syntax_data: {"ast_node_type":"other","tokens":[{"end":65,"kind":"keyword","start":0,"text":"iter::once(env_ty).chain([sig.tupled_inputs_ty]),"}]}
source_snippet:                 iter::once(env_ty).chain([sig.tupled_inputs_ty]),
context_before:             tcx.mk_fn_sig(
context_after:                 sig.to_coroutine_given_kind_and_upvars(