syntax_data: {"ast_node_type":"other","tokens":[{"end":58,"kind":"keyword","start":0,"text":"cycle_error.usage.as_ref().unwrap().0,"}]}
source_snippet:                     cycle_error.usage.as_ref().unwrap().0,
context_before:                 span_bug!(
context_after:                     "only `variances_of` returns `&[ty::Variance]`"