cyrdash is defined in T2A, T2B, T2C and some other legacy encodings but seems to escape from EU1 (xetex) and EU2 (luatex) encodings. cyrdash should, by idea, print something intermediate between \textemdash and \textendash (\cyrdash is 20% shorter than emdash). It is used in Russian, Ukrainian, Bulgarian and Mogolian languages instead of emdash, so these languages are by fact discriminated.
If I am right and \cyrdash is indeed lost, could the latex development team please extend EU1 and EU2 encodings to provide a text command for \cyrdash? It might be called \textcyrdash.
What is the codepoint of cyrdash in Unicode tables?
UPDATE: Here is Minimal Working Example. Results depend on the engine used to compile the example. I tried xelatex and pdflatex.
\documentclass{article}
\usepackage{ifxetex}
\ifxetex
\else
\usepackage[T1,T2A]{fontenc}
\usepackage[utf8]{inputenc}
\fi
\usepackage[english,russian]{babel}
\begin{document}
\Large
Russian: \\
!---!=\verb!---!\\
!\foreignlanguage{english}{---}!=\verb!\foreignlanguage{english}{---}!\\
!\cyrdash!=\verb!\cyrdash!\\
!\textemdash!=\verb!\textemdash!\\
!\textendash!=\verb!\textendash!\\
%!---!\par
\selectlanguage{english}
%!---!
English: \\
!---!=\verb!---!\\
!\cyrdash!=\verb!\cyrdash!\\
!\textemdash!=\verb!\textemdash!\\
!\textendash!=\verb!\textendash!\\
\end{document}
Babel with option russian redefines ligature --- through \cyrdash. Pdflatex takes \cyrdash and \textemdash from a same codepoint of LH font if Russian language is selected; then their sizes are equal; if Englsih is current language than \cyrdash is undefined (rises an error), and \textemdash is taken from CM fonts and is longer as compared to itself in case if Russian is selected.
In case of xelatex, \cyrdash macro is not defined in current encoding so babel fakes it as \hbox to 0.8em{--\hss--} which turns out to be approximately 80% of \textemdash.
Ulrike Fischer directed me to a valuable list of all available dashes in Unicode. I did not find a reasonable candidate for \cyrdash there although from my own experience of publishing books I know that editors of Russian publishing houses sometimes are very rigorous regarding the size of dashes. I would also agree that Microsoft Word automatically produces correct dashes between russian words but I am not sure (and cannot check right now) if it produces longer dashes between English words.
So I don't see now a good uniform solution for \cyrdash in case when it should be taken from the same font as \textemdash. Perhaps, except that soultion which is currently used in babel-russian. Can some one propose a better way?
UPDATE 2: My original vision of the problem was wrong. With the aid of respected TeX'perts it became clear that legacy LaTeX engines take \cyrdash and \textemdash from the same codepoint 22 but possibly from different fonts. As a result, ligature --- might change its length after \selectlanguage. So, now I would reformulate my question as follows:
Is is good or bad practise to mix two versions of ligature --- (which differ by their width) in the same multilanguage document?
TUname, but these are not encodings in the traditional sense. They reflect that fact that for a Unicode font there is 'nothing to do here': each font slot is defined by the Unicode Consortium. If the latter don't provide a suitable dash then it can't appear inTU, though one can of course define\cyrdashas a macro.\textthreequartersemdashwhich points to U+2012).