Add an asterisk left of an enumerate

Question

I am looking for a simple way of adding an asterisk or some small image to the left of an enumerate such that the indentation of the enumeration is not affected by the latter. I have read Add asterisk after labels in enumerate and Add an asterisk in front of section label in TOC in which the latter has the effect am looking for.

Update: egreg's solution below is nice and does answer the problem. But I want the possible newcommand to be used without introducing an newenvironment like just having \moditem inside the enumerate and working with enumitem.

Answer 1

Just modify accordingly the modenumerate environment defined in Add asterisk after labels in enumerate

\documentclass[a4paper]{article}
\newenvironment{modenumerate}
  {\enumerate\setupmodenumerate}
  {\endenumerate}

\newif\ifmoditem
\newcommand{\setupmodenumerate}{%
  \global\moditemfalse
  \let\origmakelabel\makelabel
  \def\moditem##1{\global\moditemtrue\def\mesymbol{##1}\item}%
  \def\makelabel##1{%
    \origmakelabel{\ifmoditem\llap{\mesymbol\enspace}\fi##1}%
    \global\moditemfalse}%
}

\begin{document}
\begin{modenumerate}
\item uno
\item due
\moditem{*} tre
\item quattro
\end{modenumerate}
\begin{enumerate}
\item uno
\item due
\item tre
\item quattro
\end{enumerate}
\end{document}

Instead of * you can use anything you want, as long as it can be set into an \mbox. The following enumerate is just to show that the result is the same.

---

If you are using the enumitem package, the following code allows the use of \moditem in any enumerate environment:

\usepackage{enumitem}
\setlist[enumerate]{before=\setupmodenumerate}

\newif\ifmoditem
\newcommand{\setupmodenumerate}{%
  \global\moditemfalse
  \let\origmakelabel\makelabel
  \def\moditem##1{\global\moditemtrue\def\mesymbol{##1}\item}%
  \def\makelabel##1{%
    \origmakelabel{\ifmoditem\llap{\mesymbol\enspace}\fi##1}%
    \global\moditemfalse}%
}

If you don't use enumitem, then change the first two lines into

\usepackage{etoolbox}
\appto{\enumerate}{\setupmodenumerate}