syntax_data: {"ast_node_type":"other","tokens":[{"end":60,"kind":"keyword","start":0,"text":"//"}]}
source_snippet: // or should go through `FnAbi` instead, to avoid losing any
context_before: // for `Instance` (e.g. typeck would use `Ty::fn_sig` instead),
context_after: // adjustments `fn_abi_of_instance` might be performing.