documentclass{article}
usepackage{enumitem}
usepackage{environ}

newlengthwidest
makeatletter
NewEnviron{ldescription}{%
  vbox{%
    globalsetlengthwidest{0pt}%
    defitem[##1]{%
      settowidth@tempdima{textbf{##1}}%
      ifdim@tempdima>widestglobalsetlengthwidest{@tempdima}fi%
    }%
    setbox0=hbox{BODY}%
  }
  begin{description}[
    leftmargin=dimexprwidest+0.5emrelax,
    labelindent=0pt,
    labelwidth=widest]
  BODY
  end{description}%
}
makeatother

begin{document}

begin{ldescription}
item[Short] text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text
item[A really really long label] text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text
end{ldescription}

begin{ldescription}
item[Short] text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text
item[A medium label] text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text
end{ldescription}

end{document}

documentclass{article}
usepackage{enumitem}
usepackage{environ}

newlengthwidest
makeatletter
NewEnviron{ldescription}{%
  vbox{%
    globalsetlengthwidest{0pt}%
    defitem[##1]{%
      settowidth@tempdima{textbf{##1}}%
      ifdim@tempdima>widestglobalsetlengthwidest{@tempdima}fi%
    }%
    setbox0=hbox{BODY}%
  }
  begin{description}[
    leftmargin=dimexprwidest+0.5emrelax,
    labelindent=0pt,
    labelwidth=widest]
  BODY
  end{description}%
}
makeatother

begin{document}

begin{ldescription}
item[Short] text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text
item[A label with a nested list] I would like a nested list here.
    begin{ldescription}
        item[Short] text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text
        item[A long label] text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text
    end{ldescription}
end{ldescription}

end{document}