comment-indent-function is a variable defined in `newcomment.el'.
Its value is

  • This variable may be risky if used as a file-local variable.

Function to compute desired indentation for a comment.
This function is called with no args with point at the beginning of
the comment's starting delimiter and should return either the desired
column indentation or nil.
If nil is returned, indentation is delegated to `indent-according-to-mode'.