Vertical space at the bottom of nested tabulars

I’m trying to make a macro which typeset a text "accross a vertical line", and I’m using a tabular for that :

documentclass{article}

begin{document}

makeatletter

newlengthtdv@temp

newcommandcenter@box[3]{parbox[c][#1][c]{#2}{makebox[#2][c]{#3}}} 

newcommandtxt@value@on@bar[1]{%
  settowidth{tdv@temp}{$#1$}%
  begin{tabular}{@{hspace{0pt}}c@{hspace{0pt}}}%
    rule{5pt}{8mm}%
    center@box{8mm}{tdv@temp}{#1}%
    rule{5pt}{8mm}%[-4pt]%
  end{tabular}%
}

begin{tabular}{|c|c|}
  hline
  $1$ & $2$ 
  hline
  txt@value@on@bar{0} & 2 
  hline  
end{tabular}

end{document}

There’s a tiny space at the bottom, which I think is related to arraystretch.

But if I set defarraystretch{0}, the first line is stretched (at the very worst, I could put a strut there), and if I try to uncomment the [-4pt], the cells are not aligned vertically anymore.