bug#57042: 28.1.91; c-mode autoindents cweb style lines

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 7 Aug 2022 16:16:31 +0000 (UTC)
> From: Van Ly <van.ly@sdf.org>
> 
> C Mode autoindents CWEB style lines undesirably.
> 
> Steps to reproduce
>   . starting from 'emacs -Q'
>   . open new file 'C-x f /dev/shm/explain.w'
>   . enable mode for C 'M-x c-mode'
>   . enter the C CWEB style source text below
>   . Explanation}
>       the last two lines autoindent but I expect them to be without
>   . at line 6 below at # position c-set-offset reports 'cpp-macro'
>   . at line 0 below at # position c-set-offset reports 'cpp-macro'
> 
> Expected result is for lines 6, F and 0 below to be without indent.
> 
> ```
> 1 @** Convert 511 decimal to octal.
> 2 @d _x_number_x_ 511
> 3 @d _x_banner_x_ "What is the value of %d in octal?"
> 4 @c
> 5 #include <stdio.h>
> 6 #include "u.h"
> 7 int@/
> 8 main(void)@/
> 9 {
> A   int n = _x_number_x_;
> B   printf (_x_banner_x_, n);
> C   printf ("%s! %d decimal is %o octal\n", "Right", n, n);
> D }
> E @ @(u.h@>=
> F     @h
> 0     #include <stdlib.h>
> ```

Sorry, I don't understand: why do you expect C Mode to DTRT for CWEB
files?  The above is not valid C code, it's a different language that
needs a dedicated major mode.