Set \tracinglostchars to 4 or 5 in the LuaLaTeX initialization

[eg9]

Brief outline of the enhancement

In LuaLaTeX, with the current default value of \tracinglostchars, lost chars are not traced when a box is being built, but only when the box is used.

Background

https://tex.stackexchange.com/q/736811/4427

After discussion with mickep in the TeX.SE chat, he prompted Hans to add new values for \tracinglostchars to emulate pdfTeX behavior.

Minimal example showing the current behaviour

\RequirePackage{latexbug}
\documentclass{article}

\showboxbreadth=1000
\showboxdepth=1000

\begin{document}

\sbox0{WHAT\char"4444} \showbox0
\typeout{Width is \the\wd0}

\box0

\end{document}

Here's the log file

This is LuaHBTeX, Version 1.21.0 (TeX Live 2025)  (format=lualatex 2025.3.1)  2 MAR 2025 17:32
 restricted system commands enabled.
**testbox.tex
(./testbox.tex
LaTeX2e <2024-11-01> patch level 2
L3 programming layer <2025-01-18>
Lua module: luaotfload 2024-12-03 v3.29 Lua based OpenType font support
Lua module: lualibs 2023-07-13 v2.76 ConTeXt Lua standard libraries.
Lua module: lualibs-extended 2023-07-13 v2.76 ConTeXt Lua libraries -- extended 
collection.
luaotfload | conf : Root cache directory is "/Users/enrico2013/Library/texlive/2
025/texmf-var/luatex-cache/generic/names".
luaotfload | init : Loading fontloader "fontloader-2023-12-28.lua" from kpse-res
olved path "/usr/local/texlive/2025/texmf-dist/tex/luatex/luaotfload/fontloader-
2023-12-28.lua".
Lua-only attribute luaotfload@noligature = 1
luaotfload | init : Context OpenType loader version 3.134
Inserting `luaotfload.node_processor' in `pre_linebreak_filter'.
Inserting `luaotfload.node_processor' in `hpack_filter'.
Inserting `luaotfload.glyph_stream' in `glyph_stream_provider'.
Inserting `luaotfload.define_font' in `define_font'.
Lua-only attribute luaotfload_color_attribute = 2
luaotfload | conf : Root cache directory is "/Users/enrico2013/Library/texlive/2
025/texmf-var/luatex-cache/generic/names".
Inserting `luaotfload.harf.strip_prefix' in `find_opentype_file'.
Inserting `luaotfload.harf.strip_prefix' in `find_truetype_file'.
Removing  `luaotfload.glyph_stream' from `glyph_stream_provider'.
Inserting `luaotfload.harf.glyphstream' in `glyph_stream_provider'.
Inserting `luaotfload.harf.finalize_vlist' in `post_linebreak_filter'.
Inserting `luaotfload.harf.finalize_hlist' in `hpack_filter'.
Inserting `luaotfload.cleanup_files' in `wrapup_run'.
Inserting `luaotfload.harf.finalize_unicode' in `finish_pdffile'.
Inserting `luaotfload.glyphinfo' in `glyph_info'.
Lua-only attribute luaotfload.letterspace_done = 3
Inserting `luaotfload.aux.set_sscale_dimens' in `luaotfload.patch_font'.
Inserting `luaotfload.aux.set_font_index' in `luaotfload.patch_font'.
Inserting `luaotfload.aux.patch_cambria_domh' in `luaotfload.patch_font'.
Inserting `luaotfload.aux.fixup_fontdata' in `luaotfload.patch_font_unsafe'.
Inserting `luaotfload.aux.set_capheight' in `luaotfload.patch_font'.
Inserting `luaotfload.aux.set_xheight' in `luaotfload.patch_font'.
Inserting `luaotfload.rewrite_fontname' in `luaotfload.patch_font'.
Inserting `tracingstacklevels' in `input_level_string'. (/usr/local/texlive/202
5/texmf-dist/tex/latex/latexbug/latexbug.sty
Package: latexbug 2024/07/16 v1.0p Bug-classification
)
(/usr/local/texlive/2025/texmf-dist/tex/latex/base/article.cls
Document Class: article 2024/06/29 v1.4n Standard LaTeX document class
(/usr/local/texlive/2025/texmf-dist/tex/latex/base/size10.clo
File: size10.clo 2024/06/29 v1.4n Standard LaTeX file (size option)
luaotfload | db : Font names database loaded from /Users/enrico2013/Library/texl
ive/2025/texmf-var/luatex-cache/generic/names/luaotfload-names.luc.gz)
\c@part=\count194
\c@section=\count195
\c@subsection=\count196
\c@subsubsection=\count197
\c@paragraph=\count198
\c@subparagraph=\count199
\c@figure=\count266
\c@table=\count267
\abovecaptionskip=\skip49
\belowcaptionskip=\skip50
\bibindent=\dimen140
)
(/usr/local/texlive/2025/texmf-dist/tex/latex/l3backend/l3backend-luatex.def
File: l3backend-luatex.def 2024-05-08 L3 backend support: PDF output (LuaTeX)
\l__color_backend_stack_int=\count268
Inserting `l3color' in `luaotfload.parse_color'.
\l__pdf_internal_box=\box52
)
(./testbox.aux)
\openout1 = testbox.aux

