Precision copyfitting in xcoffin

Question

Further indulging my curiosity with xcoffins, I'm playing with xcoffins-tschichold-example.tex.

In the TeX reconstruction of the front page of Elementare Typographie, the text is slightly misaligned with respect to the rule. This is obviously due to the gap (sorry, I don't know the technical term) between the right-hand edge of the glyph outline and the right-hand edge of its bounding box.

Now, in ``normal'' justified text, this gap is, I think, suppressed. How can I suppress it in this case, without losing the ability of fitting the coffin to the text automagically, and, preferably, not having to worry about wrapping individual letters (such as the n in this case) in TeX commands?

(PS I now know they're called sidebearings; thanks to @egreg.)

Answer 1

Here's how I got what I wanted (XeTeX only), thanks to a tip from @egreg. First the image, then the code.

\documentclass{article}
\usepackage{fontspec}
\usepackage{xcoffins}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand{\WidthOfCoffin}{m}{ \box_wd:N #1 }
\ExplSyntaxOff
\setsansfont{NimbusSanL}
\pagestyle{empty}
\def\kright#1{\leavevmode #1\kern-\XeTeXglyphbounds3 \the\XeTeXcharglyph`#1 }
\def\kleft#1{\leavevmode \kern-\XeTeXglyphbounds1 \the\XeTeXcharglyph`#1 #1}
\begin{document}
\NewCoffin \result
\SetHorizontalCoffin \result {}
\NewCoffin \aaa
\SetHorizontalCoffin \aaa {\fontsize{52}{50}\sffamily\bfseries mitteilungen}
\NewCoffin \rulei
\SetHorizontalCoffin \rulei  {\color{red}\rule{\WidthOfCoffin{\aaa}}{1pc}}
\NewCoffin \bbb
\SetHorizontalCoffin \bbb {\fontsize{52}{50}\sffamily\bfseries \kleft{m}itteilunge\kright{n}}
\NewCoffin \ruleii
\SetHorizontalCoffin \ruleii  {\color{red}\rule{\WidthOfCoffin{\bbb}}{1pc}}
\JoinCoffins \result \aaa 
\JoinCoffins \result[\aaa-t,\aaa-l] \rulei [b,l](0pt,2mm)
\JoinCoffins \result[\aaa-b,\aaa-l] \bbb [t,l](0pt,-10mm)
\JoinCoffins \result[\bbb-t,\bbb-l] \ruleii [b,l](0pt,2mm)
\TypesetCoffin \result
\end{document}