syntax_data: {"ast_node_type":"other","tokens":[{"end":70,"kind":"keyword","start":0,"text":"///"}]}
source_snippet: /// and thus knows full well that `data` has type `&'b mut &'a mut T`.
context_before: /// You can see that the *impl* fully specified the type of the upvar
context_after: /// (Here, I am assuming that `data` is mut-borrowed.)