The section Interface checks from Effective Go recommends
var _ json.Marshaler = (*RawMessage)(nil)
as a compile-time check that RawMessage implements Marshaler.
I get how the assignment does the type check but what does the right side actually mean?
Ok, I figured it out. It creates a new *RawMessage (pointer to RawMessage) that is nil by casting nil to *RawMessage. I would have expected
*RawMessage(nil)
but that doesn't work because the cast conversion seems to take precedence over the pointer operator, so it would become a dereference.