Make ListItemProcessor::build() throw an exception when no datasource is available

Is this processor useful at all for those kinds of fields? I'd prefer to throw a hard error instead I think.

Something like:

if (!$field->getDatasourceId()) {
  throw new InvalidConfigurationException("This field has no datasource, so there's no valid use for this processor");
}

However, that might be a little overkill. Not sure. In what scenario is it useful to have this processor enabled when no datasource is available?

I have no idea how to solve this. I'll ask drunken monkey for help.

The proposed solution doesn't seem much of an improvement – it would really just help you if that field is from a node, or by chance corresponds to a node field. It adds much more chaos than functionality, in my opinion.

If you want to make an educated guess of a datasource-independent field's option list/labels, you can try looking into the return value of $field->getDataDefinition() – I don't really know where to go from there, though. (I've been looking for a way to generically retrieve a field's option list from the definition myself, but it doesn't seem like that's possible anymore in Drupal 8.)

@ d.novikov: Is there a way to get the options list from the data definition in your case? Maybe the Facets module (and others) could just define and suppport a generic way to set an options list (callback) there? That would seem to be the cleanest solution for this to me.
But it does sound like a pretty niche feature, so just saying you won't support such functionality (and throw an exception instead) also seems like a valid solution to me.

By the way, in any case, there is also $field->getDatasource()->getEntityTypeId() for getting the entity type – hard-coding the "entity:" prefix is not necessary and should be avoided for compatibility with other contrib modules that might add entity datasources.

I agree with @drunken monkey, we could only do an educated guess. I prefer the exception.

Committed, thanks!