Function parse_when_list 

fn parse_when_list(
    key: &Ident,
    content: ParseStream<'_>,
) -> Result<InheritsHint>

The content of a parameter-level when(...) hint in inherits(...) (key is the when keyword, for error spans): its classes, spelled as in inherits(...) (each string literal and each class = "..." names one class, none split on commas), and its required message = "...".