syntax_data: {"ast_node_type":"variable","tokens":[{"end":92,"kind":"keyword","start":0,"text":"let"}]}
source_snippet:         let a_is_callsite = a.ctxt.is_root() || a.ctxt == b.span().source_callsite().ctxt();
context_before:         // `is_root` here is a fast path optimization.
context_after:         Err(if a_is_callsite { b_orig } else { a_orig })