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bug#67417: 29.1.50; c-ts-mode syntax issues with no brackets


From: Yuan Fu
Subject: bug#67417: 29.1.50; c-ts-mode syntax issues with no brackets
Date: Sun, 26 Nov 2023 17:47:49 -0800
User-agent: Mozilla Thunderbird


On 11/24/23 10:26 AM, Dmitry Gutov wrote:
On 24/11/2023 09:23, Eli Zaretskii wrote:
Date: Thu, 23 Nov 2023 12:55:31 -0800
From:  Arteen Abrishami via "Bug reports for GNU Emacs,
  the Swiss army knife of text editors"<bug-gnu-emacs@gnu.org>

This is specifically for the usage of `c-ts-mode` and is not a problem
in `c-mode`. Sometimes, when you type something like:

else
break

it won't indent the "break" until you type a semicolon. In this below
scenario, it does not indent the break at all, but `c-mode` does, and
switching from `c-mode` to `c-ts-mode` with correct indentation leaves
it fixed, but `c-ts-mode` cannot detect or fix it itself.

You can put it into a `.c` buffer all by itself and see:

```
unsigned
heap_pop(struct heapq * heap)
{
   if (heap->sz == 0)
     return -1;

   unsigned ret_val = heap->vals[0];
   heap->vals[0] = heap->vals[heap->len];
   heap->len -= 1;
   unsigned i = 0;
   unsigned lc;

   while ((lc = HEAPQ_L_CHILD(i)) < heap->len)
     {
       unsigned rc = HEAPQ_R_CHILD(i);
       /* no right child for our guy, special case */
       if (rc == heap->len)
         {
           if (heap->vals[lc] < heap->vals[i])
             SWAP(heap->vals[lc], heap->vals[i]);
           break;
         }

       if (heap->vals[lc] < heap->vals[i])
         {
           SWAP(heap->vals[lc], heap->vals[i]);
           i = lc;
         }
       else if (heap->vals[rc] < heap->vals[i])
         {
           SWAP(heap->vals[rc], heap->vals[i]);
           i = rc;
         }
       else
       break;
            }
}
```

The very last break on the else without brackets around it will not indent.c
Yuan, any comments?

My personal take on this is that as long as typing the required
semi-colons fixes the indentation, we are okay in these cases, but if
we can do better (i.e. if the problem is not that tree-sitter returns
a tree with an error node), we should fix this even without relying on
the electric semi-colon.

In the specific example above, it looks like tree-sitter does succeed
in parsing and shows a valid tree:

       alternative:
        (else_clause else
         (break_statement break ;)))))

So I wonder why we don't indent the "break;" part here.

In my testing, it indents fine when after "else" there is either:

 * some char(s) followed by closing curly
 * or (optionally) some char(s) followed by semicolon

When there is _no_ code between "else" and the closing curly, it already indents fine in my testing (whether the semicolon is added or not).

Without either, the text after "else" isn't parsed as "alternative:" -- it's parsed as a sibling of the "else" node. And, most unfortunately, when "else" is followed by a closing curly, it's just parsed as (ERROR else), so simply pressing RET does not indent the empty line properly even when one is working with electric-pair-mode enabled.

I'd personally consider the last one a more definite bug in the grammar, but maybe there is some good reason for it. I haven't found anything relevant in the bug tracker.

BTW, it seems like the latest C grammar changed how else without braces is parsed, so "break" isn't reindented even with semicolon at the end.

I pushed two commits which should fix the indentation for "break" after "else", and indentation for empty lines after if/else/for/while in general. The fix for the general case doesn't use the parse tree, since the parse tree is often incomplete when you type if (...) and hit return. Instead it uses a plain regexp match to see if the previous line starts with if/else/for/while. This seems like a reasonable heuristic to use before user types more things, at which point more accurate indentation rules would be used, since the parse tree should be more complete then.

Yuan






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