syntax_data: {"ast_node_type":"other","tokens":[{"end":54,"kind":"keyword","start":0,"text":"default_mode_span:"}]}
source_snippet:     default_mode_span: Option<(Span, ty::Mutability)>,
context_before:     /// lowered, with the span where it was introduced. `None` for a by-value default mode.
context_after:     /// Labels for where incompatibility-causing by-ref default binding modes were introduced.