syntax_data: {"ast_node_type":"other","tokens":[{"end":24,"kind":"keyword","start":0,"text":"start_offset:"}]}
source_snippet:     start_offset: usize,
context_before:     /// The length of the prefix in `out` (e.g. 2 for `_R`).
context_after:     /// The values are start positions in `out`, in bytes.