bug#69571: 29.2; csharp-mode indentation: Misaligned closing brace in blocks starting below "new"

[Eli Zaretskii <eliz@gnu.org>]

Ping!  Yuan, could help Theo figure out what's best here?

> From: Theodor Thornhill <theo@thornhill.no>
> Cc: 69571@debbugs.gnu.org
> Date: Sun, 10 Mar 2024 20:21:32 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> Date: Tue, 5 Mar 2024 22:09:51 +0100
> >> From: Carlos <carlos@cvkm.cz>
> >> 
> >> Any block starting on the line immediately below a line having the
> >> string "new" will have its closing brace aligned with the opening one.
> >> 
> >> See the following code:
> >> 
> >> public class Foo {
> >>     void Bar () {
> >> 	var x = new X(); // [1]
> >> 	for (;;) {
> >> 	    x();
> >> 		 }  // [2]
> >>     }
> >> }
> >> 
> >> Line [1] says "new". The closing brace in line [2] is aligned to the
> >> opening brace.
> >> 
> >> If you comment out the "new" (or the whole line) the problem persists.
> >> 
> >> If you remove the "new" the problem goes away and [2] is correctly
> >> aligned.
> >> 
> >> If you insert a line between line [1] and the one having the opening
> >> brace the problem goes away.
> >
> > Theo and Yuan, could you please look into this?
> 
> I have a working patch for this, but I'd like to expand it to cover an
> edge case for which I'm unable to find a good solution.  Can you suggest
> a way around this edge case?
> 
> Consider the provided code:
> ```
> public class Foo {
>     void Bar () {
>         var x = new X(); // [1]
>         for (;;) {
>             x();
>         }  // [2]
>     }
> }
> ```
> 
> Like this, the below patch doesn't work. If you remove the first
> comment, the patch works.
> 
> ```
> public class Foo {
>     void Bar () {
>         var x = new X();
>         for (;;) {
>             x();
>         }  // [2]
>     }
> }
> ```
> 
> The reason is simple, of course. What I'm struggling with here is how to
> best handle the case where there is a comment ending the line, possibly
> containing a ';' itself. I've tried some variations with save-excursion
> along with syntax-ppss to detect whether or not we're in a comment, but
> it gets verbose and ugly. Is there some simple way to do this check in
> Emacs, or should I just resort to making some best effort judgement call
> here?
> 
> Thanks,
> Theo
> 
> diff --git a/lisp/progmodes/csharp-mode.el b/lisp/progmodes/csharp-mode.el
> index 7bf57bcbe21..00278e18e51 100644
> --- a/lisp/progmodes/csharp-mode.el
> +++ b/lisp/progmodes/csharp-mode.el
> @@ -495,9 +495,10 @@ csharp-guess-basic-syntax
>         (unless (eq (char-after) ?{)
>           (ignore-errors (backward-up-list 1 t t)))
>         (save-excursion
> -         ;; 'new' should be part of the line
> +         ;; 'new' should be part of the line, but should not trigger if
> +         ;; statement has already ended, like for 'var x = new X();'.
>           (goto-char (c-point 'iopl))
> -         (looking-at ".*new.*")))
> +         (looking-at ".*new.*[^;]$")))
>       ;; Line should not already be terminated
>       (save-excursion
>         (goto-char (c-point 'eopl))
>