syntax_data: {"ast_node_type":"other","tokens":[{"end":100,"kind":"keyword","start":0,"text":"//"}]}
source_snippet:                     // For compiler developers who'd like to add new items to `Fn`/`FnMut`/`FnOnce`,
context_before:                 {
context_after:                     // you either need to generate a shim body, or perhaps return