LaTeX Font Info:    Checking defaults for OML/cmm/m/it on input line 8.
LaTeX Font Info:    ... okay on input line 8.
LaTeX Font Info:    Checking defaults for OMS/cmsy/m/n on input line 8.
LaTeX Font Info:    ... okay on input line 8.
LaTeX Font Info:    Checking defaults for OT1/cmr/m/n on input line 8.
LaTeX Font Info:    ... okay on input line 8.
LaTeX Font Info:    Checking defaults for T1/cmr/m/n on input line 8.
LaTeX Font Info:    ... okay on input line 8.
LaTeX Font Info:    Checking defaults for TS1/cmr/m/n on input line 8.
LaTeX Font Info:    ... okay on input line 8.
LaTeX Font Info:    Checking defaults for TU/lmr/m/n on input line 8.
LaTeX Font Info:    ... okay on input line 8.
LaTeX Font Info:    Checking defaults for OMX/cmex/m/n on input line 8.
LaTeX Font Info:    ... okay on input line 8.
LaTeX Font Info:    Checking defaults for U/cmr/m/n on input line 8.
LaTeX Font Info:    ... okay on input line 8.
> \box0=
\hbox(7.16+0.22)x34.47, direction TLT
.\TU/lmr/m/n/10 W
.\TU/lmr/m/n/10 H
.\TU/lmr/m/n/10 A
.\kern-0.83 (font)
.\TU/lmr/m/n/10 T
.\TU/lmr/m/n/10 䑄
.\TU/lmr/m/n/10 󰀀


! OK.
l.10 \sbox0{WHAT\char"4444} \showbox0
                                     
? 
Width is 34.47pt
(./testbox.aux)
 ***********
LaTeX2e <2024-11-01> patch level 2
L3 programming layer <2025-01-18>
 ***********
)

Here is how much of LuaTeX's memory you used:
 1821 strings out of 475506
 100000,552014 words of node,token memory allocated
 477 words of node memory still in use:
   4 hlist, 1 vlist, 1 rule, 2 glue, 4 kern, 7 glyph, 7 attribute, 48 glue_spec,
 7 attribute_list, 2 write nodes
   avail lists: 2:7,3:1,4:1,5:4,7:1,9:1
 24915 multiletter control sequences out of 65536+600000
 15 fonts using 723367 bytes
 35i,1n,49p,130b,1352s stack positions out of 10000i,1000n,20000p,200000b,200000s

warning  (pdf backend): no pages of output.

