To indicate "a collection of items," do you say "an item collection" or "items collection"? What's the rule?
For the sake of clarity, I'd suggest simply stating "a collection of items," However, to answer your question, I am inclined to think that they are both unclear in some sense.
Say I have a collection of phones. If I were to say:
- "I have a phone collection."
It is quite easy to understand, at least colloquially, but we need to understand what this means. In this sentence, phone is not a noun, but an adjective. It is describing the collection as a whole, not necessarily the constituents thereof.
- "I have a phones collection."
In this case, we are pluralizing an adjective which is not conventionally done in this scenario.
There are certain scenarios in which the latter would be more correct. For example:
- "I have a clothes basket."
"I have an arts degree."
The plural form is more appropriately used when there is little distinction between generic and specific meanings.