Add more engine names as options to latexmk

[ehudbehar]

Continuing issue #2683, I added 3 more lines to the documentation that describe how to compile a plain tex document with either pdftex, xetex or luatex.

[lervag]

Thanks!

[lervag]

Sorry, I realize I made an error here. The documentation we changed did not list an example; instead, it displays the default value of the option. We should either 1) revert this change, or 2) add the added lines to the default value as well. What do you think?

[user202729]

Just noticed this issue.

I think adding to the default value is a fine option.

Unrelated remark, I think that latexmk does not "officially" support running anything other than LaTeX
https://www.overleaf.com/learn/latex/Questions/Can_I_run_plain_TeX_on_Overleaf%3F

so "telling" latexmk that "to call the pdflatex compiler call pdftex instead" is a workaround.

That having said, there may be some performance benefit to "fake" xetex for xelatex instead of for pdflatex: https://stackoverflow.com/a/57734531/5267751 -- testing is needed.

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Note that this will be a compatibility break because currently %! TEX program = luatex will compile it with lualatex; if the change is successfully used it will become luatex.

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Another issue is, many features of vimtex is "hard coded" for LaTeX, so for example after the change is made you may have the following a.tex file

%! TEX program = pdftex
%! TEX root = a.tex

\iffalse
\documentclass{article}
\begin{document}
\fi

123

456
\bye

With %! TEX program = pdftex it works well, however vimtex's main file detection will make it not work unless there's the (fake) documentclass+begin{document} line in the file, or (better) TeX root directive line.

[lervag]

Thanks for the interesting remarks, @user202729. I think I want to revert the change instead of adding to the default list. My main point is that the option already allows to change these things for those who want/need them. This group of people is clearly small (as I have never had any similar requests before this one).