Hi,

there is a segfault produced by LuaTeX, Version 1.12.0 (TeX Live
2020/Debian) when processing the following:

\ifdefined\directlua
  \protected\def\pdfmapfile         {\pdfextension mapfile }
  \protected\def\pdfmapline         {\pdfextension mapline }
  \edef\pdfgentounicode             {\pdfvariable gentounicode}
  \protected\def\pdfglyphtounicode  {\pdfextension glyphtounicode }
\fi

\pdfglyphtounicode{A}{0041}
\pdfgentounicode=1
\pdfmapfile{}
\pdfmapline{cmr10 CMR10 34}  % no font embedding
anything
\end

PdfTex, Version 3.14159265-2.6-1.40.21 (TeX Live 2020/Debian) gives
the following assertion:

pdftex: ../../../texk/web2c/pdftexdir/writefont.c:555:
write_fontdictionary: Assertion `fo->fd->builtin_glyph_names != NULL'
failed.
Aborted

If the combination of using \pdfgentounicode and not embedding any
font is invalid, some reasonable error message should be produced.