Warning about .toc files

Question

I want to add volume in front of the part level, I've implemented this feature, but there is a warning that I have not been able to dismiss.

The MWE is as follows:

\documentclass{book}
\usepackage{titlesec}
\makeatletter
    \newcounter{volume}
    \setcounter{volume}{0}
    \def\toclevel@volume{-2}
    \renewcommand\thevolume{Vol. \@arabic\c@volume}
    \newcommand\volume{
        \if@openright
            \cleardoublepage
        \else
            \clearpage
        \fi
        \thispagestyle{empty}
        \if@twocolumn
            \onecolumn
            \@tempswatrue
        \else
            \@tempswafalse
        \fi
        \null\vfil
        \secdef\@volume\@svolume}
    \def\@volume[#1]#2{
        \ifnum \c@secnumdepth >-3\relax
            \refstepcounter{volume}
            \addcontentsline{toc}{volume}{\thevolume\hspace{1em}#1}
        \else
            \addcontentsline{toc}{volume}{#1}
        \fi
        \markboth{}{}
        {\centering
        \interlinepenalty \@M
        \normalfont
        \ifnum \c@secnumdepth >-3\relax
            \Huge\bfseries \volumename\nobreakspace\@arabic\c@volume
            \par
            \vskip 20\p@
        \fi
        \Huge \bfseries #2\par}
        \@endvolume}
    \def\@svolume#1{
        {\centering
            \interlinepenalty \@M
            \normalfont
            \Huge \bfseries #1\par}
        \@endvolume}
    \def\@endvolume{\vfil\newpage
        \if@twoside
        \if@openright
            \null
            \thispagestyle{empty}
            \newpage
        \fi
        \fi
        \if@tempswa
            \twocolumn
        \fi}
    \newcommand*\l@volume[2]{
        \ifnum \c@tocdepth >-3\relax
            \addpenalty{-\@highpenalty}
            \addvspace{2.25em \@plus\p@}
            \setlength\@tempdima{3em}
            \begingroup
            \parindent \z@ \rightskip \@pnumwidth
            \parfillskip -\@pnumwidth
            {\leavevmode
                \large \bfseries #1\hfil
                \hb@xt@\@pnumwidth{\hss #2
                \kern-\p@\kern\p@}}\par
            \nobreak
            \global\@nobreaktrue
            \everypar{\global\@nobreakfalse\everypar{}}%
            \endgroup
        \fi}
    \newcommand\volumename{Volume}
\makeatother

\usepackage{titletoc}
\setlength{\titlewidth}{0.5\textwidth}
\titlecontents{volume}
    [0pc]
    {\addvspace{1pc}\bfseries\large\titlerule\addvspace{.3pc}\centering}
    {}
    {}
    {\enspace \contentspage}
    [\addvspace{.3pc}\titlerule]

\usepackage{hyperref}
\hypersetup{colorlinks=true,linkcolor=red}
\makeatletter
    \@addtoreset{part}{volume}
    \@addtoreset{chapter}{volume}
\makeatother

\title{MWE}
\begin{document}
    \tableofcontents

    \volume{Volume.1}
    \part{Part.1.1}
    \chapter{Chap.1.1.1}
    \chapter{Chap.1.1.2}
    \part{Part.1.2}
    \chapter{Chap.1.2.1}
    \chapter{Chap.1.2.2}

    \volume{Volume.2}
    \part{Part.2.1}
    \chapter{Chap.2.1.1}
    \chapter{Chap.2.1.2}
\end{document}

A warning appears after compilation:

Package titletoc: Unknown TOC type volume. I'll set it for you with
(titletoc)  level -1000..

If you comment out the following sections, the warning disappears.

\titlecontents{volume}
    [0pc]
    {\addvspace{1pc}\bfseries\large\titlerule\addvspace{.3pc}\centering}
    {}
    {}
    {\enspace \contentspage}
    [\addvspace{.3pc}\titlerule]

Is there a way to implement functionality while eliminating warnings?

Answer 1

You mixing two different methods the facilities provided by titlesec and titletoc and those of LaTeX2e and the standard classes. You used the latter. That is why when you remove the lines you mentioned it works :) Do not mix the two methods. Use the facilities of the standard classes + LaTeX2e IMHO is always a better approach.

\documentclass{book}

\makeatletter
\newcounter{volume}
\setcounter{volume}{0}
\def\toclevel@volume{-2}
\newcommand\volume{
    \if@openright
    \cleardoublepage
    \else
    \clearpage
    \fi
    \thispagestyle{empty}
    \if@twocolumn
    \onecolumn
    \@tempswatrue
    \else
    \@tempswafalse
    \fi
    \null\vfil
    \secdef\@volume\@svolume}
\def\@volume[#1]#2{
    \ifnum \c@secnumdepth >-3\relax
    \refstepcounter{volume}
    \addcontentsline{toc}{volume}{\thevolume\hspace{1em}#1}
    \else
    \addcontentsline{toc}{volume}{#1}
    \fi
    \markboth{}{}
    {\centering
    \interlinepenalty \@M
    \normalfont
    \ifnum \c@secnumdepth >-3\relax
        \Huge\bfseries \volumename\nobreakspace\@arabic\c@volume
        \par
        \vskip 20\p@
    \fi
    \Huge \bfseries #2\par}
    \@endvolume}
\def\@svolume#1{
    {\centering
        \interlinepenalty \@M
        \normalfont
        \Huge \bfseries #1\par}
    \@endvolume}
\def\@endvolume{\vfil\newpage
    \if@twoside
    \if@openright
        \null
        \thispagestyle{empty}
        \newpage
    \fi
    \fi
    \if@tempswa
    \twocolumn
    \fi}
\newcommand*\l@volume[2]{
    \ifnum \c@tocdepth >-3\relax
    \addpenalty{-\@highpenalty}
    \addvspace{2.25em \@plus\p@}
    \setlength\@tempdima{3em}
    \begingroup
    \parindent \z@ \rightskip \@pnumwidth
    \parfillskip -\@pnumwidth
    {\leavevmode
        \large \bfseries #1\hfil
        \hb@xt@\@pnumwidth{\hss #2
        \kern-\p@\kern\p@}}\par
    \nobreak
    \global\@nobreaktrue
    \everypar{\global\@nobreakfalse\everypar{}}%
    \endgroup
    \fi}
\newcommand\volumename{Volume}

\usepackage{hyperref}
\begin{document}
    \tableofcontents
    \volume{Vol.1}
    \part{Part.1}
    \chapter{chap.1}
    \chapter{chap.2}
    \volume{Vol.2}
    \part{Part.2}
    \chapter{chap.3}
    \chapter{chap.4}
\end{document}