{"ast_node_type":"other","tokens":[{"end":78,"kind":"keyword","start":0,"text":"//"}]}
            // imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`.
            // At this point, we have deref'd `a` to `referent_ty`.  So
            // In the autoderef loop for `&'a mut Vec<T>`, we would get