syntax_data: {"ast_node_type":"function","tokens":[{"end":34,"kind":"keyword","start":0,"text":"fn"}]}
source_snippet:     fn decode(d: &mut D) -> Self {
context_before: impl<D: SpanDecoder> Decodable<D> for LocalExpnId {
context_after:         ExpnId::expect_local(ExpnId::decode(d))