bug#81533: c-ts-mode: Improved block comment indentation

Hello Stefan!

On 8/2/26 16:26, Stefan Monnier wrote:
 >>> The attached patch modifies the c-ts-mode block comment indentation
 >>> rules so that
 >>>
 >>> 1) lines beginning with "*" are aligned to the first "*",
 >>> 2) lines following the first or blank lines are indented three spaces,
 >>> 3) and other lines are aligned to the previous line.
 >
 > "when pressing TAB" is one case, "when calling
 > `indent-according-to-mode`" is another.  The first is free to mess up
 > because the users are right there to see the problem and fix it, but the
 > second needs to be more conservative because it's also used in cases
 > where the users may not see the result.

I think the "batch indentation" use case is beyond the scope of my
PR. It doesn't work in c-ts-mode and I doubt it can work. The
tree-sitter C grammar is incomplete and real world C code is just too
bastardized with macros everywhere.

 > The way `indent-line-function` is designed to handle this is that the
 > function should return `noindent` when the language's syntax is of no
 > help, such as when "indenting" the inside of a string or of a comment.

But then c-ts-mode already violates the "don't indent without language
syntax"-policy because it indents block comments. My PR only changes
(improves) the rules.

 > The rule (1) sounds right, but rules (2) and (3) should simply refrain
 > from modifying the line and return `noindent`.

Why indent lines prefixed with "*" be indented but not other lines?
While most people use "*" to draw a border, some use it for bullet
points.

Fwiw, c-ts-mode indents unprefixed lines, but not always
correctly. E.g., flying block comments are not handled well:

struct bc_frame *saved_fp;        /* previous frame pointer,
                                      NULL if bottommost frame */

becomes:

struct bc_frame *saved_fp;        /* previous frame pointer,
NULL if bottommost frame */

My PR fixes that. :)

 >>> The rules work for like 99% of all block comments and is how both
 >>> Emacs and CPython overwhelmingly indents them.
 >
 > [ It took me a while to understand what you meant here.
 >    I was confused by the idea of CPython indenting anything.  ]

My point is that you can't satisfy everyone: some people write essays,
use esoteric formatting styles, or draw ascii art in comments. But for
almost all comments the three rules are good enough.

 >> Thanks.  Yuan and Stefan, any comments?
 >
 > These issues are basically language-independent so I'm not
 > happy with the current code: these issues should be handled
 > once for all tree-sitter modes.

You could make the function public so that other modes could add it to
their default rules:

((parent-is "comment") parent c-ts-mode-common-block-comment-offset)


--
mvh/best regards Björn Lindqvist