Why does pdflatex write to toc-file \cyr-commands even with utf8 document encoding?

Question

Why does pdflatex (but not lualatex) write to toc-file \cyr-commands even with utf8 document encoding instead of normal cyrillic letters? How to prevent it? I interested in solution with redefining book.cls commands. I redefined the \addcontentsline{toc}{part}{\unexpanded\expandafter{{#1}}}% but it has no result.

% !TeX program = pdflatex
% !TeX encoding = utf8
% !TeX spellcheck = uk_UA
% !BIB program = bibtex8

\documentclass[12pt]{book}

\usepackage[a4paper]{geometry}

\usepackage[utf8]{inputenc}
\usepackage[T2A,T1]{fontenc}
\usepackage[ukrainian]{babel}

\usepackage{indentfirst}
\usepackage{misccorr}

\makeatletter
\def\@part[#1]#2{%
    \thispagestyle{empty}
    \addcontentsline{toc}{part}{\unexpanded\expandafter{{#1}}}%
    {\centering
        \interlinepenalty \@M
        \normalfont
        \Huge \sffamily\bfseries #1\par}%
    \@endpart
}
\makeatother

\begin{document}
\tableofcontents
\part{Перша}

іва
\part{Друга}
іфавіфав
\end{document}

Toc looks like

\select@language {ukrainian}
\contentsline {part}{{\IeC {\CYRP }\IeC {\cyre }\IeC {\cyrr }\IeC {\cyrsh }\IeC {\cyra }}}{3}
\contentsline {part}{{\IeC {\CYRD }\IeC {\cyrr }\IeC {\cyru }\IeC {\cyrg }\IeC {\cyra }}}{7}

Answer 1

You can try with \detokenize.

For use with hyperref: one needs both of \texorpdfstring and \phantomsection in the patch of \@part.

\documentclass[12pt]{book}

\usepackage[a4paper]{geometry}

\usepackage[T2A,T1]{fontenc}

\usepackage[utf8]{inputenc}

\usepackage[ukrainian]{babel}

\usepackage{indentfirst}
\usepackage{misccorr}

% update for hyperref compatibility

\makeatletter
\def\@part[#1]#2{%
    \thispagestyle{empty}%
    \phantomsection
    \addcontentsline{toc}{part}{\texorpdfstring{\detokenize{#1}}{#1}}%
    {\centering
        \interlinepenalty \@M
        \normalfont
        \Huge \sffamily\bfseries #1\par
    }%
    \@endpart
}
\makeatother

\usepackage[pdfencoding=auto]{hyperref}

\begin{document}
\tableofcontents
\part{Перша}

іва
\part{Друга}
іфавіфав
\end{document}

Contents of toc file:

\select@language {ukrainian}
\contentsline {part}{Перша}{3}
\contentsline {part}{Друга}{7}

and with hyperref:

\select@language {ukrainian}
\contentsline {part}{Перша}{3}{section*.2}
\contentsline {part}{Друга}{7}{section*.3}

Answer 2

In principle at least different parts of the document can be using different input encodings so the mechanism was designed to always write auxiliary files in the encoding neutral LICR ascii markup.

In practice, especially of you are using UTF-8, a single encoding is used for the whole document, but UTF-8, and UTF-8 support on LaTeX are some years younger than the design of the encoding support for toc files.