syntax_data: {"ast_node_type":"other","tokens":[{"end":59,"kind":"keyword","start":0,"text":"//"}]}
source_snippet: // to track which `SyntaxContext`s we have already decoded.
context_before: // Decodes `SyntaxContext`, using the provided `HygieneDecodeContext`
context_after: // The provided closure will be invoked to deserialize a `SyntaxContextData`