It is usually better if it can be captured in a library. But sometimes, particularly in weak languages, nothing but core language support will do. A fancy core language feature should embarrass the language designer.

Sometimes, a language evolves to the point where a core feature could be as well implemented as a library component. That is a marker of real progress, because now the feature can be offered in a dozen variations in a dozen libraries, each tailored to local conditions, where the core feature had to ruthlessly ignore all but the most generic use cases.