\Mapsto in tikz-cd

I’ve taken a look at the \Mapsto arrow from the stmaryrd package which I have used as the model for the redefinition of the cm | arrow that obeys a double line.

The 3.3 value is approximated so that the vertical line is as wide as the implies arrow (similar to how \Mapsto looks). The original maps to arrow as seen in the example has a rather short bar.

Code

\documentclass[tikz]{standalone}
\usepackage{tikz-cd}
\expandafter\let\csname pgf@arrow@code@cm |\endcsname\relax
\makeatletter
\pgfarrowsdeclare{cm |}{cm |}{%
  \[email protected]\pgflinewidth
  \ifdim\pgfinnerlinewidth>0pt\relax
    \pgfmathaddtolength\pgfutil@tempdima{-.5*\pgfinnerlinewidth}%
  \fi
  \pgfarrowsleftextend{+-\pgfutil@tempdima}%
  \pgfarrowsrightextend{+\pgfutil@tempdima}%
}{%
  \ifdim\pgfinnerlinewidth=0pt\relax
    \pgfutil@tempdima4\pgflinewidth
  \else
    \pgfutil@tempdima\pgflinewidth
    \advance\pgfutil@tempdima-\pgfinnerlinewidth
    \pgfsetlinewidth{\pgfutil@tempdima/2}%
    \multiply\[email protected]\relax
  \fi
  \pgfsetdash{}{+0pt}%
  \pgfsetroundcap
  \pgfpathmoveto{\pgfqpoint{0pt}{-\pgfutil@tempdima}}%
  \pgfpathlineto{\pgfqpoint{0pt}{\pgfutil@tempdima}}%
  \pgfusepathqstroke
}
\makeatother
\pgfqkeys{/tikz/commutative diagrams}{
  Maps to/.style={
    /tikz/commutative diagrams/Rightarrow,
    /tikz/commutative diagrams/maps to},
  Maps from/.style={
    /tikz/commutative diagrams/Leftarrow,
    /utils/exec=\pgfsetarrowsend{cm |}}
}
\begin{document}
\begin{tikzcd}
  M  \rar[Maps from] \dar[maps to]  & N \dar[Maps to] \\
  M' \rar[Maps to]    & N'
\end{tikzcd}
\end{document}

Output

As of version 0.9c (if not earlier), tikz-cd now provides a Mapsto arrow.