Re: Continuing existing comment, with proper identation

[Nikolay Kudryavtsev <nikolay.kudryavtsev@gmail.com>]

Honestly, your best bet is probably implementing literally all of the 
logic yourself, maybe reusing some code from whatever mode you're 
willing to support.

In a perfect world Emacs would understand comments as proper syntax tree 
expressions and have some kind of high level API to deal with those. But 
it does not and there isn't one.

Also, extending comments is not your only problem. Indentation is a 
factor too. Again, there's no unified API. Some major modes use homebrew 
indentation logic, few use SMIE, now there's tree-sitter too.

For example elisp-mode won't indent comments:

;;           The next line should indent up to here(according to the OP 
rules)
;; But it won't!

Then there's also the elephant in the room - comments can be code too. 
Lets say you want to comment out a very basic C-like if:

/*  if(){
  *    whatever
  */

How would your code know that offset is warranted here? You'd have to 
run the major mode's normal indentation logic to see it, right? That's 
why code that comments out stuff generally tries to avoid messing with 
indentation.