syntax_data: {"ast_node_type":"other","tokens":[{"end":87,"kind":"keyword","start":0,"text":"//!"}]}
source_snippet: //! changing a by-ref upvar to be by-value in the case that the outer coroutine-closure
context_before: //! to take the set of upvars of the parent closure by value. This at least requires
context_after: //! captures something by value; however, it may also require renumbering field indices