\box\@labels and macrocode environment

labels.tex

% \iffalse
\documentclass{article}
\usepackage{doc}
\begin{document}
  \DocInput{\jobname}
\end{document}
% \fi
%
% \makeatletter
% \tracingonline1
% \showboxdepth10000
% \showboxbreadth10000
%
% \showbox\@labels  ^^A -> void
%
%    \begin{macrocode}
\def\MACRO{}
%    \end{macrocode}
%
% \showbox\@labels  ^^A -> non-void???
%
% \begin{itemize}
% \showbox\@labels  ^^A -> non-void???
% \item a
% \showbox\@labels  ^^A -> void
% \item b
% \showbox\@labels  ^^A -> void
% \end{itemize}
%
% \showbox\@labels  ^^A -> void
%
\endinput

Issue with pLaTeX:

% \iffalse
\documentclass{article}
\usepackage{doc}
\usepackage{plext}
\begin{document}
  \DocInput{\jobname}
\end{document}
% \fi
%
%    \begin{macrocode}
\def\MACRO{}
%    \end{macrocode}
%
% \begin{minipage}<t>{10zw}
% The option `t' starts vertical writing.
% \begin{itemize}
% \item a
% \end{itemize}
% \end{minipage}
%
\endinput

! Incompatible direction list can't be unboxed.
\@item ...setbox \@labels \hbox {\unhbox \@labels 
                                                  \hskip \itemindent \hskip ...
l.17 % \item a
              
? 

https://tex.stackexchange.com/questions/548162/