begin{equation*}
xymatrix{
    A ar[r] & B = {c in C mid f(c) = 0 }
    \
    a ar@{|->}[r] & b
}
end{equation*}

begin{equation*}
xymatrix{
    A ar[r] & B & = {c in C mid f(c) = 0 }
    \
    a ar@{|->}[r] & b
}
end{equation*}

documentclass{minimal}
usepackage{xypic,mathtools}
begin{document}

begin{equation*}
xymatrix{
    A ar[r] & mathrlap{B = {c in C mid f(c) = 0 }}
    \
    a ar@{|->}[r] & b
}
end{equation*}

end{document}

begin{equation*}
  xymatrix{
    A ar[r] & B &
      save[]+<1.75em,0em>*{= {c in C mid f(c) = 0 }}restore  \
    a ar@{|->}[r] & b
  }
end{equation*}

begin{equation*}
xymatrix@C=-2pt{
    A ar[rr] &qquad& B &= {c in C mid f(c) = 0 }
    \
    a ar@{|->}[rr]& & b
}
end{equation*}

% arara: pdflatex

documentclass{article}
usepackage{mathtools}
usepackage{tikz-cd}
usepackage{calc}

begin{document}
    [
    begin{tikzcd}
    A ar{r} & mathmakebox[widthof{$b$}][l]{B = {c in C mid f(c) = 0 }}\
    a ar[mapsto]{r}& b
    end{tikzcd}
    ]
end{document}