hyphenrules no longer defined in polyglossia

The hyphenrules trick was a hack. Since version 1.45, polyglossia has a specific method for defining language specific hyphenation exceptions, namely

pghyphenation[<options>]{<language>}{<exception list>}

The <options> are for the particular language to set a variant. The <exception list> has the same syntax as the argument to hyphenation, so words separated by spaces with hyphens at the chosen break points.

As of version 1.50a, the documentation about hyphenation exceptions is inaccurate, because it is claimed that hyphenation acts globally for all language, but this is not true: the exceptions are just for the current language which, in the preamble is still language 0 (American English).

Thus hyphenation should actually never be used (unless the main language is American English).

The hyphenrules environment has not been deprecated, but it should only be used in the document body. On the other hand, as of version 1.50a the hyphenrules trick still works. This doesn't mean it should be used, but older documents using it will work.

documentclass{article}

usepackage{polyglossia}
setmainlanguage[variant=nynorsk]{norwegian}

pghyphenation[variant=nynorsk]{norwegian}{fram-halds-skulen}

begin{document}

parbox{0pt}{hspace{0pt}framhaldsskulen}

end{document}

From TeX primitive point of view you needs only to use heyphenation primitive when language primitive register is set to the selected language. So, you can write:

{language=lang.number hyphenation{fram-halds-skulen}}

A little problem is that if you are using LaTeX then you don't know the lang.number because it is auto-generated by macros. You can set the language register using LaTeX's command:

selectlanguage{nynorsk}

This command does language=lang.number internally. So, you can use the hyphenation primitive by:

{selectlanguage{nynorsk} hyphenation{fram-halds-skulen}}

Of course, you can give hyphenation exceptions for all declared languages:

 {selectlanguage{nynorsk} hyphenation{fram-halds-skulen}}
 {selectlanguage{czech}   hyphenation{výjme-čné dě-lení}}
 ...

For sake of complexity: if you are using csplain or OpTeX format then you know the number of your language, see hyphen.lan file. Norwegian Nynorsk in Unicode has language 136. So, you can do your task at primitive level only:

{language=136 hyphenation{fram-halds-skulen}}

The environment hyphenrules is deprecated in newer versions of polyglossia. To add language specific hyphenations, use pghyphenation[(options)]{(language)}{(hyphenations)}. The current documentation (v. 1.49) states incorrectly in section 5.1 that the command is xpghyphenation. This is a mistake for pghyphenation which has yet to be corrected.

documentclass{article}

usepackage{polyglossia}
setmainlanguage[variant = nynorsk]{norwegian}

pghyphenation[variant = nynorsk]{norwegian}{fram-halds-skulen}

begin{document}

parbox{0pt}{hspace{0pt}framhaldsskulen}

end{document}