How to comment a string in restructured text?

I came across this thread, looking for a more defined way to place comments in restructuredtext. Personally, I also of course don't like the one-liner .. this is a comment. To keep comments searchable and recognizable, I propose to consider using

.. only:: comment

    This is a comment

As documented (http://www.sphinx-doc.org/en/master/usage/restructuredtext/directives.html): "Undefined tags are false", such as comment.

Alternatively, one could write an extension in the style of todo, allowing syntax, such as

.. comment::
    This is a comment

Doing so without such extension of course gives error messages from the builder. But with such extension, just like todo, it would be possible to extract a list of comments from a document.

Please forgive this duplicative answer cos I'm trying to help RST newbies like me. My answer shows the CONTEXT of a comment.

I naively tried marking a line in my RST document using the answer above, DO NOT DO THIS:

    Lorem ipsum dolor sit amet, consectetur adipiscing elit.
    .. Hi everyone this line will never be seen
    Sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.

Sphinx (or other RST formatter) will not complain, but "hi everyone" will appear in the output. Instead place a blank line before and after your comment like this:

    Lorem ipsum dolor sit amet, consectetur adipiscing elit.

    .. 
        comment Hi everyone this line will never be seen

    Sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.

But the downside of this is that the paragraph is ended then restarted, so you will have whitespace between.

I have not found any equivalent in RST for the C /* */ or HTML <!-- --> comment syntax that can make some text vanish totally.

From the reference:

Arbitrary indented text may follow the explicit markup start and will be processed as a comment element.

..
   _This: is a comment!

..
   [and] this!

..
   this:: too!

..
   |even| this:: !

It is also possible to put the comment on the same line as the double dots:

.. Avoid this type of comment

This is however considered bad practice since it may lead to unintended consequences if the comment matches a proper markup construct, as pointed out by @CecilCurry in the comment below.

For comments, add 2 periods .. followed by a newline and then your comment indented.

Example:

..
  comment goes here