syntax_data: {"ast_node_type":"other","tokens":[{"end":48,"kind":"keyword","start":0,"text":"b:"}]}
source_snippet:         b: <Self::Interner as Interner>::Region,
context_before:         a: <Self::Interner as Interner>::Region,
context_after:         span: <Self::Interner as Interner>::Span,