syntax_data: {"ast_node_type":"other","tokens":[{"end":71,"kind":"keyword","start":0,"text":"iter::once(env_ty).chain(sig.inputs().iter().cloned()),"}]}
source_snippet:                 iter::once(env_ty).chain(sig.inputs().iter().cloned()),
context_before:             tcx.mk_fn_sig(
context_after:                 sig.output(),