PDF statistics: 0 PDF objects out of 1000 (max. 8388607)
 0 named destinations out of 1000 (max. 131072)
 1 words of extra memory for PDF output out of 10000 (max. 100000000)

With the last release of LuaTeX, we can set \tracinglostchar to 4 or 5

Minimal example showing the desired new behaviour

\RequirePackage{latexbug}
\documentclass{article}

\tracinglostchars=4 % <-- change here
\showboxbreadth=1000
\showboxdepth=1000

\begin{document}

\sbox0{WHAT\char"4444} \showbox0
\typeout{Width is \the\wd0}

%\box0

\end{document}

This is the new log file

This is LuaHBTeX, Version 1.21.0 (TeX Live 2025)  (format=lualatex 2025.3.1)  2 MAR 2025 17:31
 restricted system commands enabled.
**testbox.tex
(./testbox.tex
LaTeX2e <2024-11-01> patch level 2
L3 programming layer <2025-01-18>
Lua module: luaotfload 2024-12-03 v3.29 Lua based OpenType font support
Lua module: lualibs 2023-07-13 v2.76 ConTeXt Lua standard libraries.
Lua module: lualibs-extended 2023-07-13 v2.76 ConTeXt Lua libraries -- extended 
collection.
luaotfload | conf : Root cache directory is "/Users/enrico2013/Library/texlive/2
025/texmf-var/luatex-cache/generic/names".
luaotfload | init : Loading fontloader "fontloader-2023-12-28.lua" from kpse-res
olved path "/usr/local/texlive/2025/texmf-dist/tex/luatex/luaotfload/fontloader-
2023-12-28.lua".
Lua-only attribute luaotfload@noligature = 1
luaotfload | init : Context OpenType loader version 3.134
Inserting `luaotfload.node_processor' in `pre_linebreak_filter'.
Inserting `luaotfload.node_processor' in `hpack_filter'.
Inserting `luaotfload.glyph_stream' in `glyph_stream_provider'.
Inserting `luaotfload.define_font' in `define_font'.
Lua-only attribute luaotfload_color_attribute = 2
luaotfload | conf : Root cache directory is "/Users/enrico2013/Library/texlive/2
025/texmf-var/luatex-cache/generic/names".
Inserting `luaotfload.harf.strip_prefix' in `find_opentype_file'.
Inserting `luaotfload.harf.strip_prefix' in `find_truetype_file'.
Removing  `luaotfload.glyph_stream' from `glyph_stream_provider'.
Inserting `luaotfload.harf.glyphstream' in `glyph_stream_provider'.
Inserting `luaotfload.harf.finalize_vlist' in `post_linebreak_filter'.
Inserting `luaotfload.harf.finalize_hlist' in `hpack_filter'.
Inserting `luaotfload.cleanup_files' in `wrapup_run'.
Inserting `luaotfload.harf.finalize_unicode' in `finish_pdffile'.
Inserting `luaotfload.glyphinfo' in `glyph_info'.
Lua-only attribute luaotfload.letterspace_done = 3
Inserting `luaotfload.aux.set_sscale_dimens' in `luaotfload.patch_font'.
Inserting `luaotfload.aux.set_font_index' in `luaotfload.patch_font'.
Inserting `luaotfload.aux.patch_cambria_domh' in `luaotfload.patch_font'.
Inserting `luaotfload.aux.fixup_fontdata' in `luaotfload.patch_font_unsafe'.
Inserting `luaotfload.aux.set_capheight' in `luaotfload.patch_font'.
Inserting `luaotfload.aux.set_xheight' in `luaotfload.patch_font'.
Inserting `luaotfload.rewrite_fontname' in `luaotfload.patch_font'.
Inserting `tracingstacklevels' in `input_level_string'. (/usr/local/texlive/202
5/texmf-dist/tex/latex/latexbug/latexbug.sty
Package: latexbug 2024/07/16 v1.0p Bug-classification
)
(/usr/local/texlive/2025/texmf-dist/tex/latex/base/article.cls
Document Class: article 2024/06/29 v1.4n Standard LaTeX document class
(/usr/local/texlive/2025/texmf-dist/tex/latex/base/size10.clo
File: size10.clo 2024/06/29 v1.4n Standard LaTeX file (size option)
luaotfload | db : Font names database loaded from /Users/enrico2013/Library/texl
ive/2025/texmf-var/luatex-cache/generic/names/luaotfload-names.luc.gz)
\c@part=\count194
\c@section=\count195
\c@subsection=\count196
\c@subsubsection=\count197
\c@paragraph=\count198
\c@subparagraph=\count199
\c@figure=\count266
\c@table=\count267
\abovecaptionskip=\skip49
\belowcaptionskip=\skip50
\bibindent=\dimen140
)
(/usr/local/texlive/2025/texmf-dist/tex/latex/l3backend/l3backend-luatex.def
File: l3backend-luatex.def 2024-05-08 L3 backend support: PDF output (LuaTeX)
\l__color_backend_stack_int=\count268
Inserting `l3color' in `luaotfload.parse_color'.
\l__pdf_internal_box=\box52
)
(./testbox.aux)
\openout1 = testbox.aux

