Shorten fraction line
I'll be happy to remove this but you could do
\documentclass{article}
\usepackage{mathtools}
\begin{document}
\[\frac{x^{\mathrlap{-}}}{2}\hphantom{\scriptstyle -}x\]
\end{document}
My eyes bleed with your proposal. Anyway
\documentclass{article}
\usepackage{amsmath}
\makeatletter
\newcommand{\fracto}[3]{%
{\mathpalette\frac@to{{#1}{#2}{#3}}}%
}
\newcommand{\frac@to}[2]{\frac@@to#1#2}
\newcommand{\frac@@to}[4]{%
% #1 = mathstyle
% #2 = full numerator
% #3 = denominator
% #4 = reduced numerator
\begingroup
\sbox\z@{$\m@th#1\frac{#2}{#3}$}%
\sbox\tw@{$\m@th#1\frac{#4}{#3}$}%
\settowidth\dimen@{$\m@th\frac@to@demote#1#4$}%
\frac{{}\makebox[\dimen@][l]{$\frac@to@demote#1#2$}}{#3}%
\kern-\wd\tw@
\kern\wd\z@
\endgroup
}
\newcommand\frac@to@demote[1]{%
\ifx#1\displaystyle\textstyle\else
\ifx#1\textstyle\scriptstyle\else
\scriptscriptstyle\fi\fi
}
\makeatother
\begin{document}
\begin{gather*}
X + \frac{x^{-}}{2} + \frac{x^{-}}{100} + X \\
X + \fracto{x^{-}}{2}{x} + \fracto{x^{-}}{100}{x} + X \\
\end{gather*}
\end{document}
First I typeset the standard fraction, then the same but with the “reduced numerator” in two boxes, so I can use their widths. I also measure the reduced numerator.
Then I typeset the fraction with the numerator having the same width as the reduced one (aligned left). This fraction is as wide as box 2, so I back up by this amount and reinstate the width of the standard fraction, which is the width of box 0.