defcdash#1#2#3{deftempx@{#3}%
deftempa@{-}deftempb@{~}deftempc@{*}deftempd@{=}%
 ifxtempx@tempa@@Acdashelse
  ifxtempx@tempb@@Bcdashelse
   ifxtempx@tempc@@Ccdashelse
    ifxtempx@tempd@@Dcdashelse
    errmessage{Wrong usage of cdash}fifififi}
def@Acdash{ifdimlastskip>z@unskipnobreakhskip.2emfi
  cyrdashhskip.2emignorespaces}%
def@Bcdash{leavevmodeifdimlastskip>z@unskipfi
 nobreakcyrdashpenaltyexhyphenpenaltyhskipz@skipignorespaces}%
def@Ccdash{leavevmode
 nobreakcyrdashnobreakhskip.35emignorespaces}%
def@Dcdash{ifdimlastskip>z@unskipnobreakhskip.2emfi
  cyrdashnobreakhskip.2emignorespaces}

def@Dcdash{ifdimlastskip>z@unskipnobreakhskip.2emfi
  cyrdashnobreakhskip.2emignorespaces}

! Wrong usage of cdash.
cdash ...else errmessage {Wrong usage of cdash}
                                                  fi fi fi
l.38            ╨Я╤А╨╕╨▓╨╡╤В "--=
                       ╨╝╨╕╤А
?

documentclass{article}

usepackage{polyglossia}

setmainlanguage[babelshorthands = true]{russian}

defaultfontfeatures{Ligatures = TeX}

newfontfamily{cyrillicfont}{Georgia}

makeatletter

defcdash#1#2#3{deftempx@{#3}%
deftempa@{-}deftempb@{~}deftempc@{*}deftempd@{=}%
 ifxtempx@tempa@@Acdashelse
  ifxtempx@tempb@@Bcdashelse
   ifxtempx@tempc@@Ccdashelse
    ifxtempx@tempd@@Dcdashelse
    errmessage{Wrong usage of cdash}fifififi}
def@Acdash{ifdimlastskip>z@unskipnobreakhskip.2emfi
  cyrdashhskip.2emignorespaces}%
def@Bcdash{leavevmodeifdimlastskip>z@unskipfi
 nobreakcyrdashpenaltyexhyphenpenaltyhskipz@skipignorespaces}%
def@Ccdash{leavevmode
 nobreakcyrdashnobreakhskip.35emignorespaces}%
def@Dcdash{ifdimlastskip>z@unskipnobreakhskip.2emfi
  cyrdashnobreakhskip.2emignorespaces}

makeatother

usepackage{mathtools}

begin{document}

    begin{minipage}{0pt}
        Привет "--- мир

        Привет "--= мир
    end{minipage}


end{document}