Automatically set description list `labelwidth` based on widest label?

Question

How do I create a description list where the item text is automatically indented to the width of the widest label?

This is the same question as Description list with aligned descriptions but I would like the width of the widest label to be automatically calculated.

For example, I'd like the following output:

to be produced by the following input:

\documentclass{article}
\usepackage{calc}
\usepackage{enumitem}
\usepackage{lipsum}
\usepackage[margin=1em,papersize={4in,2.2in}]{geometry}

\newenvironment{mydescription}{%
  \begin{description}[
    leftmargin=!,
    labelwidth=\magicgoeshere,
    ]%
}{%
  \end{description}%
}

\newcommand{\text}{long long long long long long long long long long
  long long long long long long long long long long long long long
  long long long long long long long text}

\begin{document}
\begin{mydescription}
\item[The longest label] text
\item[Short] \text
\end{mydescription}
\hrule
\begin{mydescription}
\item[Medium label] text
\item[Short] \text
\end{mydescription}
\end{document}

Answer 1

A variation on egreg's answer to Working with margins

\documentclass{article}
\usepackage{enumitem}
\usepackage{environ}

\newlength\widest
\makeatletter
\NewEnviron{ldescription}{%
  \vbox{%
    \global\setlength\widest{0pt}%
    \def\item[##1]{%
      \settowidth\@tempdima{\textbf{##1}}%
      \ifdim\@tempdima>\widest\global\setlength\widest{\@tempdima}\fi%
    }%
    \setbox0=\hbox{\BODY}%
  }
  \begin{description}[
    leftmargin=\dimexpr\widest+0.5em\relax,
    labelindent=0pt,
    labelwidth=\widest]
  \BODY
  \end{description}%
}
\makeatother

\begin{document}

\begin{ldescription}
\item[Short] 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
\item[A really really long label] 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
\end{ldescription}

\begin{ldescription}
\item[Short] 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
\item[A medium label] 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
\end{ldescription}

\end{document}

In the first part, the width of the longest label is calculated: initially, \widest is set to 0pt; for each \item, the length of its optional argument is measured and the length stored in \@tempdima; if \@tempdima is greater than \widest, (this is always true for the first \item) then \widest is updated to be \@tempdima. This is done for all \items; \BODY is set inside a box that is never used. Then, a description environment is used, setting \labelwidth to the previously calculated value for \widest.

Answers to follow-up questions:

• Q: What is the point of \setbox0=\hbox{\BODY}?
  A: This just boxes \BODY without typesetting it.
• Q: Since \BODY is used twice, what happens if \BODY contains something that has side effects (writes to the .aux file, contains \newcommand, contains \footnote, etc.)?
  A: There's no problem with what \BODY contains (as long as it's reasonable for a standard description, so, for example, sectional unit commands won't be allowed, but they don't make sense in a description anyways).
• Q: Why is that stuff wrapped in a \vbox{}?
  A: The \vbox is used to prevent the dreadful "Something's wrong--perhaps a missing \item." error at the beginning of a list (\hbox or \mbox could have also been used instead).