Show reference label when it is undefined

Question

When a reference is undefined, LaTeX shows simply [??] which doesn't give much information. I would like it to show the label instead. This is useful when more collaborators work on the same preprint and the references are yet to be done.

I'm using neither biblatex nor natbib. I do use bibtex but it is irrelevant for this question.

MWE:

\documentclass{article}

\begin{document}

This: \cite{undef}
should look like: [{\bfseries ?? undef}]

\end{document}

Answer 1

The definition of \cite calls an internal \@citex that checks the citation and adds an \hbox{\reset@font\bfseries ?} for undefined citations. Changing it to call the argument to \cite instead of the question mark works fine.

\makeatletter
% latex.ltx, line 6121:
\def\@citex[#1]#2{\leavevmode
  \let\@citea\@empty
  \@cite{\@for\@citeb:=#2\do
    {\@citea\def\@citea{,\penalty\@m\ }%
     \edef\@citeb{\expandafter\@firstofone\@citeb\@empty}%
     \if@filesw\immediate\write\@auxout{\string\citation{\@citeb}}\fi
     \@ifundefined{b@\@citeb}{\hbox{\reset@font\bfseries 
     \expandafter\strip@prefix\meaning\@citeb}%
       \G@refundefinedtrue
       \@latex@warning
         {Citation `\@citeb' on page \thepage \space undefined}}%
       {\@cite@ofmt{\csname b@\@citeb\endcsname}}}}{#1}}
\makeatother

A much cleaner way to redefine \@citex is by patching it with xpatch:

\usepackage{xpatch}
\makeatletter
\xpatchcmd{\@citex}{\bfseries ?}{\bfseries\expandafter\strip@prefix\meaning\@citeb}{}{}
\makeatother

MWE:

\documentclass{article}
% \usepackage{lmodern}
\usepackage[T1]{fontenc}
\makeatletter
% latex.ltx, line 6121:
\def\@citex[#1]#2{\leavevmode
  \let\@citea\@empty
  \@cite{\@for\@citeb:=#2\do
    {\@citea\def\@citea{,\penalty\@m\ }%
     \edef\@citeb{\expandafter\@firstofone\@citeb\@empty}%
     \if@filesw\immediate\write\@auxout{\string\citation{\@citeb}}\fi
     \@ifundefined{b@\@citeb}{\hbox{\reset@font\bfseries 
     \expandafter\strip@prefix\meaning\@citeb}%
       \G@refundefinedtrue
       \@latex@warning
         {Citation `\@citeb' on page \thepage \space undefined}}%
       {\@cite@ofmt{\csname b@\@citeb\endcsname}}}}{#1}}
\makeatother
\begin{document}
\null\vfill\thispagestyle{empty}

This: \cite{undef} should look like: [{\bfseries ?? undef}]

This one \cite{article-minimal} shouldn't.

Ugly citation keys with special chars like \cite{_$^/:&!@*(-=+)} 
also should be printed.

\bibliography{xampl.bib}
\bibliographystyle{plain}
\end{document}

^{This answer is sponsored by texdef and (very!) special comments by tohecz and Gonzalo Medina :)}

Answer 2

biblatex provides this functionality by default:

\documentclass{article}
\usepackage{biblatex}% http://ctan.org/pkg/biblatex
\begin{document}
See~\cite{author}.
\end{document}