{"ast_node_type":"other","tokens":[{"end":79,"kind":"keyword","start":0,"text":"///"}]}
    /// but this hook allows e.g. codegen to return only `TyAndLayout` from its
    /// Most `impl`s, which propagate `LayoutError`s, should simply return `err`,
    /// `cx.layout_of(...)`, without any `Result<...>` around it to deal with