Excess vertical space when using booktabs cmidrule in math align environment

Question

I'm using booktabs cmidrule in a math align environment. It's working but it's producing too much vertical space. How can I reduce that vertical space.

Here's a simple demo

\documentclass{article}

\usepackage{amsmath}
\usepackage{booktabs}

\begin{document}

\begin{align*}
  % dummy placeholder equations. highlight spacing
  1 + 1 &= 2 \\
  1 + 1 &= 2 \\
  \cmidrule{1-2}
  2 + 2 &= 4 \\
  2 + 2 &= 4 \\
\end{align*}

\end{document}

Answer 1

In a way, I'm surprised that \cmidrule works at all outside of tabular-like and array environments. The fact the use of \cmidrule doesn't throw an immediate error message is not an indication that what you're doing can or should be expected to produce the result you're hoping to get.

If you must use \cmidrule, do typeset the equations in question in an array environment.

\documentclass{article}
\usepackage{array,amsmath,booktabs}
\newcolumntype{L}{>{\displaystyle}l}
\newcolumntype{R}{>{\displaystyle}r}

\begin{document}

\begin{minipage}[t]{0.22\textwidth}
\texttt{array} approach
\[
\renewcommand{\arraystretch}{1.28} % emulate display-style math setting
\begin{array}{@{} R @{{}={}} L @{}}
  1 + 1 & 2 \\
  1 + 1 & 2 \\
  \cmidrule{1-2}
  2 + 2 & 4 \\
  2 + 2 & 4 
\end{array}
\]
\end{minipage}\quad
\begin{minipage}[t]{0.22\textwidth}
\texttt{align*} approach
\begin{align*}
  1 + 1 &= 2 \\
  1 + 1 &= 2 \\
  \cmidrule{1-2}
  2 + 2 &= 4 \\
  2 + 2 &= 4 \\
\end{align*}
\end{minipage}
\end{document}

Answer 2

If you want to reduce vertical space in this example or in a similar case add negative space before, for example with \\[-2ex]

In addition to the excessive space you notice booktabs adds vertical space both sides of the \cmidrule so it is harder to correct.

\documentclass{article}

\usepackage{amsmath}
%\usepackage{booktabs}

\begin{document}

\begin{align*}
  % dummy placeholder equations. highlight spacing
  1 + 1 &= 2 \\
  1 + 1 &= 2 \\[-1.5ex]
  \cline{1-2}
  2 + 2 &= 4 \\
  2 + 2 &= 4 \\
\end{align*}

\end{document}