syntax_data: {"ast_node_type":"function","tokens":[{"end":68,"kind":"keyword","start":0,"text":"//"}]}
source_snippet:     // optimization for some large types produced by async fn trees.
context_before:     // Cache of all expansions we've seen so far. This is a critical
context_after:     expanded_cache: FxHashMap<(DefId, GenericArgsRef<'tcx>), Ty<'tcx>>,