{"ast_node_type":"variable","tokens":[{"end":63,"kind":"keyword","start":0,"text":"let"}]}
    let principal_pred = poly_trait_ref.map_bound(|trait_ref| {
    assert!(!poly_trait_ref.has_non_region_param());
        ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref))