There are exceptions (eg singles bar; systems analyst; travellers cheque; dogs home; working mens club [yes, usually without apostrophes]), but most nouns when used attributively are in the singular form. This 'rule' is not inviolate; a minority of examples of 'donuts eaters' can be found on the internet.
– Edwin AshworthJul 13 '23 at 11:02
1
Nobody says I eat cornflake, but there are far more written instances of singular [cornflake* eaters](https://www.google.com/search?tbm=bks&q=%22cornflake+eaters%22) in Google Books than there are of plural [cornflakes eaters*.](https://www.google.com/search?tbm=bks&q=%22cornflakes+eaters%22)
– FumbleFingersJul 13 '23 at 11:30