I want to write with `tikz-cd` a map, in such a way that the two arrows are aligned and have the same length. I got this by adding `\phantom{}` everywhere, but I wonder if there's a easier (smarter) way.

My attemp:

```
\documentclass{article}
\usepackage{amsmath,amsthm,amssymb,amsfonts,amscd}
\usepackage{tikz}
\usepackage{tikz-cd}
\usetikzlibrary{arrows}

\begin{document}

\begin{tikzcd}[row sep = 0ex]
\tau : I \arrow[rightarrow]{r} & \mathbb{R} \phantom{---------} \\
\phantom{\tau :. }  t \arrow[mapsto]{r} & \tau (t) := \displaystyle \int_{t_0}^{t} \sqrt{T \cdot T} \ \mathrm{d} s
\end{tikzcd}
\end{document}

```

[![enter image description here](https://i.stack.imgur.com/7SIEN.png)](https://i.stack.imgur.com/7SIEN.png)