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.