{"ast_node_type":"other","tokens":[{"end":67,"kind":"keyword","start":0,"text":"mapped_regions:"}]}
    mapped_regions: IndexMap<I::PlaceholderRegion, I::BoundRegion>,
    // the `var` (but we *could* bring that into scope if we were to track them as we pass them).
    mapped_types: IndexMap<I::PlaceholderTy, I::BoundTy>,