Patch expl3.sty regression on texlive 2020

[dginev]

Fixes #1454 .

Now that I updated my texlive and can reproduce locally, I traced this issue to one of my older patches in Package::InputDefinitions which hardcoded a little too closely the LaTeX 2 behavior of a standalone Digest(T_CS('\@pushfilename')) while loading e.g. a new .sty package with definitions.

In LaTeX 3 that internal macro gets extended to do more token interplay with the rest of the input stream, so my standalone Digest call creates the errors reported in the issue - and I could reproduce those clearly.

However, the expl3 metadata fishing is a bit of a hack in all honesty: It relies on the exact placement of \@pushfilename in the original latex.ltx. You move that macro e.g. a line or two down, or worse - change the definition pattern on the line after - and it will completely break its behavior. So my instinct again was that we shouldn't be emulating that directly. Which lead me to the comment we have had in expl3.sty.ltxml:

# Redefine aux macro to avoid reliance on argument ordering artefacts in latex.ltx
# The purpose here was originally to set `\g_file_curr_name_str`,
#  but I am unsure if it is used anywhere currently?

Which deactivated the \@pushfilenameaux in previous texlives. I think the original point still holds, so I also deactivated the equally hacky \@expl@push@filename@aux@@, making the surface error disappear.

Happily, our expl3 tests had not regressed, and the core interpretation is still solid on texlive 2020 (for the degree we've tested it).

[brucemiller]

Ah, ok.