syntax_data: {"ast_node_type":"other","tokens":[{"end":80,"kind":"keyword","start":0,"text":"For"}]}
source_snippet: For the diagnostic output (in `Value::from_cycle_error`), we want to detect that
context_before:     -> representability(Foo)
context_after: the `Foo` in the *second* field of the struct is culpable. This requires