LaTeX Font Info:    Checking defaults for OML/cmm/m/it on input line 8.
LaTeX Font Info:    ... okay on input line 8.
LaTeX Font Info:    Checking defaults for OMS/cmsy/m/n on input line 8.
LaTeX Font Info:    ... okay on input line 8.
LaTeX Font Info:    Checking defaults for OT1/cmr/m/n on input line 8.
LaTeX Font Info:    ... okay on input line 8.
LaTeX Font Info:    Checking defaults for T1/cmr/m/n on input line 8.
LaTeX Font Info:    ... okay on input line 8.
LaTeX Font Info:    Checking defaults for TS1/cmr/m/n on input line 8.
LaTeX Font Info:    ... okay on input line 8.
LaTeX Font Info:    Checking defaults for TU/lmr/m/n on input line 8.
LaTeX Font Info:    ... okay on input line 8.
LaTeX Font Info:    Checking defaults for OMX/cmex/m/n on input line 8.
LaTeX Font Info:    ... okay on input line 8.
LaTeX Font Info:    Checking defaults for U/cmr/m/n on input line 8.
LaTeX Font Info:    ... okay on input line 8.

Missing character: There is no 䑄 (U+4444) in font [lmroman10-regular]:+tlig;!
> \box0=
\hbox(7.16+0.22)x34.47, direction TLT
.\TU/lmr/m/n/10 W
.\TU/lmr/m/n/10 H
.\TU/lmr/m/n/10 A
.\kern-0.83 (font)
.\TU/lmr/m/n/10 T
.\TU/lmr/m/n/10 䑄
.\TU/lmr/m/n/10 󰀀

! OK.
l.10 \sbox0{WHAT\char"4444} \showbox0
                                     
? 
Width is 34.47pt
(./testbox.aux)
 ***********
LaTeX2e <2024-11-01> patch level 2
L3 programming layer <2025-01-18>
 ***********
)

Here is how much of LuaTeX's memory you used:
 1821 strings out of 475506
 100000,552014 words of node,token memory allocated
 477 words of node memory still in use:
   4 hlist, 1 vlist, 1 rule, 2 glue, 4 kern, 7 glyph, 7 attribute, 48 glue_spec,
 7 attribute_list, 2 write nodes
   avail lists: 2:7,3:1,4:1,5:4,7:1,9:1
 24915 multiletter control sequences out of 65536+600000
 15 fonts using 723367 bytes
 35i,1n,49p,130b,1352s stack positions out of 10000i,1000n,20000p,200000b,200000s

warning  (pdf backend): no pages of output.

PDF statistics: 0 PDF objects out of 1000 (max. 8388607)
 0 named destinations out of 1000 (max. 131072)
 1 words of extra memory for PDF output out of 10000 (max. 100000000)

If the value is set to 5, an error is raised (like for the value 3 in pdflatex).

Missing character: There is no 䑄 (U+4444) in font [lmroman10-regular]:+tlig;!
.
\sbox  ...box {\color@setgroup #2\color@endgroup }

l.10 \sbox0{WHAT\char"4444}