Side by side minipage and varwidth

Question

I'm looking for a way to combine the minipage and varwidth environments side by side, so that the width of minipage automatically adapts to that of varwidth, to fill the entire line.

\begin{minipage}{\linewidth-\varwidth}
Content of minipage.
\end{minipage}\hfill%
\begin{varwidth}{.5\linewidth}
Content of varwidth.
\end{varwidth}

A two-step compilation may be required, but thanks for any suggestion. I don't know how to get the width of varwidth.

Answer 1

The \MeasureVarwidth[<maximum width of varwidth>]{<text>} command will measure the natural width of a text and save the content for later use.

If the measured content of text is greater than the optional <maximum width of varwidth> (default= 0.5\linewidth), the width of the varwidth part will be set to that value.

The width of the minipage will then be set to fill the entire line.

The minipages were enclosed in a \fbox for better visualization.

\documentclass{article}
\usepackage{showframe} % show the margins
\usepackage{calc}% needed 
\usepackage{varwidth}
\newsavebox{\ToVarwidth}
\newlength{\WidthofVarwidth}
\newlength{\miniwidth}
% save box and measure its width
\newcommand{\MeasureVarwidth}[2][0.5\linewidth]{% #1 optional max width of varwidth, #2 varwidth text
    \savebox{\ToVarwidth}{#2}%

    \settowidth{\WidthofVarwidth}{\usebox{\ToVarwidth}}%
    \setlength{\miniwidth}{\linewidth-\WidthofVarwidth}%
    \ifdim\WidthofVarwidth>#1%
    \savebox{\ToVarwidth}{\parbox{#1}{#2}}%

    \setlength{\miniwidth}{\linewidth-#1}%
    \fi
}
\begin{document}
\setlength{\fboxsep}{0pt}% only for \fbox
\MeasureVarwidth{Some text}
\noindent\fbox{%% \fbox only to visualize 
    \begin{minipage}{\miniwidth} 
        Content of minipage.
    \end{minipage}%
            }\hfill
    \begin{varwidth}{\linewidth}
    \usebox{\ToVarwidth}
    \end{varwidth}
\bigskip
\MeasureVarwidth{Some text Some text Some text}
\noindent\fbox{%
    \begin{minipage}{\miniwidth}
        Content of minipage.
    \end{minipage}%
    }\hfill
    \begin{varwidth}{\linewidth}
        \usebox{\ToVarwidth}
    \end{varwidth}
\bigskip
\MeasureVarwidth{Some text   Some text  Some text Some text  Some text Some text Some text }
\noindent\fbox{%
    \begin{minipage}{\miniwidth}
        Content of minipage.
    \end{minipage}%
}\hfill
\begin{varwidth}{\linewidth}
    \usebox{\ToVarwidth}
\end{varwidth}
\bigskip
Changing the maximum width of \verb|varwidth| \medskip
\MeasureVarwidth[0.7\linewidth]{Some text    Some text  Some text Some text  Some text Some text Some text }
\noindent\fbox{%
    \begin{minipage}{\miniwidth}
        Content of minipage.
    \end{minipage}%
}\hfill
\begin{varwidth}{\linewidth}
    \usebox{\ToVarwidth}
\end{varwidth}
\end{document}

Answer 2

First possibility

\documentclass{article}
\usepackage{varwidth}
\usepackage{lipsum}
\ExplSyntaxOn
\NewDocumentEnvironment{doubleminipage}{+b}
 {
  \mbfore_doubleminipage:n { #1 }
 }{}
\seq_new:N \l__mbfore_doubleminipage_parts_seq
\cs_new_protected:Nn \mbfore_doubleminipage:n
 {
  \seq_set_split:Nnn \l__mbfore_doubleminipage_parts_seq { \BREAK } { #1 }
  \hbox_set:Nn \l_tmpa_box
   {
    \begin{varwidth}{0.5\linewidth}
    \seq_item:Nn \l__mbfore_doubleminipage_parts_seq { 2 }
    \end{varwidth}
   }
  \noindent
  \begin{minipage}{\dim_eval:n { \linewidth - \box_wd:N \l_tmpa_box - 1em }}
  \seq_item:Nn \l__mbfore_doubleminipage_parts_seq { 1 }
  \end{minipage}
  \hfill
  \box_use:N \l_tmpa_box
 }
\ExplSyntaxOff
\begin{document}
\begin{doubleminipage}
\lipsum[1][1-4]
\BREAK
Some contents \
on the right
\end{doubleminipage}
\end{document}

Second possibility

\documentclass{article}
\usepackage{varwidth}
\usepackage{lipsum}
\ExplSyntaxOn
\NewDocumentEnvironment{doubleminipage}{m}
 {
  __mbfore_doubleminipage:n { #1 }
  \noindent
  \begin{minipage}{\dim_eval:n { \linewidth - \box_wd:N \l_tmpa_box - 1em }}
 }
 {
  \end{minipage}\hfill\box_use:N \l_tmpa_box
}
\cs_new_protected:Nn __mbfore_doubleminipage:n
 {
  \hbox_set:Nn \l_tmpa_box
   {
    \begin{varwidth}{0.5\linewidth}
    #1
    \end{varwidth}
   }
 }
\ExplSyntaxOff
\begin{document}
\begin{doubleminipage}{
  Some contents \
  on the right
}
\lipsum[1][1-4]
\end{doubleminipage}
\end{document}

Output for both

Note

I used a 1em gutter in order to separate the two boxes.