{"ast_node_type":"variable","tokens":[{"end":41,"kind":"keyword","start":0,"text":"let"}]}
    let icx = ItemCtxt::new(tcx, def_id);
fn impl_trait_header(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Option<ty::ImplTraitHeader<'_>> {
    let item = tcx.hir_expect_item(def_id);