The sole purpose to add Dev::Trace::Inspector module is to make custom inspection possible and efficient while tracing. For example, ActiveRecord::Relation#inspect makes additional queries to fetch top 10 records and generate the output everytime. To avoid this, Inspector will not call inspect method when it finds such objects (deeply nested anywhere). Instead, it’ll call AR::Relation#to_sql to get plain SQL query which doesn’t make additional queries and is better to understand in tracing output.