syntax_data: {"ast_node_type":"function","tokens":[{"end":71,"kind":"keyword","start":0,"text":"fn"}]}
source_snippet:     fn merge_spans(&self, sp_lhs: Span, sp_rhs: Span) -> Option<Span> {
context_before:     ///    * the LHS span must start at or before the RHS span.
context_after:         // Ensure we're at the same expansion ID.