sitter as a declaration.
> >
> > Can we teach c-ts-mode to recognize FOR_EACH_TAIL_SAFE and
> > FOR_EACH_TAIL for what they are, perhaps conditioned on
> > c-ts-mode-emacs-sources-support being non-nil?
> 
> I’m aware of this issue, but the truth is there isn’t a good solution to
> it. We need to recognize FOR_EACH_TAIL_SAFE (not hard) and fix arbitrary
> code after it (hard). In this case it’s a if statement, with macro calls
> and AND operation in it’s condition, it’s already three things we need
> to recognize and somehow handle. It can also be a for loop, a switch
> case, a function call, a while loop. If we want to fix FOR_EACH_TAIL we
> would need to handle every possible thing, at that point we might as
> well have wrote a parser :-)

Sorry, I don't understand the difficulties.  The body of FOR_EACH_TAIL
and a few similar macros we use could be on of the following:

  . a single simple statement
  . an 'if' clause
  . a 'while' loop
  . a 'do' loop
  . a 'for' loop
  . a brace-delimited block (this one already works, AFAICS, so we
    perhaps need not anything about it)

(In practice, only the first 2 and the last one are used, AFAICS.)

Doesn't tree-sitter tell us enough to figure out which of the above is
in the body?  If so, why would we need to write a full parser?

> We can probably fix this very particular case, but it’s still work and
> overhead, and doesn’t mean much.

Please understand: good support for editing Emacs C sources is from my
POV imperative for c-ts-mode to gain traction.  One of my gripes about
CC Mode was insufficient support for our macro system and for various
GCC attributes; that improved recently to some extend, but not enough,
and at a price of introducing ugly lists of strings that cause trouble
when used in file-local variables.  We must do better in c-ts-mode!

So please make an effort of providing reasonable built-in solutions
for these idiosyncrasies of the Emacs C sources, conditioned on the
new variable c-ts-mode-emacs-sources-support, at least for those of
them that are used widely enough.  It is very important.

TIA