Added an autowidest key

enumitem.sty

@@ -12,6 +12,8 @@
 \NeedsTeXFormat{LaTeX2e}
 \ProvidesPackage{enumitem}[2019/06/20 v3.9 Customized lists]
 
+\RequirePackage{eqparbox}% Needed by autowidest
+
 % Notes
 % ~~~~~
 %
@@ -145,6 +147,7 @@
 \newif\ifenit@sepfrommargin
 \newif\ifenit@lblfrommargin
 \newif\ifenit@calcwidest
+\newif\ifenit@autowidest
 \newif\ifenit@nextline
 \newif\ifenit@boxdesc
 
@@ -277,6 +280,7 @@
   \fi\fi}
 
 \def\enitkv@enumitem@widest#1{%
+  \enit@autowidestfalse% Manually using widest overrides autowidest
   \ifcase\enit@type  % enumerate
     \expandafter\let\csname enit@cw@\@enumctr\endcsname\relax
     \@namedef{enit@widest@\@enumctr}##1{\enit@format{#1}}%
@@ -289,12 +293,34 @@
   \expandafter\let\csname enit@widest@\@enumctr\endcsname\relax}
 
 \enitkv@key{}{widest*}{%
+  \enit@autowidestfalse% Manually using widest* overrides autowidest
   \setcounter{enit@cnt}{#1}%
   \expandafter\edef\csname enit@cw@\@enumctr\endcsname
     {\the\c@enit@cnt}%
   \expandafter\edef\csname enit@widest@\@enumctr\endcsname##1%
     {##1{\the\c@enit@cnt}}}
 
+\enitkv@key{}{autowidest}[]{%
+	% \eqboxwidth gets the widest width of the calls to \eqp@compute@width with the same argument
+	\enitkv@setkeys{widest}{\hbox to \eqboxwidth{enit@autowidest@\EnumitemId}{}}
+	\enit@autowidesttrue}
+% \enit@autowidest needs to be called after \makelabel is set up properly
+% (thus it is not called as part of \enit@calcleft since that gets called in the wrong place for descriptions)
+\newcommand\enit@autowidest{%
+	\ifenit@autowidest\ifenit@calcwidest% autowidest only makes sense if used with an =* option
+		\let\enit@makelabel=\makelabel% Save the original \makelabel
+		% Override \makelabel so that it also keeps track of label widths
+		\def\makelabel##1{
+			\savebox\enit@inbox{\enit@makelabel{#1}}% Stop \eqp@compute@width from typseting this twice
+			% \eqp@compute@width will compute the widest width by typesting \usebox\enit@inbox
+			% and then returns \eqp@produce@box{<widest width>}{\usebox\enit@inbox}
+			% \eqp@produce@box is globally defined by each of eqparbox's various \eq commands so it's safe to modify
+			% It's modified below to simply ignored the <widest width>, as this will be handled by the
+			% widest key itself
+			\global\let\eqp@produce@box=\@secondoftwo%
+			\eqp@compute@width{enit@autowidest@\EnumitemId}{\usebox\enit@inbox}}
+	\fi\fi}
+
 \enitkv@key{}{labelindent*}{%
   \enit@lblfrommargintrue
   \ifnum\enit@calc=\z@
@@ -936,6 +962,7 @@
    \enit@tagid
    \enit@sepfrommarginfalse
    \enit@calcwidestfalse
+   \enit@autowidestfalse
    \let\enit@widest@\relax
    \let\enit@resuming\z@
    \let\enit@series\relax
@@ -1376,6 +1403,7 @@
        \enit@normlabel\@itemlabel\@itemlabel
        \enit@ref
        \enit@calcleft
+       \enit@autowidest
        \enit@before
        \enit@negwidth}%
     \enit@keyfirst
@@ -1413,6 +1441,7 @@
         \def\makelabel##1{\enit@align{\enit@format{##1}}}%
         \enit@preset{#2}{#1}{#4}%
         \enit@calcleft
+		\enit@autowidest
         \enit@before
         \enit@negwidth}%
     \enit@keyfirst
@@ -1461,6 +1490,7 @@
            \enit@svlabel{\enit@format{##1}}%
            \aftergroup\enit@postlabel
          \fi}%
+       \enit@autowidest% This needs to go after the \makelabel definition above
        \enit@before
        \enit@negwidth}%
      \enit@keyfirst

enumitem.tex

@@ -21,6 +21,7 @@
 \definecolor{notes}{rgb}{.75, .3, .3}%
 
 \makeatletter
+\expandafter\def\csname [email protected]\endcsname{} % Because it's loaded by eqparbox but breaks the decl environment
 \newenvironment{desc}
   {\if@nobreak
      \vskip-\lastskip
@@ -573,7 +574,7 @@ \subsection{Horizontal spacing of labels}
 \end{note}
 
 \begin{desc}
-|widest=<string>|\qquad|widest*=<integer>|\qquad|widest|
+|widest=<string>|\qquad|widest*=<integer>|\qquad|widest|\qquad|autowidest|
 \end{desc}
 
 To be used in conjunction with the \texttt{*}-values, if desired. It
@@ -597,6 +598,9 @@ \subsection{Horizontal spacing of labels}
 you might need change the computed parameter (eg, |itemindent=*| with 
 |align=left|).
 
+\New{3.X} You can use |autowidest| to have \textsf{enumitem}
+work out the actual widest label for you, however it requires at least two runs of \LaTeX.
+
 \begin{desc}
 |labelsep*=<length>|
 \end{desc}
@@ -863,21 +867,7 @@ \subsection{Injecting code}
 assigned to each list, returned by |\EnumitemId|.
 
 \begin{example}
-  Here is an example of how to combine a |\label| with |\EnumitemId|,
-  and |after| to automatically set the width of the list label to the
-  widest one (provided the ref is the same as the label):\footnote{See
-  \texttt{https://tex.stackexchange.com/questions/29322/%
-  how-to-make-enumerate-items-align-at-left-margin}.}
-\begin{verbatim}
-\SetEnumitemKey{widestlabel}
-  {labelwidth = \widthof{\ref{enum-\EnumitemId}},
-   after      = \label{enum-\EnumitemId}}
-\end{verbatim}
-Then just use the key |widestlabel|.
-\end{example}
-
-\begin{example}
-  Reverse counting is also doable, but somewhat trickier, and we need
+  Here is an example of reverse counting, but it is somewhat tricky, and we need
   some ``external'' help. Here is a possible solution, but not the
   only one (and very likely not even the best -- for example, |start|
   is in fact no-op).