\tracingnone resets too much

[PhelypeOleinik]

Brief outline of the enhancement

Currently \tracingnone resets all TeX's tracing parameters to zero, which may be a nuisance if a document wants them on throughout, so changing \tracingnone should restore the tracing parameters to what they were before the \tracingall instead.

Minimal example showing the current behaviour

See how \nullfont b doesn't throw an error (whereas it probably should due to the setting in the preamble).

\RequirePackage{latexbug}
\documentclass{article}
\tracinglostchars=3 % I want \tracinglostchars=3 throughout the document
\begin{document}
\nullfont a

\tracingall
% <some debugging>
\tracingnone

\nullfont b % no longer errors
\end{document}

Minimal example showing the desired new behaviour

\tracingall/none could take an approach similar to what \ExplSyntaxOn/Off do for catcodes: Normally \tracingnone is a no-op, then \tracingall defines \tracingnone on-the-fly to reset the tracing parameters to what they were when \tracingall was issued (with proper handling of two consecutive \tracingall). Here's a sample for some of the parameters:

\RequirePackage{latexbug}
\documentclass{article}
\tracinglostchars=3 % I want \tracinglostchars=3 throughout the document

\let\tracingnone\relax
\def\tracingall{%
  \edef\tracingnone{%
    \tracinglostchars=\the\tracinglostchars\relax
    \tracingmacros=\the\tracingmacros\relax
    \tracingonline=\the\tracingonline\relax
    % and more
    \let\noexpand\tracingnone\relax
  }
  \tracinglostchars=3
  \tracingmacros=2
  \tracingonline=1
  % and more
}

\begin{document}
\nullfont a

\tracingall
% <some debugging>
\tracingnone

\nullfont b % no longer errors
\end{document}

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[eg9]

\tracingnone does what the name implies. Maybe something like \tracinglostchars might be retained, because it's a somewhat different kind of tracing.

Maybe \tracingrevertstatus?

I don't think it's a very important subject anyway: if one needs \tracingall, they're in trouble, but probably know what tracing is about.

[muzimuzhi]

Or introducing a new pair of commands \TracingAllOne and \TracingAllOff? Currently trace package provides \traceon and \traceoff.

[FrankMittelbach]

I'm not a fan of introducing new command names for tracing that aren't really going to provide much benefit if any. I asked for not resetting tracinglostchars simply because until now \tracingnone was reverting the tracing to the LaTeX default state while after the change done some time ago it reverted to the initex state which in the LaTeX situation is not useful.
In my opinion it should revert to whatever LaTeX considers the default non-tracing status even if technically it is not exactly "none".

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[FrankMittelbach]

This will get resolved with #1687 so we can close here