\leaders in redefining \headrule cause "A <box> was supposed to be here"

Question

After stripping away hundreds of line of code to reach this MWE, I found that my "A «box» was supposed to be here" seemed to be caused by a \renewcommand{\headrule} in conjunction with fancyhdr.

Moreover, the error goes away if I delete \leaders from the \box specification.

This redefinition was something that I cobbled together at some point, undoubtedly inspired by examples I didn't fully understand.

I'd appreciate insight into why I'm getting this error and how to rewrite it to squash that error.

\documentclass[12pt,final]{article}
\usepackage{fancyhdr}
\pagestyle{fancy}
\renewcommand{\headrule}{%
        \hbox to\headwidth{\leaders\rule[5pt]{\headwidth}{0.4pt}\hfill}%
%       \hbox to\headwidth{\rule[5pt]{\headwidth}{0.4pt}\hfill}%
    }%
\begin{document}
Hello, world.
\end{document}

P.S., the error message isn't really "«box»" but rather the « and » should be replaced with less-than and greater-than signs. Those were rejected when I entered them here.

Answer 1

Note that the rule is set in vertical mode and that's where you're running into problems with your \hbox to \headwidth (which is similar to \makebox[\headwidth]) approach.

This is what fancyhdr's \headrule looks like (using \show\headrule):

\def\headrule{{\if@fancyplain\let\headrulewidth\plainheadrulewidth\fi
    \hrule\@height\headrulewidth\@width\headwidth
    \vskip-\headrulewidth}}

Apart from the condition an \hrule is inserted and the rule thickness is underdone (vertically). I would just replicate the behaviour using something like

\makeatletter
\renewcommand{\headrule}{%
  \vskip -5pt % Move head rule up by 5pt
  \hrule \@height 0.4pt \@width \headwidth % Place head rule
  \vskip \dimexpr-0.4pt + 5pt\relax % Remove space inserted by raise and rule
}
\makeatother