Function generate_foreign_item 

fn generate_foreign_item(
    ecx: &mut ExtCtxt<'_>,
    eii_attr_span: Span,
    item_span: Span,
    item_kind: &ItemKind,
    vis: Visibility,
    attrs_from_decl: ThinVec<Attribute>,
) -> Box<Item>

Generates a foreign item, like

extern "…" { safe fn item(); }