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From: Eli Zaretskii <eliz@gnu.org>
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Subject: Re: Underline the current line (function inside)
Date: Fri, 11 Jun 2021 20:55:25 +0300
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In-Reply-To: <YMOCWV9xZvDE4aoD@protected.localdomain> (message from Jean Louis
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> Date: Fri, 11 Jun 2021 18:33:45 +0300
> From: Jean Louis <bugs@gnu.support>
> Cc: help-gnu-emacs@gnu.org
> 
> (defun underline-line ()
>   "Underline the current line."
>   (interactive)
>   (let* ((start (line-beginning-position))
> 	 (end (line-end-position))
> 	 (length (string-width (buffer-substring-no-properties start end))))
>     (end-of-line)
>     (newline)
>     (insert (make-string length ?=))
>     (newline)))

Right.  I suggest to try the alternative that uses current-column, it
will allow you to avoid consing a string, and I think will also be
faster.