deriver returned an unsupported value type

Why this fires¶

The @derive_for function returned a value that is not a supported derive result. The compatibility contract accepts str; the structured contract accepts Syntax<.Items> and Syntax<.Item>.

Fix it¶

Prefer a structured item quote:

@derive_for(Tag)
fn derive_tag(info) -> Syntax<.Items> {
  return quote items { impl ${info.ref} {} }
}