Misplaced \noalign because \input before booktabs rule

Question

In a large project, I reuse with \input tabular rows stored as child files. This has worked perfectly in the past, but in my texlive 2021 this fail when a booktabs rule follow the input. MWE:

\documentclass{article}
\usepackage{booktabs}
\begin{document}
\begin{tabular}{ccc}
\toprule
    1 & 2 & 3\\
\midrule    
\input{foo}
\bottomrule % This causes the error
\end{tabular}
\end{document}

Where foo.tex is simply a & b & c \\.

It fails also ending the tabular rows with \tabularnewline or \cr. The funny point is that \bottomrule works fine inside foo.tex (a & b & c \\\bottomrule) but this is not good a solution because foo.tex could be needed in another table, and not necessarily as the last row. AFAIK, unlike \include, \input add strictly the content of the file, so it should be irrelevant if \bootomrule is inside or outside the child file, but is it not. I wonder if there is a better fix that \input{foot}\\[-12pt]\bottomrule.

Answer 1

Your issue is the same as here and similar to this one. Newer \input adds unexpandable tokens in the way of \noalign (from \bottomrule), then when the \noalign is seen it is too late and you get the error. With no changes to the document body, you can add this to your preamble:

\ExplSyntaxOn
\cs_new:Npn \expandableinput #1
  { \use:c { @@input } { \file_full_name:n {#1} } }
\AddToHook{env/tabular/begin}
  { \cs_set_eq:NN \input \expandableinput }
\ExplSyntaxOff

The code above will redefine \input to be \expandableinput in all tabular environments. To add that to more environments, simply replicate the two lines and change the environment name:

\AddToHook{env/tabularx/begin} % also for tabularx
  { \cs_set_eq:NN \input \expandableinput }
\AddToHook{env/tabulary/begin} % and for tabulary
  { \cs_set_eq:NN \input \expandableinput }

Answer 2

I will provide an alternative solution with tabularray package of version 2022B (2022-06-01), by using evaluate option in functional library:

\begin{filecontents*}[overwrite]{foo.tex}
  a & b & c \\
\end{filecontents*}
\documentclass{article}
\usepackage{tabularray}
\UseTblrLibrary{booktabs,functional}
\begin{document}
\begin{booktabs}[evaluate=\fileInput]{ccc}
\toprule
  1 & 2 & 3\
\midrule
  \fileInput{foo}
\bottomrule
\end{booktabs}
\end{document}

where \fileInput function is provided by functional library.