Vertical line from top margin of page to current positon

Question

I'm struggling with this really simple (vanilla plainTeX) trick, which I'm not able to crack...

How does one draw a vertical (i.e. perpendicular to the horizon) line that leads from the top (or top margin or as topmost as possible) of the current page to the current position (specifically the position of text at the time where the macro solving this problem is called)?

P. S. It may have unclear but the current position is meant including both the horizontal and vertical position. I. e.

Here I \myruler am...

Another paragraph...

would produce sth like

       |
       |
       |
       |
       |
Here I | am...

Another paragraph...

Answer 1

One possibility is to use the \pagetotal dimension which holds the accumulated natural height of the current page:

\newdimen\ruleht
\def\myrule{\par
  \ruleht=\pagetotal
  \vskip-\the\ruleht\llap{\vrule height\the\ruleht}
}

\def\Text{%some filler text for the example
text text text text text text text text text text text text text text text text 
text text text text text text text text text text text text text text text text 
text text text text text text text text text text text text text text text text 
text text text text text text text text text text text text text text text text 
text text text text text text text text text\par}

\Text\Text\myrule

\bye

Answer 2

Building on Gonzalo's answer, maybe something like

\hsize=24pc
\def\lipsum{Lorem ipsum dolor sit amet. }
\def\llipsum{\lipsum\lipsum}
\def\lipsumV{\llipsum\llipsum\lipsum}
\def\pgtotrule{\dimen0=\pagetotal
  \advance\dimen0 by\ht\strutbox
  \advance\dimen0 by\baselineskip
  \smash{\vrule height\dimen0 depth\dp\strutbox}}

\lipsumV
\lipsumV

\llipsum \lipsum \pgtotrule \lipsumV

\bye