syntax_data: {"ast_node_type":"other","tokens":[{"end":67,"kind":"keyword","start":0,"text":"//"}]}
source_snippet:                 // `Subtype`   :Only transmutes the type, so moves.
context_before:                 // `Downcast`  :Only changes information about a `Place` without moving.
context_after:                 // So it's safe to skip these.