syntax_data: {"ast_node_type":"impl","tokens":[{"end":68,"kind":"keyword","start":0,"text":"mut"}]}
source_snippet:         mut encode_ctxt: impl FnMut(&mut T, u32, &SyntaxContextKey),
context_before:         encoder: &mut T,
context_after:         mut encode_expn: impl FnMut(&mut T, ExpnId, &ExpnData, ExpnHash),